Alternative imaging techniques in vascular surgery

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JOURNAL OF VASCULAR NURSING

DECEMBER 1998

Alternative imaging techniques in vascular surgery Kathleen E Verta, BSN, RN, CVN, and Michael J. Verta Jr, MD, FACS

Progress in vascular surgery has led to the need for more sophisticated methods of imaging the vascular system. Although conventional contrast angiography is still the primary method of visualizing the vascular system, it has problems and limitations that occasionally render it unsafe or inadequate. When conventional angiography cannot provide the needed information, 3 newer imaging methods--3-dimensional (3D) spiral computed tomographic scanning, computed tomographic angiography, and magnetic resonance angiography--are being used more widely to supplement or replace contrast angiography. The advantages, disadvantages, and clinical application of each method will be described. These methods have fundamentally changed the practice of vascular surgery and a thorough knowledge of them is essential. (J Vasc Nurs 1998;16.'78-83.)

Since the first successful aortogram by Dos Santos in 1929, contrast angiography has been the mainstay of vascular imaging. It is readily available in any hospital doing vascular surgery, interpretation criteria are well-established, and technologic advances, such as digital subtraction, have made obtaining good images possible even in adverse circumstances. 1-3 Nevertheless, problems exist with contrast angiography, which may preclude its use or render it inadequate for the task (Table I). All but the most severe problems of contrast sensitivity and toxicity can be managed by good patient preparation. However, bypasses to the ankle and foot are now common, and delivering sufficient contrast material to visualize these very distal arteries in a patient with severe multisegmental occlusions or poor cardiac output may not be possible. Finally, to see complex anatomic relationships, use of multiple injections of contrast with filming from several different angles is often necessary, which increases the likelihood of problems from contrast material and exposes the patient to more radiation. To overcome the deficiencies of conventional angiography, 3 new techniques have emerged--spiral computed tomographic (CT) scanning with 3-D reconstruction, CT angiography, and magnetic resonance angiography. Each has its advantages and

PROBLEMS OF CONVENTIONAL ANGIOGRAPHY Contrast sensitivity

Poor visualization

Angioneurotic edema Anaphylaxis Contrast toxicity

Multisegmental disease Poor cardiac output Poor spatial resolution

Nephrotoxicity Neurotoxicity

Multiplane filming Multiple injections

COMPARISON OF ALTERNATIVE TECHNIQUES 3-D CT

CT Angiography

MRA

Noninvasive

Yes

Yes

Yes

X-ray exposure

Yes

Yes

No

Contrast required

No

Yes

No

Scan time

Short

Short

long

3-D resolution

Good

Good

Good

Visceral artery resolution

--

Good

Better

Small artery resolution Tortuous vessels

---

Fair Fair

Good Good

disadvantages (Table II), but each also contributes unique and important clinical information not provided by conventional angiography. This article briefly describes each of these techniques and uses clinical examples to show how they have become integrated into current vascular surgical practice.

SPIRAL c T SCANNING In spiral (also called volumetric or helical) CT, the scan head moves circumferentially around the patient as the patient passes through the scanner, thus creating a series of overlapping slices that are connected to one another in a continuous spiral. By using sophisticated computer software, these slices are reconstructed into 3-D images which can be rotated and viewed from multiple angles. Thus the anatomic relationships of adjacent structures easily can be seen. Although scan times are very short, the patient is exposed to x-rays, and contrast infusion usually is necessary to see blood vessels well. 4,5

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Figure 1. Preoperative chest radiograph. A small stump of cervical rib (white arrow) can be seen on the right.

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Figure 2. 3-D CT of the thoracic outlet, viewed from behind the right shoulder. The arrow points to the junction of the cervical and first ribs. Note the unusual shape of the cervical rib.

Clinical Example

A 26-year-old hairdresser and competitive bodybuilder had pain and tingling in the shoulder and arm when working on hair or lifting weights above her head. The pain was primarily in the deltoid and biceps area with some radiation to the triceps and lateral forearm. Physical examination revealed normal pulses in all positions, tenderness of the brachial plexus to percussion, and a positive elevated arm stress test suggestive of thoracic outlet syndrome. Because the deltoid and biceps pain that the patient described was in the C5-7 nerve root distribution rather than in the C8-T1 distribution expected in thoracic outlet and the initial chest radiograph was not helpful (Figure 1), a 3-D CT scan was ordered. The scan clearly showed a well-developed cervical rib with an unusual "bent" configuration that suggested an explanation for the atypical symptoms (Figure 2). A transaxillary resection of the first and cervical ribs was done. At operation, the C5-7 nerve roots were seen to be stretched taut over the cervical rib, and the C8-T1 root was only mildly compressed. After the operation, the patient was completely relieved of her symptoms and was back at work in 10 days and lifting weights again in 3 weeks. A CT scan after the operation showed complete resection of the first and cervical ribs (Figure 3).

Figure 3. Postoperative 3-D CT of the thoracic outlet, viewed from the anterior. Note the absence of the first rib on the right.

Comment

In this case, the ability to see anatomic structures and their relationships in 3 dimensions was essential to understanding the patient's symptoms and critical in planning the operative approach.

CT A N G I O G R A P H Y CT angiography is performed by using the spiral CT scanner and rapid intravenous infusion of contrast material. Very thin (1 mm) slices are taken through the area of interest and are

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DECEMBER 1998

Figure 4. A, CT angiogram of the AAA, viewed from the posterior. Note that the right (white arrow) and left (black arrow) renal arteries are involved in the aneurysm. B, CT angiogram of the renal arteries. Note the stenosis just distal to the right renal artery origin (arrow).

20 to 30 seconds but large volumes of contrast must be given, which limits the usefulness in patients with contrast sensitivity or significantly impaired renal function. Moreover, when used in full 3-D mode, only a 2.5-cm long segment can be imaged with each contrast infusion. 4-6 Clinical Example

Figure 5. MRA of the aortoiIiac segment. Note the severe occlusive disease in the left iIiac artery (arrow). The right iliac is obscured by branches of the superior mesenteric artery.

reassembled in 3 dimensions, which gives an image that closely resembles a regular angiogram. CT angiography shows the aorta and the main visceral arteries quite nicely, but smaller branch vessels are often not seen well. Scan times are usually only about

A 60-year-old man on multiple antihypertensive drugs was given an angiotensin-converting enzyme inhibitor to achieve better blood pressure control. When his kidney function deteriorated on the medication, an ultrasound showed a small fight kidney and an unsuspected 6.2 cm juxtarenal abdominal aortic aneurysm. A radionuclide renogram showed decreased total renal perfusion and nearly absent blood flow to the fight kidney. A C T angiogram showed that the neck of the aneurysm involved both renal arteries and that a critical fight renal artery stenosis existed (Figure 4). The aneurysm was repaired with a Dacron aortobifemoral bypass. A simultaneous fight aorto-renal bypass with autogenous saphenous vein corrected the renal artery lesion. After the operation, the blood pressure became normal without any medications, and renal function returned to normal. A repeat radionuclide renogram now showed normal perfusion to both kidneys. Comment

CT angiography permitted identification of a critical right renal artery stenosis and simultaneously evaluated the relationship of the aneurysm neck to the renal arteries, which in turn

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Figure 6. Conventional angiogram of the distal calf and ankle. Note the absence of visible arterial branches.

allowed the planning of an operation that would definitively correct both problems with a single procedure. MAGNETIC R E S O N A N C E A N G I O G R A P H Y Magnetic resonance angiography (MRA) is done with a standard MRI scanner but with image processing software specifically programmed to isolate the blood vessels. The images are then reconstructed to resemble a standard angiogram, hut because the images are reassembled in 3-D, they can be rotated and viewed from multiple angles. No contrast material is necessary, making it useful in patients with poor renal function or contrast sensitivity. Scan times are quite long and motion artifacts are rather common, restricting its use to relatively short segments of the vascular system. Finally, the experience of the radiologist and his or her skill in choosing the proper algorithm for processing the data acquired is critical to image quality.6, 7 Clinical Example A 78-year-old man with diabetes and known coronary artery disease complained of severe leg pains after walking 100 feet or less, which rendered him essentially housebound. He also had severe diabetic nephropathy with moderate azotemia and a creatinine clearance of less than 20. Physical examination showed barely palpable femoral puls-

es bilaterally, no distal pulses on either side, and dependent rubor of both feet. The preoperative ankle/brachial index (ABI) measured 0.30-0.35 bilaterally. To plan therapy, the aortoiliac segment had to be accurately visualized, but the severe impairment of the patient's renal function made the use of contrast material unacceptably hazardous. Therefore, an MRA of the aorta and iliacs was done; it showed severe bilateral iliac artery occlusive disease with reconstitution of the common femoral arteries in the groin (Figure 5). The patient underwent a Dacron aortobifemoral bypass, which resulted in considerable improvement of his walking distance and a rise in his AB! to 0.50-0.52 after the operation. Comment In this case, MRA showed the status of the aortoiliac segment with remarkable precision without the use of any contrast material. Thus the definitive surgical procedure could be selected without risking further compromise of the patient's renal function. Clinical Example A 65-year-old man with a previous right below-knee amputation had been followed for 4 years with known occlusive disease of his left leg. He was asymptomatic, and his ABI

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Figure 7. MRA of the same area as in Figure 6. The peroneaI artery (asterisk) is seen, and a patent dorsalis pedis artery is clearly visible (arrows).

was stable at 0.40-0.45. He came to the office with a 48-hour history of the sudden onset of coldness and numbness of his left foot. Physical examination revealed a strong left femoral pulse but no distal pulses. The left foot had decreased motion and sensation, and the toes were cool and cyanotic. His ABI now measured 0.20 at the dorsalis pedis. An emergency contrast angiogram failed to show any suitable vessels in the foot (Figure 6). An MRA showed a patent anterior tibial-dorsalis pedis artery that was suitable as a bypass target (Figure 7). A left femoral-distal tibial bypass with in-situ saphenous vein was done. After the operation, the foot became warm and pink, and a strong dorsalis pedis pulse was palpable. The patient continues to do well with a patent bypass graft at the time of this report. Comment

In this patient, successful salvage of his limb depended on identification of a vessel in his foot that was suitable for use as a bypass target. Because conventional angiography had been unable to show any usable vessels and the patient already had received a large volume of contrast material, MRA offered the best way to image the distal vessels without risking nephrotoxicity. DISCUSSION From Dos Santos' first crude aortogram through today's digital subtraction studies, contrast angiography has remained the

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"gold standard" for imaging the vascular system. It is widely available, safe and simple to perform, and in the large majority of cases provides sufficient information for making clinical decisions. However, progress in vascular surgery has been accompanied by new imaging requirements that underscore the limits of conventional angiography. As bypasses to arteries of the ankle and foot in patients with severe multisegmental disease become common, the ability to adequately demonstrate these very distal arteries has become more important. As improved perioperative care has permitted older, sicker patients to undergo vascular operations safely, the necessity of avoiding contrast-related complications has become greater. As complex thoracoabdominal aneurysms are being repaired with increasing success, the need to clearly visualize complex anatomic relationships has grown. In this context, 3 new alternative imaging techniques have begun to play a larger clinical role. 3-D spiral CT scanning is the most widely used of the 3 methods. Its ability to exquisitely depict complex 3-D relationships has made it invaluable in areas such as the thoracic outlet, in which a clear understanding of the anatomy is essential for proper diagnosis and therapy. 4 In the first patient, 3-D CT was invaluable not only in reconciling the atypical symptoms with the anatomy, but also in planning the proper operative approach. CT angiography is a technique that combines the familiarity of angiography with the sophisticated imaging ability of spiral CT. In the case of abdominal aortic aneurysm, the relatively small risk of contrast infusion is more than offset by the abundant information obtained. The true diameter of the aneurysm, its distal extension into the iliac arteries, the relationship of the aneurysm to major visceral branch vessels, the detection of associated renal artery stenoses, the presence of accessory renal arteries, and the patency of the inferior mesenteric artery all can be determined with a single spiral CT scan. 4,5,12,13 All this information, combined with the ability to turn and view the image from multiple angles, makes CT angiography an invaluable preoperative tool in aneurysm repair. In the second patient, identification of the right renal artery stenosis and awareness that the aneurysm involved the origins of both renal arteries made it possible to expeditiously carry out a definitive surgical correction of both problems at one time. In addition to its obvious value in aneurysms, more recent reports have suggested that CT angiography is impressively accurate in examining the pulmonary vessels when compared with conventional pulmonary angiography) 4,15 The newest of the alternative imaging techniques to be applied to the vascular system M R A - - i s potentially the most useful. Whereas the use of MRA in assessing carotid artery disease is well known, more recent reports have suggested that it is comparable with conventional angiography in evaluating the aorta, iliac, and femoral arteries.5, 8,9 In the third patient, in whom the administration of contrast material was excessively hazardous, the ability to precisely visualize the aortic bifurcation allowed us to plan a definitive procedure without jeopardizing the patient's renal function. However, MRA has its greatest potential in peripheral arterial disease. Several recent reports 1°,11 have shown that it is supe-

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rior to conventional angiography in identifying usable target arteries in patients with severe occlusive disease. Furthermore, bypass grafts done to these "angiographically occult" arteries show normal longterm patency rates. 1° In our last patient, limb salvage was only possible because we could identify a patent run-off vessel through MRA. CONCLUSIONS Conventional angiography is still the diagnostic technique of first choice in vascular disease because of its safety, availability, and reliability. When it cannot be used or cannot provide sufficient information, alternative imaging techniques are available. 3-D spiral CT, CT angiography, and MRA avoid many of the problems of conventional angiography and show anatomic relationships better. However, these techniques are more expensive, are not as readily available, and clinical experience with them is still somewhat limited. Nevertheless, these alternative imaging techniques have fundamentally changed the practice of vascular surgery, and it behooves everyone involved in the care of vascular patients to be familiar with the techniques and their use.

Our special thanks to Jeffrey A. Blonder, MD, of the Department of Radiology, Highland Park Hospital, for his indispensable assistance in providing the superb images used in this article as well as his enthusiastic support for this project. REFERENCES 1. Altman SD, Kumpe DA, Redmond PL, et al. Principles of angiography. In: Rutherford R, editor. Vascular surgery. 4th ed. Philadelphia (PA): WB Saunders; 1995. p. 161-78. 2. Subber SW, Kumpe DA, Rutherford R. Contrast arteriography. In: Rutherford R, editor. Vascular surgery, 4th ed. Philadelphia (PA): WB Saunders; 1995. p. 178-203. 3. Paul LW, Juhl JH, editors. Essentials of roentgen interpretation. 3rd ed. Hagerstown (MD): Harper and Row; 1983. 4. Rubin GD, Dake MD, Semba CE Current status of threedimensional spiral CT scanning for imaging the vasculature. Radiol Clin N Am 1995;3(1):51-70. 5. Haaga JR, et al, editors. Computed tomography and magnet-

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ic resonance imaging of the whole body. 3rd ed. St. Louis (MO): Mosby; 1994. 6. Pearce WH, Salyapongse AN, Fitzgerald S. Computed tomography and magnetic resonance imaging in vascular disease. In: Rutherford R, editor. Vascular surgery. 4th ed. Philadelphia (PA): WB Saunders; 1995. p. 130-41. 7. Stanson AW, Breen JE Imaging of major veins by computed tomography and magnetic resonance imaging scanning. In: Yao JST, Pearce WH, editors. Progress in vascular surgery. Stamford (CT): Appleton & Lange; 1997. p. 447-85. 8. Caputo GR, Anderson CM, Saloner D. Magnetic resdnance angiography. In: Rutherford R, editor. Vascular surgery. 4th ed. Philadelphia (PA): WB Saunders; 1995. p. 141-61. 9. Carpenter JR Owen RS, Holland GA, Baum RA, Barker CF, Perloff L J, et al. Magnetic resonance angiography of the aorta, iliac, and femoral arteries. Surgery 1994; 116:17-23. Hoch JR, Tullis M J, Kennell TW, McDermott J, Acher CW, 10. Turnipseed WC. Use of magnetic resonance angiography for the preoperative evaluation of patients with infrainguinal arterial occlusive disease. J Vasc Surg 1996;23:792-801. 11. Carpenter JR Golden MA, Barker CE Holland GA, Baum RA. The fate of bypass grafts to angiographically occult runoff vessels detected by magnetic resonance angiography. J Vasc Surg 1996;23:483-9. 12. LaRoy LL, Comfier PJ, Matalon TA, Patel SK, Turner DA, Silver B. Imaging of abdominal aortic aneurysms. Am J Roentgenol 1989;152:785-92. 13. Papanicolaou N, Wittenberg J, Ferrucci JT, Stauffer AE, Waltman AC, Simeone JF, et al. Preoperative evaluation of abdominal aortic aneurysms by computed tomography. Am J Roentgenol 1986;146:711-5. 14. Remy-Jardin M, Remy J, Wattinne L, Giraud E Central pulmonary thromboembolism: diagnosis with spiral volumetric CT with the single-breath-hold technique--comparison with pulmonary angiography. Radiology 1992;185:381-7. 15. Schwickert HC, Schweden F, Schild HH, et al. Chronic thromboembolic pulmonary artery obstruction: CT diagnosis and follow-up after thromboendarterectomy. RAdiology 1994;191:351-7.

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