Digital radiography

was born in Tucson, Arizona in 1950. He received his B.A. degree in Applied Mathematics in 1972 from the University of California at Berkeley, and his M.D. degree in 1975 from the University of Arizona College of Medicine. Dr. Chernin completed an internship in Internal Medicine in 1976 at St. Louis University Hospitals. Subsequently he served for two years as a commissioned officer in the United States Public Health Service in California. Following this, he completed his residency in Diagnostic Radiology and a one year fellowship in Special Procedures Radiology, Ultrasound and Computed Tomography at the University of Arizona. Dr. Chernin joined the faculty of the University of Arizona in 1982, where he is an Assistant Professor in the Department of Radiology. Since October 1983 he has been Chief of Radiology at the Tucson Veterans Administration Medical Center. His primary professional interests are ultrasound, body CT, and special procedures.

DIGITIZATION has penetrated many aspects of our lives. This trend has been aided by the development of the digital computer with its ability to process vast amounts of data at high speed. Radiology has been affected by this phenomenon perhaps more than other medical subspecialities because of the newly emerging technologies that rely on digital acquisition of data: computerized tomography (CT), ultrasound, magnetic resonance imaging, and digital fluoroscopy. Digital subtraction arteriography (DSA) is part of the larger phenomenon of the digitization of radiology. This monograph discusses first, digital radiography as applied to diagnostic imaging in general, followed by a more detailed discussion of DSA. The basic technology is described and the clinical applications and limitations are reviewed and illustrated by numerous examples. DIGITAL

RADIOGRAPHY

Digital radiography may be defined as the acquisition, ulation, and storage of radiographic images in digital

manipformat. 7

Terms with similar meaning include photoelectronic radiography and computed radiography. These names all refer to the new generation of radiographic imaging systems that use digital recording and processing techniques, in particular the digital computer, as opposed to the older analog and film-screen techniques. There are several advantages to having medical imaging data in digital form. The data can be manipulated by computer software in ways that are not possible with analog information. For example, precise subtraction of images can be performed and the edges of faint structures can be visually enhanced. Digital images can be reproduced any number of times without loss of fidelity, and filing and retrieval systems can be automated. Transmission to remote viewing sites can be achieved by means of cables or telephone lines, permitting persons in geographically distant sites to view the same images simultaneously. Digital radiography consists of three fundamentally different processes (Fig 1). Data from the patient can be measured either digitally or conventionally. If measured digitally, the image can be reconstructed or assembled, whereas the conventional image is digitized with an analog to digital converter (ADO. In either event, the digital image is then processed by computer and displayed. An example of each process is given in Figure 1. CT scanning entails acquisition of data in digital form and reconstruction of the image by backprojection. In scanned projection radiography, used, for example, in the scout view on CT machines, the image is assembled line by line. Finally, in DSA the Fig l.-The

three

types

of digital

radiography,

with

clinical

examples

of each.

Digital Radiography J

\

Digital Measurement of Object Data

Conventional Image Generation

‘I

/

Reconstruction of Assembly of Digital Image Digital Image (e.g., Computed (e.g., Scanned Tomography) Projection Radiography, CT Localizing Mode) \

1 Image Processing, Display, Archiving

Image Digitization (e.g., Digital Video Angiography)

/

image is produced conventionally, appears on an image intensifier screen, and is transmitted by a television camera to an ADC. CT scanning is the most widely used digital imaging procedure today. Scanned projection radiography is largely experimental, except in its CT application (localization technique). Digitizing of conventional images has been most widely applied to angiography but is potentially usable in nearly any radiographic study. Ultrasound and nuclear medicine studies have been digitized with increasing frequency as well, and a completely digitized radiology service is not unthinkable. The savings from lower film costs are expected to be substantial, Use of the scanned projection technique for general radiography is currently under investigation.’ Early results with digital imaging of the chest indicate that pathologic states are seen as well as on conventional films, and mediastinal structures are seen better. Confidence levels are also higher than for conventional radiography. An additional advantage of experimental digital systems is that the entire dynamic range of information can be stored and used. Thus, by changing window levels and width, as on a CT console, the viewer is able to examine all densities that are present, from air to the densest body part. When viewing a bone lesion in an extremity, for example, one could set the monitor to examine boney structures and then quickly reset the dials to see the soft tissues. This is conceptually the same as varying the technical factors after the radiograph is made. This complete capture of information would eliminate the need to repeat radiographs because of inadequate technique. DiGITAL

SUBTRACTION

ARTERIOGRAPHY

Many synonyms for DSA can be found in the radiographic literature, each highlighting one or more features of this new methodology. Some of these terms include computerizedcomputed fluoroscopy, digital video subtraction angiography, digital vascular imaging, digital videoangiography, and computerized electronic radiography. The typical intravenous (IV) DSA study uses either continuous. or pulse-mode fluoroscopy equipment. Digital computing devices are essential for image processing. Subtraction, as described below, is a standard feature of data manipulation, and most commercial units produce a video image prior to digitization. Manual photographic subtraction has been known for many decades in radioloq. It was originally described in the German literature in 1935 and has been used as a standard angiographic tool since then. However, it is an analog technique and is quite laborious and time-consuming. Results are dependent, to a large extent, on the skill of the technologist. 9

Utilizing IV contrast injection for angiography is also an old concept, first described in the English literature by Robb and Steinberg3 for visualization of the cardiac chambers, pulmonary circulation, and great vessels (Fig 2). The more recent development of photoelectronic systems and digital computers has made the combination of these two techniques-IV contrast injection and image subtraction-much more powerful.

SYSTEM COMPONENTS The basic components of a typical digital system are illustrated in Figure 3. The images are produced at the image intensifier, viewed by a high-resolution television camera, and transmitted by closed circuit to an ADC. Each image is converted point by point to a number which encodes the position of each pixel or picture element and its shade of gray. The digitized image is then processed by the computer, stored digitally, and displayed. The computer processes the image using subtraction and enhancement techniques. Temporal (time) subtraction is in some ways analogous to manual subtraction. An image frame is selected prior to the arrival of contrast material at the area of interest. This mask or reference frame (Fig 4,A) is placed in a digital memory. After arrival of the contrast, one or more later Fig 2.-Peripheral venous injection of contrast was used historically for visualizing the pulmonary vasculature.

looerator’s

I

I Converter

I

I

I

I

-

I Storage

I

ShortTerm

I

I

m Data Flow -Control Lines

Fig 3.--Schematic of a digital video system showing flow of data (heavy and system control (light lines).

lines)

frames are acquired and placed in a second digital memory (Fig 4,B). The mask frame is then digitally subtracted point by point from the contrast frame, resulting in a picture in which background is eliminated and only the vessels remain (Fig 4,C). Because only very small amounts of contrast agent are present, electronic enhancement is necessary. In essence, electronic enhancement consists of expanding the dynamic range of the subtracted image and making the vessels more visible (Fig 4,D). These images are available within seconds and can then be manipulated in several other ways (postprocessing), as discussed below. The contrast of the image may be reversed to give the study an appearance which is more similar to that of a conventional angiogram (i.e., white vessels on a black background) (see Fig 8). Understanding the role of each component of the digital imaging system4 clarifies the limitations of digital technology and points to areas where further development may be useful. Cenerally, each individual component should be assessed with four factors in mind: contrast sensitivity, x-ray exposure, temporal resolution, and spatial resolution. The first two components of the system, the x-ray tube and generator, can be discussed together. A large focal spot size (1.2 mm) is generally used for DSA since the system as a whole has relatively low spatial resolution and a high photon flux is needed. Standard angiographic tubes and generators are usually adequate, except for 0.3-mm magnification focal spots, which produce insufficient photon flux. Tube life may be somewhat 11

Fig 4.-Steps in basic mask mode DSA image production. A, mask frame (no contrast) is acquired just before contrast injection. B, raw data frame is obtained seconds later when contrast has reached a peak. Contrast is barely visible, C, digital subtraction of A from B eliminates background, leaving only the vessels. D, computer enhancement clarifies vascular edges and makes smaller branches more visible.

shortened because of the high photon demands. The generator ideally should be one which is computer-controlled, since this allows rapid determination of technical factors with a minimum of test exposures. The exposure is estimated based on area of interest, patient size, and the type of image processing to be used. After an area is chosen, a test exposure is made and the final exposure factors are computed so that the intensifier light output falls within the correct range for optimum pick-up by the television camera. 12

The image intensifier must not lose contrast or resolution with exposure rates of l-2 R per image, a requirement more vigorous than that for conventional fluoroscopy or cinefluoroscopy. For proper functioning of the television camera, high light levels secondary to high radiation levels must be moderated by aperture controls or variable filtration. Intensifier size is an important consideration. The larger (14- to 16-inch) units offers wider fields of view and better contrast resolution but are much more expensive and, because of fixed computer matrix size, have lower spatial resolution. The television camera converts the optical image into an electronic one. Overall system resolution is determined by the quality of the television component, which therefore must be state of the art. Conventional radiographic and fluoroscopic systems have a signal to noise (SNR) ratio of 100: 1 to 200: 1, which accounts for their poor low-contrast sensitivity. By comparison, DSA system televisions usually have SNRs on the order of 800: 1, which allows excellent low-contrast detectability. The SNR can be further increased to 1,200: 1 by averaging a series of television frames and by other techniques. Most current television systems use the standard 525~raster line format. Newer units have 1,000~raster line or higher television camera monitors. These result in better resolution only if computer matrix sizes are increased to at least 1,024 x 1,024 pixels. The television signal must be amplified either before or after it is digitized. This may be done in linear, logarithmic, or square root modes. The linear mode is best when tissue density is uniform throughout the field, as with abdominal imaging. Logarithmic amplification is the most common mode and is usually used in carotid imaging, since it prevents high- or low-density structures from obscuring contrast-filled vessels. The square root mode is undergoing evaluation at the University of Arizona. The image must be converted to digital form so that the computer can manipulate it. This is the job of the ADC, which assigns specific, discrete digital values to the output of the television camera in a matter of microseconds. The exactness of the assigned value is known as the depth of digitization and directly affects the number of shades of gray that are recorded. Most commercial systems provide 256 or 512 gray levels, corresponding to 8 bits or 10 bits, respectively. The computer, or image processor, is the heart of DSA. Systems may be either hard-wired or programmable, the latter having considerable flexibility and postprocessing capability. For example, reregistration consists of shifting the contrast image up and down or left and right in relation to the mask image to compensate for motion that occurred between the two images. In remasking, another heavily used technique, a new mask is se13

lected that is temporally closer to the contrast image, thereby allowing less opportunity for movement between images. The optimum form and devices to be used for data storage as well as the selection of which data to store are unresolved issues at this point. Even though it would be advantageous to store all data in digital form for ease of transmission and later image manipulation, using present technology (digital disk), the cost would be very high. Magnetic tape is an inexpensive, but cumbersome, alternative. Therefore, analog storage in the form of film transparencies taken from video monitor displays is widely used. As a general rule, approximately 90% of IV DSA studies are diagnostically adequate. When failures occur, they are usually due to patient motion. To address the problem of motion, several alternative methods of forming subtraction images have been developed.5 Subtraction can be performed with respect to variables other than time, i.e., energy or depth (tomography). A first-order subtraction method uses one physical variable to form the difference between images, a second-order method uses two. Mask mode subtraction, as described above, is a first-order method. Another first-order method is integrated remasking, in which two or more images are added together to form a contrast image or a mask image. These summed images are then subtracted to form the final difference image. Another first-order method using time as a variable is matched filtering. Here, images are given different weights, depending on when they are acquired relative to the passage of the contrast bolus. Those images that “match” the bolus, i.e., are obtained near the contrast peak, are given the greatest positive weight, while those farthest from the peak receive the greatest negative weight. The final image is the algebraic sum of all of the images obtained. A first-order method using the physical variable energy is dualbeam energy subtraction. With this technique, two images are obtained virtually simultaneously but at different energies, one at the K-edge of iodine and the other at a non-K-edge level. Hybrid subtraction is a second-order technique involving temporal subtraction of two dual-energy images. X-ray exposure with digital arteriography in comparison with conventional techniques is a matter of controversy. In one study,6 the dose to the lens and to the marrow during carotid DSA was about 20 times less than during conventional angiography. This reduction was due primarily to a small fluoroscopic field size, diminished fluoroscopy time, and the use of a belowtable x-ray tube. Others4 point out that exposure reductions with DSA have been exaggerated. Recent literature’ indicates that for studies with high contrast and spatial resolution, exposures as high as 14 rad per image may be expected. 14

INTRA-ARTERIAL INJECTION Contrast material can be injected intraarterially (IA) and the images processed in exactly the same manner as with IV injections.8 This can be useful for visualizing small vessels that cannot be seen by conventional angiography and may be clinically important, as in evaluating the potential for salvaging an ischemic limb. Several additional advantages result from IA injections. Contrast resolution is enhanced because of the higher concentrations of contrast material. This occurs even though the dose and rate of administration of contrast medium are lower in comparison with the IV technique and conventional angiography. The lower dose is especially important in diabetics and patients with impaired renal function. Concentration of contrast material can also be reduced by a factor of 4 or 5, which results in much less patient discomfort, particularly with extremity injections. There is also less hazard of spinal cord neurotoxicity during bronchial, lumbar, or intercostal injections. Subtracted images are displayed on the CRT screen almost immediately, which helps to reduce procedure time compared with conventional studies. As with IV DSA, film costs per case are considerably lower. Studies can be done more safely because of reduced catheter time and catheter size compared with conventional angiography. IA DSA is expected to replace conventional angiography for many applications. For renal imaging, the IA technique is helpful because it eliminates overlap from the celiac and superior mesenteric branches. It is particularly useful for evaluations in which multiple injections are needed, e.g., renal neoplasm workups, for which aortic, selective renal, and. inferior vena cava OVC) injections are indicated. To look for small synchronous tumors on the contralateral side, conventional angiography with its superior spatial resolution has been recommended.’ For upper extremity studies, IA injections of the aorta are o&en adequate for visualization of the distal arm circulation, but subclavian or axillary injections are better. With selective celiac or superior mesenteric artery injections, major branches are visible, but small branches and bleeding sites are not. As with IV DSA, large amounts of bowel gas or respiratory motion can produce an indeterminate study. For traumatized patients, IA digital studies are highly advantageous because results are quickly available without the need to wait for scouts or film processing. Several drawbacks of IA DSA must also be noted. Because of poorer spatial resolution, fine details of the a&o-architecture are lost. However, in most cases fine resolution is not needed for diagnosis. Furthermore, better contrast resolution provides partial compensation by augmenting visibility of small vessels. The 15

small field size (9 inches) of most DSA image intensifiers is a particular disadvantage for aortic runoffs. Usually only one extremity can be viewed at a time, and as many as five injections may be needed from the aortic bifurcation to the trifurcation. However, the speed of the digital technique may allow more runs to be performed in less time and with less contrast material than a conventional study. The advent of larger (16~inch) image intensifiers will help diminish the number of injections needed, but at the price of a further reduction in spatial resolution. Significant patient motion requires the use of conventional angiography, but this is not as difficult a problem as with IV DSA, probably because of the much smaller boluses of injected contrast. In a recent review,’ 96% of IA studies (75 of 78) were technically satisfactory. EXAMINATION

TECHNIQUE

Steps in performing IV arteriography are generally the same, regardless of the portion of the vascular anatomy to be studied. Typically the patient is positioned on a table which can be moved in the cephalocaudad direction, over which one or two C-arm x-ray tube image intensifier units are positioned (Fig 5). These may be angled to any degree of obliquity, eliminating the need to tilt the patient, and can also be angled in the cephalocaudad direction. Studies are typically done on outpatients; fasting for a minimum of 2 hours is usually the only preparation needed. Most adults require no premeditation; however, giving sedatives and analgesics to pediatric patients may diminish the chance of havFig S-The table top can be translated C-arm can be rotated in two planes.

16

in the cephalocaudad

direction,

while

the

ing the study ruined by motion artifact. Children may be given Phenergan (promethazine, 1 mg/kg), Demerol (meperidine, 2 mg/kg) and Thorazine (chlorpromazine, 1 mg/kg) orally.g Onehalf the above doses should be used for intramuscular administrati0n.l’ An alternative is rectal sodium pentothal (thiopentothal sodium, 25 mg/kg). A No. 5 to No. 7 French pigtail catheter is passed from the antecubital vein, preferably the basilic vein, into either the superior vena cava (SVC) or right atrium (RA) under fluoroscopic control. The femoral vein or IVC can be used as an alternative route, if necessary. The pigtail configuration, although somewhat more uncomfortable for the patient during catheter insertion and withdrawal, protects against the possible complications of mediastinal extravasation or cardiac perforation with pericardial injection of contrast material, which are risks associated with straight catheters.” It has been suggested that use of a straight catheter with ten or more side holes, positioned at the junction of the SVC and the RA, eliminates the risk of mediastinal extravasation.12 In children, a preferred method is placement of a No. 5 French catheter in the femoral vein or IVC.” In many centers, injection is instead accomplished via a 14- to 13 gauge peripheral sheath catheter, 20 cm long, placed in an arm vein.13 Nursing personnel and technologists can be trained to perform many of these catheter techniques, thereby saving radiologists’ time. After the patient is positioned in a way that allows optimum visualization of the appropriate portion of the vascular anatomy, a 25 to 45-ml bolus of contrast material (meglumine sodium diatrizoate 76%) is power-injected at the rate of 15-30 ml/second for patients with central catheter placement. Peripheral injections are typically given at the rate of 8-12 ml/second for a total contrast volume of about 40 ml and may be followed with a flushing bolus of 20-30 ml of dextrose. In children, 0.5-1.0 mg/ kg, diluted 50% by saline, is given.” Total contrast volume for the entire study should be kept below 3-5 ml/kg in patients with normal renal function. After image acquisition and processing, the images are displayed on a video screen. Hard copy can be obtained by using a multi-image camera. It is helpful to include mask views alongside subtraction views to assist in orientation by using bony landmarks. Additional runs are performed if necessary after moving the table, angling the tube and image intensifier, and recollimating. For most examinations, the limiting factor for the number of runs is the total volume of contrast to be administered. Image acquisition at a rate of l-2 per second is adequate for most purposes, although many systems allow acquisition rates as high as 30 per second. 17

CLINICAL

APPLICATIONS

During the development of DSA at the Universities of Arizona 4 and Wisconsin,15 most examinations were performed for evaluation of head and neck lesions, particularly disease of the carotid artery. The indications have expanded steadily since that time (Table 1). Moat portions of the vascular anatomy are now being examined by IV injection, but if selective injections are necessary, the digital technique can easily be adapted for IA studies, as described above. HEAD AND NECK Carotid

Arteries

A number of indications for IV DSA of the carotid arteries have been defined, including nonspecific, potentially ischemic symptoms; asymptomatic carotid bruits; clinically diagnosed ischemic events; postoperative evaluation; and patients with contraindications to conventional angiography. These patients are often elderly and so are at higher risk during conventional angiography. They can be screened safely by the IV techni ue. In large series of patients referred for carotid evaluation, 46 IV DSA has provided diagnostic quality images in at least 90% (Figs 6 and 7,AL M OSt inadequate studies are due to patient motion and swallowing artifacts. Occasionally patients are unable to cooperate with the examination or have insufficient access for placement of a venous catheter. Diminished cardiac output may TABLE

l.-APPLICATIONS

CERVICOCEREBROVASCULAR

Carotid artery disease Intracranial Aneurysms Arteriovenous malformations Tumors Surgical bypass Extracranial Glomus tumors Parathyroid adenomas RENAL

Renovascular hypertension Unexplained hematuria Renal masses Prospective renal donors Posttransplant evaluation PULMONARY

Embolic disease Arteriovenous malformations Sequestration 18

OF

DSA

AORTIC

Congenital anomalies Aneurysms and pseudoaneurysms Atherosclerotic disease CARDIAC

Left ventricular function Congenital anomalies PERIPHERAL

Atherosclerotic disease Graft evaluation VISCERAL

Mesenteric insufficiency Hepatic masses and portography

Fig &-IV (arrow).

DSA examination showing stenosis of the left common carotid artery

Flg 7.-W DSA. A, the right internal carotid artery is completely occluded just beyond its origin and only a smooth stump remains (so/id arrow). Heavily calcified plaque at the left common carotid bifurcation (open arrows) causes misregistration artifact. A web-like stenosis or ulcerating plaque could thus be obscured. 8, intracranial study showing rapid opacification of left internal carotid artery with collateral circulation through anterior communicating artery.

account for an inadequate contrast bolus despite central venous injection. Patients who have difficulty holding their heads still or refraining from swallowing during contrast medium injection may be assisted by a bite bar.17 Projecting the bifurcation of interest over the cervical spine may also help reduce swallowing artifact.” Further development of hybrid (temporal/energy) subtraction is expected to largely resolve the problem of soft issue motion artifacts. Isometric exercise (pushing against fixed resistance) may transiently increase heart rate and cardiac output, thereby enhancing contrast density. For carotid evaluation, venous reflux of contrast will occasionally obscure the carotid vessels. This problem can be eliminated with right atria1 contrast injection or by maintaining an injection rate of 20 ml/second or less in the SVC. When compared with conventional angiography, technically adequate IV DSA studies have a sensitivity, specificity, and overall accuracy of about 95% for clinically significant (over 60% diameter stenosis) lesions.16 Clinical decisions regarding operative versus nonoperative treatment can be made following IV DSA in about 80% of cases.14 Ideally, the carotid bifurcations should be assessed in two projections, either a 45- to 55degree oblique and an anteroposterior (AP) view, or two opposite oblique views. In one recent series,” slightly less than half of the bifurcations (126/260) were seen adequately in more than one projection, partly due to contrast volume limitations. Approximately 15% of these were normal in one projection and abnormal in the other. These figures underscore the importance of obtaining more than one view. Vascular overlap is a major cause of inability to adequately visualize both bifurcations. In fact, a true lateral view is unobtainable without superimposition of the bifurcations, and therefore the posterior wall of the bifurcation cannot be clearly seen. Use of a 75-degree oblique projection (45 degrees of beam angulation combined with 30 degrees of head and neck angulation) is equivalent to a lateral off-axis view and may yield information similar to that provided by the true lateral view.18 Motion of densely calcified atherosclerotic plaques with subsequent misregistration artifact is another frequent cause of inadequate IV studies (see Fig 7Aj.l’ The misregistration between precontrast and postcontrast images obscures vessel margins and makes it difficult or impossible to determine the degree of stenosis. Increasing the frame rate from 1 per second to 2 per second increases the chance of obtaining adequate registration; theoretically, ECG-gating should also help. IA studies are less susceptible to plaque misregistration, probably due both to the higher imaging frame rate as well as to the more temporally compact contrast bolus (Fig 8). It is important to distinguish the nearly occluded from the oc20

Fig 8.-lntra-arterial DSA study showing a web of the right internal carotid artery (arrow) and complete occlusion of the left.

eluded internal carotid so that the former patients can undergo endarterectomy rather than bypass procedures. Very high-grade stenosis of the internal carotids can be diagnosed by IV DSA1’ despite the limitations imposed by motion artifact, inferior spatial resolution, and lack of selectivity. These disadvantages seem to be offset by several other factors, including the availability of immediate subtraction, computer enhancement of dilute contrast material, and the availability of frontal projections for easy comparison with the contralateral side. Most important, perhaps, is the thorough mixing of contrast material with blood prior to its arrival in the carotid. This reduces the tendency toward layering of contrast medium distal to a high-grade stenosis, which may occur following selective arterial injection. Therefore, IV DSA may be more efficacious than conventional angiography in studying some patients with nearly complete internal carotid occlusions. Vertebrobasilar

Arteries

Although the vertebrobasilar system is less amenable to operative correction than are the carotids, evaluation of this portion of circulation is useful for decision-making regarding antiplatelet and anticoagulant therapy. IV DSA demonstrates vertebral arteries well in 90% of patients (Fig 91, the basilar artery in about 75% (Fig 101, and the posterior cerebral arteries 21

Fig 9.-A normal right artery arising from the right subctavian is welt demonstrated on this IV study. An aberrant left vertebral artery (arrow), arising directly from the aorta, is visualized. vertebral

in less than c~O%.~’In technically adequate vertebral artery images, IV DSA tends to underestimate the degree of disease, although there is no statistically significant difference compared with conventional angiography. A straight lateral view is optimum for viewing the basilar artery by DSA. Fig basilar

lO.-Slightly oblique (lo-degree) artery aneurysm (arrow). IV DSA.

view

of the

base

of the

skull

showing

a

Intracranial

Vessels

The intracranial vessels can be evaluated by IV technique.‘i IV DSA usually provides answers to specific clinical questions and almost always adds useful, although sometimes limited, information. An acquisition rate of 1 or 2 frames per second is adequate, and an AP view is obtained routinely, with lateral, oblique, and basal projections obtained as necessary. Oblique views provide better definition of the cavernous segments of the internal carotids since they are obscured on the AP view and overlap on the lateral. Middle cerebrals are best shown when the petrous ridges and orbital roofs are superimposed; proximal posterior cerebrals are better evaluated with the petrous ridges bisecting the orbits. For assessing intracranial collateral circulation in the setting of extracranial occlusion, the AP view is the most useful (Fig 7,B). Although the caliber of the M-l segments of the middle cerebral arteries can be assessed following an IV injection, diagnosis of a focal stenosis of an intracerebral vessel based on an IV study alone is risky. However, hemodynamically significant information, which is not available from a unilateral selective injection, such as delayed flow in the carotid siphon and distal intracranial vessels secondary to a unilateral carotid stenosis, can be obtained (see Fig 7,B). IV DSA has been used effectively for aneurysm evaluation in patients whose CT findings are ambiguous but suggestive of aneurysm, to follow untreated aneurysms (see Fig 10) for changes in size and shape, and for postsurgical follow-up. Patients with undiagnosed hemorrhage (intracerebral or subarachnoid) probably should not be evaluated by IV DSA, since vascular superimposition could hide small aneurysms, and these patients are usually too ill to remain motionless during an IV study. However, it is useful for documentation and follow-up of vasospasm. In addition to findings of diminished arterial caliber and poor visualization of vessels, the cerebral circulation time, measured from first appearance in a parietal vein, may be prolonged.12 For arteriovenous malformations, the IV technique is useful both for screening and for postoperative follow-up, since these lesions are easily detected. It is not adequate as the primary preoperative study, however, since the contribution of individual arteries is difficult to judge due to simultaneous opacification of all intracranial arteries. Venous bypass grafts and superficial temporal to middle cerebral artery anostomoses can be seen, but multiple projections may be needed to see the anastomoses to best advantage, and careful positioning of the patient’s head is required. IV DSA is indicated to evaluate the following situations involving possible intracranial tumors: to substantiate a lesion 23

seen on CT and suspected of being a neoplasm, to assess a vascular tumor at the base of the skull where dense bone limits the utility of CT, to evaluate secondary effects on the carotid arteries and cavernous sinuses in patients with known sellar tumors, and to rule out aneurysm as the etiology of suprasellar or intrasellar masses. The typical homogeneous blush of meningiomas has been observed, and glomus tumors are easily detected.21, 22 Consistently useful information has been obtained by IV DSA in patients who have undergone therapeutic embolization procedures.12 Posterior fossa studies provide useful but often limited information. Because of simultaneous bilateral filling of all intracranial arteries the dural sinuses and intracranial veins are well visualized.” Confusing filling defects, often seen in conventional angiographic studies due to unopacified blood entering the dural sinuses, are avoided. Therefore, the degree of venous displacement, invasion, or occlusion is clearly shown. The superior sagittal sinus, vein of Galen, straight sinus, and transverse sinus are visualized in AP and lateral views in all cases; the internal cerebral veins are consistently seen in the lateral view. The inferior petrosal sinus, cavernous sinus, and basilar vein of Rosenthal are all seen in the lateral projection in the majority of patients. Less consistently seen are the inferior sagittal sinus and thalamostriate veins. IV DSA is now the procedure of choice for evaluating the jugular bulb region in patients with suspected pathology in this area, replacing venography and conventional angiography with subtraction. If a tumor blush is seen, then selective injection must be performed to delineate the exact arterial supply prior to therapy. IV DSA can again be used postoperatively for followup* A suspected carotid-cavernous sinus fistula can be confirmed by IV DSA. The most reliable signs are early fillings of the cavernous and inferior petrosal sinuses. Retrograde superior ophthalmic vein filling is also frequently seen.22 IA DSA of the cerebral vasculature compares quite favorably with conventional angiography23 with regard to image quality and yield of diagnostic information and promises to eliminate many, if not most, film-screen studies. IA DSA is equivalent to conventional angiography for large-vessel (> 1 mm) resolution. Although small-vessel (< 1 mm) resolution is somewhat inferior, assessment of these vessels is usually not necessary for diagnosis, especially with the advent of CT scanning. Contrast density is generally superior on IA DSA studies, even though these images typically are obtained with smaller volumes of contrast medium and slower injection rates than those used for conventional 24

angiography. Because of high contrast sensitivity, subtle tumor blushes may be more evident than on film-screen studies. Patient motion is much less of a problem than with IV studies, probably because much smaller volumes of motion-inducing contrast material are used. Further, the high concentrations of contrast material that are available for IA injections enhance the quality of the subtraction. IA DSA studies are usually faster than conventional studies, since the subtracted images are available immediately for viewing. Reducing catheter time, contrast volumes, and rates is an important safety benefit in patients with unstable neurologic deficits. The efficiency of IA DSA is particularly important when multiple injections must be performed, as in spinal angiography. For embolization cases, subtraction of all bony and soft tissue structures may be performed in real time, allowing an image consisting exclusively of catheter, contrast material, and embolic material to be displayed.24 Miscellaneous Approximately 5% of patients with primary hyperparathyroidism and enlarged parathyroid glands will have negative initial neck explorations. If the parathyroids cannot then be localized by CT or ultrasound, IV DSA may be useful as a screening procedure25 and has been estimated to be capable of finding about 50% of glands. If this is unrewarding, IA DSA, using either an aortic arch injection or injections into the innominate artery, left subclavian artery, or left common carotid artery, may be helpful. Finally, selective conventional arteriography and venous sampling may be necessary when the digital studies are negative or equivocal. Screening for various other neoplasms of the head and neck can be performed by IV DSA (Fig 11); IA studies are generally required prior to surgery. AORTA Despite brisk pulsations, the thoracic aorta13 and great vessel origins are well visualized by IV DSA. Straight anterior and left anterior oblique-equivalent projections are routinely utilized, in addition to any other views needed for diagnosis. Congenital (Figs 12 and 13) and acquired lesions can be accurately diagnosed in 92% of patients, usually eliminating the need for conventional aortography. Grossman and colleagues13 found that IV DSA correlated well with conventional angiographic findings, surgery, CT, and subsequent clinical course in 95% (41/43) of cases. Their series included patients with primary aortic disease (coarctation, pseudocoarctation, double arch, Marfan syndrome, 25

Fig Il.--IV (A) and lateral

DSA showing (8) views.

Fig 12.-Five-year-old aortic coarctation clearly shown

26

juvenile

boy with (arrows), by IV DSA. Lateral view.

nasopharyngeal

angiofibroma

(arrows)

in AP

flg 13.-A, anteriorly angled, subcostal two-dimensional echocardiogram of an infant shows the aorta bifurcating 4 cm above the valve. B, IV DSA showing a double arch with a smaller left component (arrow), functionally an innominate artery. The distal pat-l of the left arch is not seen. At surgery it was an atretic segment which completed a vascular ring.

cervical arch, aneurysm, and dissection) and with mediastinal tumors adjacent to the aorta mimicking aneurysm. Type II aortic dissections were well demonstrated, but in type I cases the coronary arteries could not be visualized and conventional aortography was necessary. Guthaner and Mille? have shown that IV DSA provides useful and unique information regarding dissections of both the thoracic and the abdominal aorta. Diagnostic studies were performed in all six of their patients, one of whom underwent DSA preoperatively. Unlike conventional angiography, in which typically only the injected lumen is clearly seen, DSA allows visualization of both the true and false channels with a single contrast injection. In addition, IV DSA can demonstrate dynamic pathophysiologic traits of dissection, much as cineangiography. For example, movement of an intimal flap over time may be identified, or a jet of contrast may be seen linking the two channels. Determination of which lumen supplies major aortic branches can also be made, particularly with fast (i.e., 4 frames per second) framing rates. Several limitations for dissection imaging are apparent, however. Aortic regurgitation cannot be assessed because of the forward transit of the contrast bolus. The region of the aortic root and the ascending aorta is particularly difficult to assess because of overlap by contrast-filled major pulmonary vessels. This problem is partially ameliorated by 20 degrees of caudal image intensifier angulation. Small field size (maximum, 16 inches) makes several injections mandatory for visualizing the entire length of the aorta. The most critical limitation from a clinical standpoint relates to the ability to visualize intimal disruption. Since intimal disruption can be a subtle 27

finding on conventional angiography, it could certainly be overlooked on a digital video study with its poorer spatial resolution and degradation by motion artifacts arising from vascular pulsations and involuntary respiratory motion (despite breath-holding). Extensive prospective trials comparing IV DSA with conventional aortography and CT scanning will be necessary to clarify the role of IV video studies in evaluating aortic pseudoaneurysm and dissection. IV DSA in conjunction with esophagography is adequate for screening patients with vascular rings preoperatively’ and in most patients can replace cardiac catheterization. Although the barium esophagogram can be used as the only tool for preoperative assessment, DSA is valuable since an anatomical road map is thereby made available. The surgeon can be alerted to the need for a right (lo%-20% of patients) rather than the usual left thoracotomy, and congenital heart disease, which occurs in up to 15% of patients with vascular rings, can be diagnosed. Imaging of the abdominal aorta by IV DSA is useful primarily for assessment of atherosclerotic changes, including aneurysm formation. These applications are discussed further in the section on peripheral DSA. RENAL In evaluating the renal vasculature, several technical problems may occur (Fig 14). These include motion from respiratory, cardiac, and intestinal sources, the presence of numerous overFig 14.-Normal IV DSA of the renal arteries, showing a single renal artery on each side. Note artifact overlying right kidney from bowel gas motion.

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lying vessels, and limitations due to patients’ body habitus. To move intestinal gas away from the kidney, a balloon compression device, similar to that utilized in uroradiography, may be used. IV glucagon, given 2 minutes before contrast injection, will reduce bowel peristalsis artifact in 60% of patients and eliminate it entirely in 40%.27 IV DSA has been shown to be effective for the evaluation of renal allografts, renal masses, potential renal donors, unexplained hematuria, and following therapeutic interventions (embolization, vascular bypass, endarterectomy).28 Schwarten2’ has recently reported that IV DSA is useful for follow-up of renal artery transluminal angioplasty. No disagreement was found between IV DSA and aortography in normal subjects and in patients with less than 50% stenosis on a-year follow-up studies. There was some disagreement regarding the severity of higher grade restenosis, but none with regard to their presence. Technically satisfactory studies were achieved in 92% of patients, with inadequate studies resulting from heavy vascular calcifications, motion artifact, and poor cardiac output. The advent of DSA has made feasible a direct test for assessing the possibility of a renovascular etiology in hypertensive patients. Preliminary studies3’, 31 suggest that IV DSA is more sensitive and more specific for this problem than is rapid-sequence IV urography, which is an indirect means of evaluating the renal vasculature. Video IV studies are safer and more costeffective than conventional angiography, and thus constitute a true screening test for patients with essential hypertension. IV DSA has also been shown to be useful for evaluating hypertension in other clinical settings.31 Twenty-five percent of patients who have had renal allografts develop hypertension, in half of cases because of renal artery stenosis. The failure rate of renal artery graft reconstruction is 6%-g%, with hypertension a not uncommon result. IV DSA can help clarify any of these situations. Renal DSA raises the interesting possibility of extension of digital radiography techniques beyond angiography on a large scale. Early investigation by Hillman and co-workers32 has shown that imaging renal parenchyma and collecting structures immediately following IV DSA of the renal vasculature provides clinically usable information. Nephrographic and pyelographic images obtained within 5 minutes of contrast injection, in some cases “raw” data rather than subtracted images, reliably depict renal anatomy and qualitative function. In animal experiments, renal conspicuity is actually superior to that achieved with filmscreen technology. Whether IV DSA can replace conventional angiography for evaluation of potential renal donors remain debatable.33 The principal difficulty is determining the number of renal arteries; 29

accessory arteries are missed in 17%-18% of cases.28933 Although using a larger imaging intensifier is expected to reduce this error rate,28 superimposition of vessels is likely to remain a problem. Accuracy is improved by routinely obtaining oblique views and viewin images at a variety of different window widths and levels. ii Faster frame rates may prove to be useful since renal vessels should fill prior to visceral branches. PERIPHERAL VESSELS IV DSA is ideal for evaluation of the Leriche syndrome (atherosclerotic occlusion of the distal aorta and proximal iliac arteries).34 Conventional arteriography by the femoral approach is generally impossible in these patients and the axillary approach entails a mortality risk three times that of the femoral approach and a twofold to threefold increase in CNS complications. Full evaluation of these patients by conventional means often necessitates an aortic arch injection in order to opacify all collateral vessels. The IV method accomplishes such opacification automatically with each injection. IV DSA is useful for diagnosing localized occlusions or stenosis of bypass grafts.35 The use of IV injection is particularly beneficial in patients who have femoral prosthetic graft material in place in order to avoid puncture of the graft itself and the associated complications (Fig 15). Other indications for IV studies have included suspicion of focal lesions based on prior studies Fig 15.-W DSA. Proximal anastomosis of aortobifemoral graft is normal, A, while a pseudoaneutysm of the right femoral anastomosis, 6, is present.

(Fig 16), ev a 1ua t ion of percutaneous transluminal angioplasty, and detection of peripheral vessels not seen on conventional runoff studies.36 However, IV DSA has a much wider application in assessing patients with peripheral vascular disorders,37 particularly when the examination is properly tailored to the patient’s specific clinical problem. 38 This tailoring should take into account the history, physical findings, pertinent laboratory tests, and the results of noninvasive forms of vascular interrogation such as Doppler. In particular, segmental limb arterial pressure measurements combined with Doppler pulse-velocity waveforms in the symptomatic extremity will help to pinpoint the region for angiographic study.38 Narrowing the portion of the vasculature to be studied is important in these patients because the standard g-inch intensifier permits only a restricted field of view. This situation promises to be at least somewhat alleviated by the newer 16-inch intensifiers.36 Because of the narrow standard field and because moving tabletops are not feasible for IV DSA at this point, the number of runs becomes an important limiting factor. Usually a maximum of four or five contrast injections can be made without exceeding 3 ml/kg. When a large (16-inch) intensifier is used, all the vessels from the renal arteries to the trifurcations can generally be visualized with four ima es: aortoiliac, iliofemoral, femoropopliteal, and popliteotibial. 3f Another promising possibilFig 16.-W DSA. Atheromatous changes of the distal aorta and a tight focal stenosis of the left iliac artery (arrow) are seen.

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ity, even using a smaller (g-inch) intensifier, is that of acquiring up to 4 images per second and integrating as many as 16 images for each subtraction picture.3g This greatly improves the SNR of the final image and allows the injected contrast amounts to be halved (total of 160-200 ml per patient). Thus, multiple runs, which provide images from the renal arteries to the trifurcations, can be performed using the same amount of contrast medium as during conventional aortic runoffs. When IV DSA is used to image the distal aorta and iliac artery, bowel gas artifacts may become a problem. Glucagon injections and abdominal compression may be helpful, as with renal DSA (see above). Visualizing the femoral arteries simultaneously is difficult in most instances with the g-inch intensifiers but is not generally a problem if a 16-inch intensifier is available. Popliteal arteries and trifurcation vessels are routinely seen, and major tibia1 branches often can be seen as well. The oversaturation artifact caused by the air gap between the legs can be reduced by using a wedge filter or by crossing the patient’s ankles and angling the C-arm. Patients with suspected thoracic outlet syndrome can be evaluated in neutral position or with the arm held in a position which provokes symptoms and obliterates the distal pulse. A 20degree left anterior oblique projection is good for seeing the subclavian origin (Fig 17).

Fig 17.-A 59-year-old patient with a pulsatile neck mass was studied by IV DSA. No AVM was found. Subclavian artery dilation and tortuosity (arrows) was the cause.

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PULMONARY Pulmonary embolism is a serious and common disease whose treatment is not innocuous; therefore, firm diagnosis is crucial. Radionuclide scanning is often definitive, but many patients require pulmonary angiography. The safety advantage of IV DSA is magnified in this setting since conventional pulmonary angiography is associated with the following major complications in 4% of patients: arrhythmias induced by catheter contact with the interventricular septum, cardiac perforation and tamponade, and acute car pulmonale resulting from pulmonary artery injections. Mortality from the procedure is as high as 0.67%. IV DSA, for patients with major pulmonary emboli, has an overall accuracy of 75%-91%40p * and is accurate in up to 97% of the patients in whom an adequate examination can be performed.*l Since a limited number of IV runs can be performed due to contrast volume limitations, a preliminary radionuclide perfusion scan provides a useful guide to the areas of greatest suspicion. In the patient whose IV DSA study is nondiagnostic, a conventional pulmonary angiogram can be obtained immediately following the IV study by advancing the catheter across the tricuspid valve and into the pulmonary outflow tract (Fig IS>. Fig 18.-Normal pulmonary angiograms. A, IV DSA of the left pulmonary artery and branches. 8, comparable IA digital study obtained moments later.

The most useful projection for the IV study is the anterior oblique of the side with suspected emboli, since this view separates the pulmonary vessels and reduces interference from cardiac pulsation. The AP view is helpful in some cases. Emboli larger than about 2 mm, which could produce segmental or even subsegmental occlusion, are detectable. Most patients, even those who are debilitated or have moderate or severe dyspnea, can be usefully studied IV. Those who are uncooperative or cannot suspend respiration for 15 seconds should have conventional studies instead. In addition, patients with normal DSA examinations who are suspected of having small emboli (e.g., by the presence of subsegmental-peripheral-defects by radionuclide perfusion scanning) should probably have conventional magnification angiography4’ to exclude emboli beyond third-order pulmonary branches. This is a question for which further research is needed. CARDIAC

One of the initial motivations for developing DSA was to have available a relatively noninvasive method for investigation of the coronary arteries. Although research continues, the IV use of DSA for this common clinical problem remains unfilled to date because of physiologic motion, vascular overlap, inability to resolve very small vessels, and superimposition of contrast-filled arteries and heart chambers. IV DSA has, however, proved to be useful for other cardiac applications, including quantitation of the cardiac ejection fraction (EF).42 EF has historically been measured either by conventional angiography or gamma camera imaging, both of which have significant drawbacks. Conventional angiography utilizes large-volume left ventricular contrast injections which alter ventricular function and are hazardous to patients who are diabetic or have impaired renal function. Further, a particular geometric shape, usually a prolate ellipsoid, of the left ventricle is assumed for purposes of calculating the EF. While this assumption may be correct for a normal ventricle, it is often incorrect in a diseased heart with asymmetric wall motion. Radionuclide methods are limited by high quantum noise and poor spatial resolution, which make left ventricular boundary detection difficult. Accurate EF determinations require acquisition of multiple ECG-gated images, and significant inaccuracies may result in patients with arrhythmias. DSA videodensitometry eliminates many of the negative features of the other techniques and offers a means of measuring EF with good anatomical resolution and without assuming a particular ventricular shape. A number of technical considerations must be addressed, however, in order to obtain good re34

sults. These include corrections for tissue iodine accumulation, veiling glare, x-ray scatter, beam hardening, and pincushion distortion. Pulmonary circulation time has been used to assess the effects of various pharmacologic and physiologic interventions on cardiac hemodynamics. This parameter is of clinical interest since specific changes can occur during exercise in patients who have ischemic heart disease. Most of these studies have been performed using radionuclide techniques, which are plagued by the limitations discussed above. Therefore, it is advantageous to be able to measure the parameter using videodensitometric DSA techniques. Such a methodology has been described43 and changes in pulmonary transit time have been demonstrated following isoproterenol injections in humans. Application of video digital image processing to replace cineangiocardiography may be feasible by using a temporal filtration (moving mask) methodology.** Preliminary investigation suggests that this technique may provide several advantages over routine film-based cineangiography. During ventriculography, the ventricular contour can be seen simultaneously in both systole and diastole owing to an afterglow artifact that results from the moving mask technique. This allows superb visualization of ventricular wall motion. The DSA method provides better contrast resolution than conventional cineangiography and allows for better suppression of background structures. Some suppression of background occurs even during panning while viewing distal portions of the coronary arteries, due to the temporal averaging of the subtraction mask. Contrast material dose reductions of up to two thirds may be possible for ventriculography, and coronary injections can probably be done with 4 ml rather than 5 ml of contrast solution. In evaluating congenital cardiac disease, DSA can be used to obtain both anatomical and physiologic information9 which is in good agreement with that obtained by echocardiography, radionuclide cardiac studies, and conventional cardiac catheterization. Interatrial septal defects are particularly well suited to IV DSA detection, but ventricular septal defects are seen with only moderate sensitivity. Information from IV DSA can significantly shorten or modify cardiac catheterization in up to 85% of patients, and surgically placed great vessels grafts can be evaluated in 95% of patients (Fig 19). Physiologic data including EF, cardiac output, and quantitation of intracardiac recirculation may be obtained. VISCERAL

As elsewhere, a common indication for abdominal vascular imaging is the need to identify atherosclerotic changes. Major 35

Fig lg.--IV DSA study in a patient with a history of tetralogy of Fallot. The left pulmonary artery is revascularized through a Blalock-Taussig shunt (arrow). The right pulmonary artery filled earlier in the run through a Waterson shunt.

mesenteric vessels and first-order branches can be readily seen with IV DSA, although smaller branches are frequently obscured due to overlap and peristaltic activity. A promising technique is the use of videodensitometry in conjunction with DSA to assess the degree of mesenteric stenosis. Although this method requires further evaluation it probably will ultimately lead to outpatient evaluation of a substantial portion of patients with mesenteric vascular insufficiency. The intra-arterial use of digital vascular imaging has been shown to be useful for portal and hepatic studies.45T 46 Use of a large (14~inch) image intensifier allows simultaneous visualization of the entire liver after a celiac or hepatic artery injection, but this advantage must be weighed against a loss of spatial resolution. A study evaluating primary hepatic tumors, metastasis, and hemangiomas found that the main advantfge of IA DSA is better visibility during the parenchymal phase. Lesions are enhanced after background subtraction, and subtle tumor blushes can be appreciated. By remasking, the arterial and parenchymal phases can be superimposed, which further improves detection of tumor blush. The left lobe is well visualized by IA DSA without the need for performing subselective left hepatic catheterization. The spine is a common impediment to good left lobe visualization during conventional angiography, but this, 36

like all bony structure impediments, is eliminated with DSA. Since smaller contrast volumes (X-20 ml rather than 50-60 ml) are used with IA DSA, the probability of catheter recoil is diminished, a useful benefit, especially in patients with indwelling perfusion catheters. Portal venous anatomy, flow direction, and portal-systemic shunts are visualized in the venous phase after celiac or superior mesenteric artery injections. DSA imaging of the venous anatomy is effective in 80%-90% of cases and is therefore useful for preoperative assessment of hemodynamics and for postoperative evaluation of shunt function.46 As with conventional angiograms, images may be improved by use of IA vasodilators or balloon occlusion techniques. DISCUSSION

The advantages and drawbacks of DSA, both IV and IA, have been widely discussed in the radiologic literature (Table 2). The fundamental advantages are safety, lower cost, and better acceptability by patients and referring physicians. Lower radiation dosage may prove to be an advantage as well. The use of venipuncture for IV DSA eliminates the small but definite risks associated with arteriotomy, such as thrombosis and stroke. Of nearly equal importance, it allows angiography to be an outpatient rather than an in-patient procedure. Even when IA injections are deemed desirable, the catheter size can be reduced significantly (3 French) and most patients can safely go home a short time after the arteriogram is obtained. Discomfort during conventional arteriography, especially aortic runoffs, can be severe. This is only partially resolved by using the newer nonionic contrast agents. When IA DSA is used instead, discomTABLE INTRAVENOUS

P.-COMPARISON OF CONVENTIONAL ANGIOGRAPHY WITH AND INTRA-ARTERIAL DIGITAL SUBSTRACTION ARTERIOGRAPHY CONVENTIONAL

Injection site Discomfort Relative morbidity Hospitalization requirement Film use Procedure time Contrast volume cost Relative contrast resolution Relative spatial resolution Vascular overlap problems Motion-induced image degradation

Artery Moderate-severe High In-patient High Long Moderate High Low High Mild Mild

IV DSA Vein Minimal Minimal Outpatient Low Short High Low High Low Moderate

IA DSA Artery Moderate Moderate Variable Low Short Low Low High Low Mild Moderate

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fort is reduced considerably because smaller volumes of contrast media are injected. Using the IV approach, discomfort is no greater than that associated with bolus injections for excretory urography. As indicated above, catheter-related morbidity is less with IA DSA because of the smaller catheter size that can be used. There is even less morbidity during IV injections. During peripheral IV injections, extravasation of contrast material produces pain and the danger of skin sloughing; however, there are no reported instances of the latter due to IV DSA. Central injections have been complicated by mediastinal extravasation of contrast, but this problem can be eliminated if a pigtail configuration is used. A theoretical risk of IV DSA, which has not been reported to be a major problem, is that of allergic reactions. These are known to have a higher incidence following IV administration of contrast compared with IA administration. Cost of IV DSA has been estimated to be one quarter to one third that of conventional arteriography. This is largely a result of film savings; on average three or four sheets of film are used, compared with 50 to 100 for a conventional study. Cost is further greatly reduced by avoiding hospitalization. Subtraction films are available immediately and procedure time is generally shorter with either IV or IA DSA than with conventional studies. Patient throughput is enhanced if peripheral contrast injections are used since no fluoroscopy time is necessary for catheter placement. The main technological reason for the success of DSA is related to improved contrast resolution. Although this comes at the expense of spatial resolution, the trade-off is a reasonable one since final images have sufficient resolution for diagnosis in the vast majority of cases. As larger matrix sizes (e.g., 1,024 x 1,024) become the industry standard, problems with poor spatial resolution will diminish. One of the main disadvantages of IV DSA is vascular overlap, an inevitable difficulty with a projection imaging format. The area of interest can usually be adequately studied by employing multi-angled projections. Contrast volume limitations become important when large areas must be imaged. However, the total contrast dose is limited for any angiographic study, particularly in patients with limited renal reserve. Therefore, tailoring the study to areas of particular clinical concern must be done. This problem will be somewhat ameliorated for IV DSA as larger image intensifiers become standard equipment. Further improvement can be anticipated if moving tabletop systems and dual energy subtraction techniques are developed to the point of clinical utility. Perhaps the most difficult technical problem is that of motioninduced image degradation. Here again, technical innovations 38

hold the key to improved performance. Postimaging reprocessing techniques such as remasking, integration, and reregistration have proved useful. As with many new technologies, digital angiography was greeted with enthusiasm initially and exaggerated expectations emerged, compounded by aggressive marketing of the earlier systems, many of which were of the “add-on” type, which lack the all-important ingredient of system integration. With time and increasing clinical experience, limitations became better known and ultimately many physicians became disillusioned.47 Even though IV DSA may not reach some of its initial aims, such as evaluating coronary arteries after an IV injection, or evaluating organ function, it already has broad clinical applicability (see Table 1) and its advantages far outweight its limitations. Image quality should continue to improve as technical advances are made in intensifiers, video cameras, ADCs, and postprocessing software. The advent of DSA and the continuing development of general digital radiography has laid the groundwork for a completely digitized radiology service in the future. There would be several advantages to such a service. Costs would be reduced, primarily because of reduced utilization of film. With completely digitized storage, films would not be lost, and multiple users could have access to a patient’s study simultaneously without having to go to the radiology department. All of the patient’s imaging studies would be centrally located and immediately available for correlation with the current examination. SUMMARY

DSA has quickly made the transition from an experimental examination performed in a research institution to a routine procedure performed in a community hospital. Diagnostic quality images for screening applications are obtained in over 90% of patients, and in many instances DSA is the only angiographic study that need be done. The applications of IV DSA have broadened to encompass many angiographic procedures for which selective injections are not mandatory. When IA studies must be performed, digital equipment offers the advantages of speed, safety, and lessened patient discomfort. New technological developments will increase the advantages and applications. REFERENCES 1. Fraser R.G., Breatnach E., Barnes G.T.: Digital radiography of the Clinical experience with a prototype unit. Radiology 148:1, 1983. 2. Des Plan& B.G.Z.:. Subtraktion: Eine rotgenographische Methode zur aten Abbidung betimmter Teile des Objekts. ROEFO 52:67, 1935.

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