Renal MR Angiography

cr. Principles, Techniques, and Clinical Applications. Philadelphia: Lippincott-Raven, 1998, pp. 183196. 11. LaRoy LL, COimier PJ, Matalon TAS, et al. Imaging of abdominal aortic aneurysms. AJR 1989; 152:785-792. 12. Papanicolaou N, Wittenberg J, Ferrucci JT, et al. Preoperative evaluation of abdominal aortic aneurysms by computed tomography. AJR 1989; 1986; 146:711-715. 13. Rubin GD, Dake MD, Napel S, Jeffrey RBJ. Threedimensional cr angiography as an alternative to conventional arteriography in planning and in vivo evaluation of aortic stent grafts. Radiology 1993; 189(P):112. 14. Schwarcz TH, Planigan DP. Repair of abdOminal aortic aneurysms in patients with renal, iliac, or distal arterial occlusive disease. Surg Clin North Am 1989; 69:845-857. 15. Rubin GD, Dake MD, Napel S, Jefferey RBJ. Renal stent position and patency: evaluation with spiral cr angiography. Radiology 1992; 185(P)181. 16. Boijsen E. Angiographic studies of the anatomy of Single and multiple renal arteries. Acta Radiol 1959; 183(Suppl):1-99. 17. Rubin GD, Dake MD, Napel S, et al. Spiral CT of renal artery stenosis: comparison of three-dimensional rendering techniques. Radiology 1994; 190: 181-189. 18. Prokop M, Schaefer CM, Leppert AGA, Galanski M. Spiral cr angiography of thoracic aorta: femoral or antecubital injection site for intravenous administration of contrast material? Radiology 1993; 189(P) 111. 19. Remy-Jardin M, Remy .I, Wattinne L, Giraud F. Central pulmonary thromboembolism: diagnosis with spiral volumetric cr with the single-breath-hold technique-comparison with pulmonary angiography. Radiology 1992; 185:381-387. 20. Costello P,Ecker CP, Tello R, Hartnell GG. Assessment of the thoracic aorta by spiral cr. AJR 1992; 158:1127-1130. 21. Sommer T, Holzknechnt N, Smekal A, et al. Thinsection spiral cr in the evaluation of aortic dissection: comparison with MR imaging and transesophageal echocardiography. Radiology 1993; 189(P) 112. 22. Prokop M, Schaefer CM, Leppert AGA, Galanski M. Spiral cr angiography for diagnosis and follow-up of chronic aortic dissection. Radiology 1993; 189(P): 112.

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23. Costello P, Dupuy DE, Ecker CP, Tello R. Spiral CT of the thorax with reduced volume of contrast ma-

terial: a comparative study. Radiology 1992; 183: 663-666. 24. Prince MR. Gadolinium-enhanced MR aOitography. Radiology 1994; 191:155-164. 25. Litt AW, Eidelman EM, Pinto RS, et al. Diagnosis of carotid artery stenosis: comparison of 2DFT time-offlight MR angiography with contrast angiography in 50 patients. AJNR 1991; 12:149-154. 26. Ecklund K, Hartnell GG, Hughes LA, Stokes KR, Finn JP. MR angiography as the sole method in evaluating abdominal aortic aneurysms: correlation with conventional techniques and surgery. Radiology 1994; 192:345.

4:45 pm Renal MR Angiography Martin R. Prince, MD, PhD, Qian Dong, MD, Stefan O. Schoenberg, MD, and Ruth C. Carlos, MD Department of Radiology University ofMichigan Medical Center Ann Arbor, Michigan Abstract Renal MR angiography has advanced tremendously in the past several years and it can now accurately evaluate patients suspected of renal artery stenosis. There are many techniques available that proVide information about kidney morphology, renal arterial anatomy, and blood flow. Many of these techniques provide complementary information. This article reviews some of the more common techniques, illustrates how several sequences can be combined into a comprehensive examination, and presents examples of renal vascular pathology. Renal artery stenosis is the most common cause of secondary hypertension. When bilateral, renal artery stenosis can also cause progressive renal failure. Multiple treatment options, including percutaneous transluminal angioplasty, stenting, or surgical revascularization, may cure renovascular hypertension and preserve renal function. Accordingly, detecting renovascular disease early may prevent the morbidity of hypeltension and renal failure. Renovascular disease is implicated as the underlying cause in approximately 5% of patients with poorly controlled hypertension on medical therapy 0,2). 'The incidence of renal artery stenosis is higher in selected groups of patients. In a large autopsy study, diabetes mellitus in combination with hypertension had a renal artelY stenosis incidence of 10% (3). Diabetic patients with renal artery stenosis had bilateral renal artery stenosis 43% of the time. Patients with abdominal aoItic aneurysm had an incidence of 22% (4). Patients with peripheral vascular disease had up to 45% incidence of renal artery stenosis (5). Although using conventional angiography to evaluate renovascular disease is currently the gold standard, it is

Table 1 Sensitivity and Specificity of MRA (without Gadolinium) in Detection of Proximal Renal Artery Stenosis

Investigator

Year

Number of Patients

Kim

Debatin

1990 1991

25 33

Kent Gedroyc Grist Yucel Hertz Loubeyre Servais De Cobelli de Haan Loubeyre Schoenberg Duda

1991 1992 1993 1993 1994 1994 1994 1996 1996 1996 1997 1997

23 50 35 16 16 53 21 50 38 46 23 22

Technique

Sensitivity

Specificity

Degree of Stenosis

2DTOF 2D PC 2DTOF 2DTOF 3D PC 3D PC + 2D TOF 2D + 3D TOF 2D TOF 3D TOF 2D TOF 3D PC 3D PC 3D TOF + 3D PC 2D PC Flow 2D TOF 3D PC

100% 80% 53% 100% 83% 89% 100% 91% 100% 70% 94% 93% 100% 100% 73% 78%

92% 91% 97% 94% 97% 95% 93% 94% 76% 78% 94% 95% 90% 93% 47% 76%

>50% NA NA >50% NA >50% >70% >50% >50% NA NA NA >50% >50% NA NA

Preliminary data presented on the meeting of the International Society for Magnetic Resonance in Medicine at New York, 1996. Preliminary data presented on the meeting of the International Society for Magnetic Resonance in Medicine at Vancouver, 1997. NA = not available, any stenosis >0% was included.

an expensive, invasive examination with important risks, including arterial dissection, thrombosis, hemorrhage, life-threatening allergic reaction, or reduction in renal function from nephrotoxic contrast media. For these reasons, there has been considerable interest in developing less expensive, noninvasive alternatives. In this article, we review one of these noninvasive alternatives, magnetiC resonance angiography (MRA), for the evaluation of renal vascular pathology. Renal MRA may be used to assess patients with hypertension or deteriorating renal function. In addition, there has been more widespread use of MRA to evaluate renal arterial and venous structures to provide important information for planning repair of abdominal aortic aneurysms, for partial nephrectomy in patients with neoplastic disease, to assess renal bypass grafts or renal transplant anastomoses, and extension of renal tumors into the inferior vena cava (rvC). Techniques Many MR imaging approaches have been used to evaluate patients suspected of having renal vascular disease. Tl- and T2-weighted spin echo imaging has been used to evaluate kidney size, cortico-medullary differentiation, and parenchymal thickness. These findings all correlate with renal ischemia (6). High-temporal resolution mapping of gadolinium enhancement in the renal cortex and medulla has also been used to diagnose renal ischemia (7-9). In addition, several MRA techniques have been used to directly image the renal arteries and veins (Tables 1 and 2). Kim et al. in 1990 evaluated the accuracy of using two-dimensional (2D) time-of-flight (TOF) to evaluate the renal artery origins (0). A sensitivity of 100% and specificity of 92% were found. The images, however,

were of limited quality especially in older patients with slow flow. TOF images were difficult to interpret and did not show the entire renal artery. Three-dimensional (3D) TOF achieved higher resolution and eliminated slice misregistration. This improved the quality of reformations and subvolume maximum intensity projection (MIP) images. 3D TOF also permitted use of a shorter echo time that reduced signal dephasing a11ifacts at stenoses. However, 3D TOF suffered from long imaging time, too long to allow breath holding. There also was saturation of blood signal distally in the renal artery so only proximal and midrenal al1eries could be imaged well (1). Signal saturation in the distal renal artery was improved by using multiple overlapping thin slab acquisitions (MOTSA) (2) and with tilted, optimized nonsaturating excitation (TONE) (13). Even greater improvement was achieved with signal targeting with alternating radiofrequency (STAR) 04,15) and gated techniques (6). Another improvement came with 3D phase contrast 07-20). Phase contrast uses flow encoding gradients to achieve additional suppression of signal in stationalY background tissues. Phase contrast is also improved post-gadolinium (21). However, the best use of gadolinium contrast is for acquiring arterial phase 3D spoiled gradient echo arteriograms, which are somewhat analogous to spiral computed tomographic angiography (CTA) (22-29). This approach depicts the renal arteries along with the entire abdominal aorta, iliac arteries, and mesenteric arteries in a 30-second acquisition that can be performed during breath holding. By repeating the exam during a second breath hold, equilibrium phase images are produced which show the renal veins and rve. Clever mapping of k-space can allow high temporal resolution imaging (30). Gadolinium can be

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Table 2 Sensitivity and Specificity of MRA (with Gadolinium) in Evaluating the Entire Renal Artery for Stenotic Disease Number of Patients

Investigator

Year

Prince Grist' Holland

1995 1996 1996

Silverman Snidow Leungt Steffens Hany De Cobelli

1996 1996 1997 1997 1997 1997

19 28 63 63 37 47 65 50 39 55

Rieumont Baker

1997 1998

30 50

Technique

Sensitivity

Specificity

3D-Gd 3D Gd 3D Gd 2DTOF Cine PC 3D Gd 3D Gd 3D Gd 3D Gd 3D Gd 3D PC 3D Gd 3D Gd

100% 88% 100% 74% 100% 100% 100% 96% 93% 100% 94% 100% 97%

93% 88% 100% 98% 93% 89"16 94% 95% 98% 97% 96% 71% 92%

Degree of Stenosis 75% >50% >50% NA NA >60% NA >50% >50% >50% >50% >50%

'Preliminary data presented on the meeting of the International Society for Magnetic Resonance in Medicine at New York, 1996. tPreliminary data presented on the meeting of the International Society for Magnetic Resonance in Medicine at Vancouver, 1997. NA = not available, any stenosis >0% was included.

used safely, even at high doses, in patients with renal failure (31). Perhaps the most important advance is the development of 20 cine phase contrast (PC) for renal blood flow measurements (32,33). This sequence is acquired perpendicular to the renal artery with EKG monitoring. A flow curve is produced showing how renal blood flow varies over the cardiac cycle. Flattened low-resistivity patterns are suggestive of hemodynamically significant stenosis. The flow data can also be integrated to determine the volume flow rate to each kidney. Evaluation of changes in renal blood flow with various pharmacological interventions has been proposed but not yet extensively studied.

342

Example of Imaging Protocol There are many useful approaches to the renal MRA examination, and centers with real-time monitoring of cases may choose to customize the examination in every patient based on how well the renal arteries are seen and any pathology that shows up on each successive pulse sequence. We have found it is easier, however, to have a standardized exam that we know works well in 95% of patients, then it is not necessary to monitor every case. The standardized exam can be done emergently without requiring the radiologist until it is complete. In addition, the technologist becomes expert at performing this single test. This may seem somewhat excessive but it is based on years of experience with renal MRA and has been refined during implementation in over 1000 examinations. Limited quality on anyone sequence may be compensated for by the other sequences (34). The standardized examination starts with a large field-of-view Tl-weighted sagittal sequence. This can be achieved by using either a spin echo technique with interleaved acquisition or gradient echo imaging with one or two breath holds. These images provide spatial

Figure 1. Coronal maximum intensity projection of 3D Gd MRA shows normal renal arteries bilaterally with an accessory right renal artery arising from the distal aorta (arrow).

localization for the renal artely 3D gadolinium (Gd) MRA and 3D PC sequences, and also an assessment for mass lesions of the adrenal glands, kidneys, and retroperitoneum. Next, acquire axial T2-weighted images with fat saturation. This will help characterize any masses that are present, and it is especially useful for discriminating benign, simple renal cysts from more ominous complex or solid renal masses. Then, 3D dynamic gadolinium-enhanced spoiled gradient echo imaging is performed in the coronal plane positioned to include the aorta and renal arteries. This sequence is repeated three times: pre-contrast, during the arterial phase, and during the venous or equilibrium phases of contrast administration. To ensure coverage of

A

B

Figure 2. CA) Sagittal Tl-weighted spin echo image reveals normal kidney size and parenchymal thickness. CB, C) Kidney length can also be measured from subvolume oblique reformation of the 3D Gd MRA arterial phase.

c

the relevant anatomy, posItIon the image volume to include anterior to the venD-al border of the distal aorta, superior to the celiac trunk, and extend posteriorly to encompass as much of the kidneys as possible. The field-of-view should be set to approximately the width of

the patient's torso, typically 28-32 cm. This will help minimize aliasing and also should be large enough to include the iliac arteries so that all accessory renal arteries can be detected (Fig. 1). To make the acquisition fast enough to complete in a breath hold, use the shortest

343

A

B

Figure 3.

(A) Coronal subvolume maximum intensity projection of 3D Gd MRA shows normal renal arteries bilaterally (arrows), There is atherosclerotic narrowing of the left common iliac artery, (B) The renal alteries are also normal on the 3D phase contrast MRA (arrows),

Figure 4.

Sagittal subvolume maximum intensity projection of 3D Gd MRA shows normal celiac (CA) and superior (SMA) and inferior mesenteric (IMA) arteries,

344

possible repetition time and partial acquisition (0,5 averages), if available, The echo time should be <3 msec and ideally close to 2,1 msec where fat and water are out-of-phase so that there is some additional fat sup-

pression, A flip angle of apprOXimately 45° is generally appropriate because the technique is not very flip angledependent. Larger flip angles provide greater fat suppression and a greater signal-to-noise ratio if the contrast dose is large enough, Correctly estimating the breath-hold capaCity of the patient is important because multiple breath holds are reqUired for each phase of the contrast bolus (35). Usually, older patients, smokers, and patients with cardiopulmonary disease can suspend breathing for only 20-25 sec or less, Younger, nonsmoking patients with no cardiac or pulmonary disease can easily hold their breath 30-40 sec or longer. Oxygen by nasal cannula can increase a patient's breath-holding duration (36). The slice thickness, number of slices, and number of phase encoding steps should be adjusted to ensure the exam can be completed within a breath hold, Although the acquisition time can also be shortened by increasing the bandwidth, this results in a significant signal-to-noise ratio penalty. It is preferable to shorten the scan time by using 0.5 averages, fewer phase encoding steps, or fewer slices. Before contrast material injection, image once with breath holding to make sure the aorta and kidneys are included without excessive aliasing artifact. This precontrast data can also be subsequently subtracted from the dynamic contrast enhancement data set. Next, repeat the 3D image with data acquisition synchronized to the arterial phase of the bolus. Optimal bolus timing can be determined by using a test bolus (37), by using a gadolinium detection pulse sequence (SmartPrep, GE Medical Systems, Milwaukee, WI) (38,39), or empirically based on the patient's age and cardiovascular status. Typically, a delay of 10 seconds between starting injecting and starting scanning works for the majority of patients. Inject at 2 cc/sec. Another technique to ensure correct

A

B

Figure 5. (A) Coronal oblique subvolume MIP may be obtained in the plane of the course of the artery using a subvolume MIP as a guide. (B) A second image in the axial oblique orientation may be reconstrlJcted from a coronal oblique subvolume MIP to display the artery in its entirety.

A

B

c Figure 6. (A) Coronal subvolume maximum intensity projection of 3D Gd MRA shows atherosclerotic plaque near the right renal artery origin (arrow). (B) Axial subvolume MIP shows mild stenosis «50%) of the right renal artery (arrow) and (C) has a normal appearance on 3D phase contrast (arrow).

bolus timing is to image repeatedly so rapidly that the arteIial phase is synchronized with at least one acquisition. Afterward venous and equilibrium phases can be

performed with brief delays to allow the patient to catch his or her breath. The bolus does not have to be as long at the acquisition as long as the peak arterial gadolinium

345

A

c

B

D

Figure 7. (A) Sagittal Tl-weighted image shows that the left kidney has markedly reduced length and parenchymal thickness and right kidney is normal in size. (B) Coronal subvolume maximum intensity projection of 3D Gd MRA demonstrates a stenosis of the right renal artery (arrow), the left renal artery is occluded (arrowhead), and the aorta is irregular with atherosclerotic change. (C) Axial maximum intensity projection of 3D phase contrast (PC) images show dephasing in the region of stenosis (arrow) indicating the left renal artery stenosis is hemodynamically significant. There is no 3D PC signal corresponding to the occluded left renal artery (arrowhead). (D) Conventional arteriogram confirms MR findings and demonstrates a 60 mm Hg pressure gradient across the right renal artery stenosis (arrow).

346

A

B

C Figure 8. (A) Coronal maximum intensity projection, (B) subvolume maximum intensity projection, and (C) axial subvolume maximum intensity projection of 3D Gd MRA images show beaded, irregular appearance of renal arteries bilaterally in a patient with FMD (aLTows).

concentration coincides with acquisition of a substantial portion of central K-space data (40). The image quality tends to vary directly with gadolinium dose. The larger the dose, the greater the probability of high-quality images. Some experts advocate doses of 0.2 or 0.3 mmoVkg. However, it may be simpler to give evelY patient the same volume of contrast (40 mL: two bottles of 20 mL each). This makes it easy to become comfortable with a standard pattern of hand injection. It may also be helpful to use a standardized IV tubing set (SmaltSet, TopSpins Inc, Plymouth, MI). Typical injection rates are 2-3 cc/sec. It is important to flush the gadolinium through the rv tubing and veins with at least 20 cc of normal saline to ensure delively of the entire gadolinium dose and to ensure rapid venous return. Finally, axial 3D PC angiography can be obtained to further characterize renal altery stenoses. The venc should be set to 50 em/sec for patients expected to have normal renal artelY flow. It can be reduced to 30 em/sec in patients with heart failure, renal failure (creatinine >2.0 mg/dL), aortic anewysm, or in patients >60 years of age; t11ese patients are expected to have slower renal artelY flow. Young «30 years of age) or athletic patients

may require a venc of 60 em/sec. Performing this sequence at the end of the study takes advantage of the extra signal-to-noise produced by the gadolinium injected for coronal dynamic gadolinium-enhanced scan.

Image Analysis The sagittal TI-weighted pulse sequence is used to measure kidney length and parenchyma thickness (Fig. 2). The kidney length and parenchyma tI1ickness are reduced in patients with chronic renal artery stenosis. The axial T2-weighted images are examined for the presence of renal masses or other abdominal pathology. Pathologic findings can also be furtl1er assessed by reviewing the single slices of the 3D gadolinium-enhanced dataset. The 3D dataset can be postprocessed on a computer workstation with MIPs and reformations. In this way, the abdominal aOlta, tI1e renal arteries, as well as the common iliac arteries are evaluated (Fig. 3A). Also, it is necessary to assess the celiac axis and superior mesenteric arteIY origins by using a sagittal subvolume MIP (Fig. 4). For ·the renal alteries, a subvolume MIP is performed encompassing each renal artelY in the coronal oblique and axial oblique planes so that each renal

347

A

Figure 9. (A) Coronal and (B) axial subvolume maximum intensity projection of 3D Gd MRA show bilateral renal artery saccular aneurysms (arrows), the larger being on the left.

B

Figure 10. (A) Coronal and (B) axial subvolume maximum-intensity projection of 3D Gd MRA images show a fusiform aneurysmal dilatation of the left renal artery (arrows) and a normal right renal artery (arrowheads). Note there has been graft repair of an infrarenal abdominal aortic aneurysm.

A

B

348

A

B

c Figure 11. (A) Coronal maximum intensity projection image and (8) oblique reformatted image show normal aorta, iliac, and transplant arteries (an·ows). (C) Stenosis of the transplant renal artery distal to the anastomosis in another patient (arrow).

artery origin is evaluated with perpendicular views (Fig. 5). The grading of renal artery stenosis can be decided on 3D Gd MRA. Commonly, renal arteries are graded as follows: normal (0-24%), mild (25-49%), moderate (5075%), severe (75-99%), or occluded. Renal artery stenosis grades are illustrated in Figures 3, 6, and 7. The evidence of hemodynamically significant stenosis can be further evaluated on 3D PC by the presence of signal dephasing. Normal renal artery caliber on 3D PC indicates normal renal arteries (Fig. 3B) or, at most, mild renal artery stenosis (Fig, 6c). Severe signal dephasing, however, is evidence of a hemodynamically significant or severe renal artery stenosis (Fig. 7C). If a stenosis is present on both 3D Gd and 3D PC but there is no

dephasing, then it is graded as moderate. Poststenotic dilatation, loss of cortico-medullary differentiation, delayed renal enhancement, and asymmetric concentration of gadolinium in the collecting systems are additional signs of hemodynamic significance (41). Renal veins, IVC, and the portal venous system are evaluated on the venous and the equilibrium enhancement images.

Pathology Renal A rtery Stenosis Atherosclerotic stenosis is the most common renal arterial pathology and the number one reason for angiographic evaluation of the renal arteries. Atherosclerosis is

349

A

B

c Figure 12. (A) Coronal maximum intensity projection image of a patient with abdominal aorta coarctation, (B) the region of coarctation is magnified, and (C) sagittal subvolume maximum intensity projection shows the left renal artery (arrow) originates from the tapered aorta above the occlusion.

350

the cause of renal artery stenosis in most cases of renovascular hypertension. It typically presents in middle or old age. Men are affected twice as often as women. Atherosclerotic disease is exacerbated by cigarette smoking, hyperlipidemia, and diabetes. Atherosclerotic renal

artery stenosis is most corrunonly a manifestation of generalized atherosclerosis also involving coronary, cerebral, and peripheral vessels, although in 15-20% of all cases it is not associated with disease elsewhere. In patients with renovascular hypertension, atherosclerosis

c

A

B

Figure 13. (A) Coronal maximum intensity projection, (8) magnification view, and (C) axial subvolume maximum intensity projection of 3D Gd MRA images show aortic dissection. The right renal artery arises from the true lumen (arrows), and left renal artery arises from the false lumen (arrowheads).

is usually present in the aorta and it typically compromises the ostium and/or proximal 1-2 cm of one or both renal arteries (Fig. 7B). Rarely, atherosclerosis may also involve the distal renal artelY or renal artery branches. When left untreated, it may progress to renal altery occlusion. Fibromuscular dysplasia (FMD) is the second most common cause of renal artery stenosis. It is a nona theromatous vascular lesion that can involve medium-sized and smaJJ alteries (42). This disease most commonly affects renal, carotid, and intracerebral arteries, although it has been reported in other arterial beds, including the subclavian, axillary, mesenteric, hepatic, splenic, and iliac arteries. The etiology of FMD is unknown. Several hypotheses for the cause have been proposed, including humoral factors, mechanical factors, genetic factors, and

ischemia. The majority of patients are female, and FMD almost always presents at a young age «40 years of age). Typical clinical manifestations depend on the vessels involved; renovascular hypertension, stroke, abdominal angina, or other symptoms of vascular insufficiency are all common presentations. The histologic classification of FMD is based on the angiographic appearance and the layer of arterial wall that is primarily affected. In this way, lesions can be divided into intimal fibroplasia, medial fibromuscular dysplasia, and perimedial (adventitial) fibroplasia. Medial FMD is the most common type and is fUither subdivided into medial fibroplasia (common), medial hyperplasia, and perimedial dysplasia (both rare). TypicaJJy, the appearance on angiography of medial fibroplasia is the classic string-of-beads, with sites of web-like

351

A

B

Figure 14. (A) Coronal maximum intensity projection image during arterial phase and (B) reformatted image during equilibrium phase from 3D Gd MRA in a patient with large renal cell carcinoma show a large heterogeneously enhancing mass within the upper and midportion of the left kidney (arrows), the bilateral renal arteries are patent, the tumor extends into the proximal left renal vein (arrow), and the main portal vein and its main branches are patent.

stenoses alternating with small fusiform or saccular aneurysms. Usually the distal two thirds of the main renal artery is involved, sometimes with extension into segmental vessels. Bilateral involvement is common (Fig. S). FMD may be complicated by focal aneurysm formation and also distal embolization of small thrombi that form in the aneurysmal segments. Compared with medial fibroplasia, angiographic findings in perimedial fibroplasia also include a beaded appearance; however, these beads are not aneurysms but are spaces between concentric deposits of collagen between the media and adventitia smaller than the diameter of the normal vessel lumen. Most commonly, the distal main renal artery is involved without extension into the segmental branches. Prominent collateral circulation is common. Intimal fibroplasia usually presents as a smooth, focal stenosis with involvement of the distal renal artery. Proximal lesions can be seen in association with abdominal aorta hypoplasia and coarctation. Angiographic findings include web-like stenoses and spontaneous dissection.

352

Renal Artery Aneurysm Aneurysms resulting from atherosclerosis usually occur in the infra renal aorta and common iliac arteries. However, they also can involve the renal arteries. Most aneurysms of the renal artery have been found in individuals between 50 and 70 years of age. Congenital aneUlysms and those associated with fibromuscular disease, however, are usually seen in young adults or even children (43). The complications of aneurysms, such as rupture, thrombosis, embolization, and dissection, could occur on renal artery aneurysms. Renal artery aneurysms

are subject to the same complications as aneurysms elsewhere, including rupture, thrombosis, embolization, and dissection. Because of the 3D nature of the technique, MRA is helpful in evaluating the anatomic characteristics of an aneurysm including the neck of the aneurysm, its relationship to other vascular structures, the diameter in all orientations, and aneurysm type, such as saccular (Fig. 9) or fusiform (Fig. 10).

Transplant Renal Arteries Whenever the creatinine rises in a renal transplant patient, the possibility of a transplant renal artery stenosis may be considered. An arterial stenosis may be more likely when there is associated hypeltension and clinical features or renal biopsy results that are not consistent with rejection. Doppler ultrasound may also identify a pulsus tardus waveform, which may be seen with hemodynamically Significant stenosis. Typically, stenoses occur at the surgical anastomosis connecting the transplant artery to the iliac artery (Fig. 11). There may also be atherosclerotic narrowing of the iliac artery proximal to the transplant alterial anastomosis. Rarely, a stenosis may be located distal to the anastomosis. Renal vein thrombosis or venous anastomotic narrowing can also occur. Coarctation Aortic coarctation reduces the renal perfusion pressure and thereby causes renovascular hypertension. If coarctation is suspected, the imaging field-of-view should be expanded to include the entire thoracic aorta because the most common site of coarctation is just distal to the origin of the left subclavian anery. Coareration may also

A

B

c

D

Figure 15. Renal cell carcinoma is seen on (A) coronal Tl-weighred spin echo, (B) axial T2-weighred, and (C) axial 2D rime-of-flighr images to exrend into the right renal vein (black arrow) and expand rhe rve (white arrows). (D) Venous phase image of a renal contrasr arteriogram confirms the !vIR findings.

involve the abdominal aorta (Fig. 12). Abdominal coarctation is often associated with narrowing of the renaJ artery origins.

Aortic Dissection Renal anery flow can be compromised in patients with aortic dissections that extend down into the abdominal aona (Fig. 13), There are several mechanisms that can compromise flow (44). First, the dissection may extend into a renal artery. Second, a normaJ renal artery arising from ~he true lumen may have reduced flow as a result of collapse of the true lumen. The intimal flap may intermittently cover the renal origin, interrupting renal blood flow. Reduced perfusion pressure- in the true lumen may also reduce renal blood flow. Inlerestingly, when the

renal artery origin is sheared off by the dissection and it arises from the false lumen, there is a protective effect because the false lumen often has a higher pressure than the true lumen. One additional cause of increasing serum creatinine in patients with aonic dissection results from the antihypertensive medication llsed in the medical management of dissection. A normal blood pressure may not be sufficient to adequately perfuse kidneys damaged by years of uncontrolled hypertension and dependent on elevated pressure.

Renal Cell Carcinoma Renal cell carcinoma tends to involve the renal vein (Fig. 14) and grow up the inferior vena cava. The extent of tumor invasion of the lye may be difficult to assess on

353

imaging sequences, Frank Londy for assistance with preparing the figures, and Karen Travis for assistance with manuscript preparation.

References 1. Prince MR, Grist TM, Debatin ]. 3D Contrast MR Angiography. Berlin, New York: Springer, 1997. 2. Lewin A, Blaufox MD, Castle H, Entwisle G, Langford H. Apparent prevalence of curable hypertension in the hypertension detection and follow-up program. Arch Intern Med 1985; 145:424-427. 3. Sawicki PT, Kaiser S, Heinemenn L, et al. Prevalence of renal artery stenosis in diabetes mellitus-an autopsy study. ] Intern Med 1991; 229:489-492. 4. Brewster DC, Retana A, Waltman AC, et al. Angiography in the management of aneurysms of the abdominal aorta. Its value and safety. N Engl ] Med 1975; 292:822-825. Figure 16. Coronal maximum intensity projection linage during the equilibrium phase shows a retroaortic left renal vein (arrow).

5. Missouris CG, Buckenham T, Cappuccio FP, et al. Renal artery stenosis: a common and important problem in patients with peripheral vascular disease. Am] Med 1994; 96:10-14

ultrasound and even on contrast enhanced CT. T1- and T2-weighted spin echo sequences, in combination with axial TOF images, provide a comprehensive assessment of patients presenting with solid renal masses (Fig. 15). Axial T1- and T2-weighted images should cover from the diaphragm to below the lower poles to assess for adenopathy, invasion of adjacent organs, and for unusual liver metastases. Kallman et al. has shown that MR evaluation has 100% sensitivity for detection of tumor thrombus beyond the distal renal vein (45). If a mass has a T1-bright region, a fat saturation sequence is useful to determine if the mass is a hemangiomyolipoma. Cystic masses that do not meet all the requirements for benign lesions (homogeneously, T2 bright; imperceptible thin margins, T1 dark) may require a gadolinium injection to ensure that the mass is not enhancing. The renal vein may also be assessed during the equilibrium phase of a gadolinium bolus.

6. Prince MR, Schoenberg SO, Ward ]S, Londy F], Wakefield TW, Stanley]C. Hemodynamically significant atherosclerotic renal artery stenosis: MR angiographic features. Radiology 1997; 205:128-136.

Anatomic Variation

It is important to be aware of the possibility of accessory renal arteries that occur in an estimated 10% of patients (46) and mostly originate from the abdominal aorta. Normally, the left renal vein passes anterior to the aorta just inferior to the superior mesenteric arteries. However, it is also possible to have a retroaortic renal vein (Fig. 16). When the normal and retroaortic veins are present, it is called a circuma011ic renal vein.

Summary Safe, accurate MR assessment of renal vascular pathology is now practical.

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Acknowledgments: The authors thank Drs. Chenevert and Maki for assistance with the development of the

7. Mukai], Kershaw G, Hees PS. Application of magnetic resonance imaging to physiologic and morphologic characterization of renal artelY stenosis before and after angioplasty. Am j Physiol Imaging 1992; 7:220-229. 8. Ross PR, Gauger], Stoupis C, et al. Diagnosis of renal artery stenosis: feasibility of combining MR angiography, MR renography, and gadopentetate-based measurements of glomerular filtration rate. A]R Am] Roentgenol 1995; 165:1447-1451. 9. Vosshenrich R, Kallerhoff M, Grone H], et al. Detection of renal ischemic lesions using Gd-DTPA enhanced turbo FLASH MRI: experin1ental and clinical results.] Comput Assist Tomogr 1996; 20:236-243. 10. Kim D, Edelman RR, Kent KC, Porter DH, Skillman ]). Abdominal aorta and renal artery stenosis: evaluation with MR angiography. Radiology 1990; 174: 727-731. 11. Grist TM. Magnetic resonance angiography of the aorta and renal arteries. MagnetiC Resonance Imaging Clinics of North America 1993; 1:253-269. 12. Yucel EK, Kaufman]A, Prince M, Bazarj H, Fang LS, Waltman AC. Time of flight renal MR angiography: utility in patients with renal insufficiency. Magn Reson Imaging 1993; 11 :925-930. 13. Fellner C, Strotzer M, Geissler A, et al. Renal arteries: evaluation with optimized 2D and 3D time-of-flight MR angiography Radiology 1995; 196:681-687

14. Edelman R, Siewert B, Adamis M, Gaa ], Laub G, Wielopolski P. Signal targeting with alternating radiofrequency (STAR): application to MR angiography. Magn Reson Med 1994; 31:233-238. 15. Wielpolski PA, Adamis M, Prasad P, et al. Breathhold 3D STAR MR angiography of the renal arteries using segmented echo planar imaging. Magn Res Med 1995; 33:432-438. 16. Borrello ]A, Li D, Vesely TM, Vining EP, Brown ]], Haacke EM. Renal arteries: clinical comparison of three-dimensional time-of-flight MR angiographic sequences and radiographic angiography. Radiology 1995; 197:793-799. 17. de Haan MW, Kouwenhoven M, Thelissen GR, et al. Renovascular disease in patients with hypertension: detection with systolic and diastolic gating in threedimensional, phase-contrast MR angiography. Radiology 1996; 198:449-456 18. Duda SH, Schick F, Teufl F, et al. Phase-contrast MR angiography for detection of arteriosclerotic renal artelY stenosis. Acta Radiol 1997; 38:287-291. 19. Gedroyc WM, Neerhut P, egus R, et al. Magnetic resonance angiography of renal artery stenosis. Clin Radiol 1995; 50:436-439 20. Loubeyre P, Trolliet P, Cahen R, Grozel F, Labeeuw M, Minh VA. MR angiography of renal artelY stenosis: value of the combination of three-dimensional time-of-flight and three-dimensional phase-contrast MR angiography sequences. A]R Am ] Roentgenol 1996; 167:489-494. 21. Bass ]C, Prince MR, Londy FJ, Chenevert TL. Effect of gadolinium on phase-contrast MR angiography of the renal arteries. A]R Am J Roentgenol 1997; 168: 261-266. 22. Holland GA, Dougherty L, Carpenter JP, et al. Breath-hold ultrafast three-dimensional gadoliniumenhanced MR angiography of the aorta and the renal and other visceral abdominal arteries. AJR Am ] Roentgenol 1996; 166:971-981. 23. Johnson DB, Lerner CA, Prince MR, et al. Gadolinium-enhanced magnetic resonance angiography of renal transplants. Magn Reson Imaging 1997; 15:1320. 24. Leung DA, McKinnon GC, Davis CP, et al. Breathhold, contrast-enhanced, three-dimensional MR angiography. Radiology 1996; 200:569-571. 25. Prince MR. Gadolinium-enhanced MR aortography. Radiology 1994; 191:155-164. 26. Prince MR, Narasimham DL, Stanley JC, et al. Breathhold gadolinium-enhanced MR angiography of the abdominal aorta and its major branches. Radiology 1995; 197785-792. 27. Rieumont MJ, Kaufman JA, Geller SC, et al. Evalua-

tion of renal artelY stenosis with dynamic gadolinium-enhanced MR angiography. A]R Am] Roentgenol 1997; 169:39-44 28. Steffens ]C, Link J, Grassner J, et al. Contrast-enhanced, k-space-centered, breath-hold MR angiography of the renal arteries and the abdominal aorta. ] Magn Reson Imaging 1997; 7:617-622. 29. WiJman AH. Riederer SJ, Grimm RC, et al. Multiple breath-hold 3D time-of-flight MR angiography of the renal arteries. Magn Res Med 1996; 35:426-434. 30. Korosec FR, Frayne R, Grist TM, Mistretta CA. Timeresolved contrast-enl1anced 3D MR angiography. Magn Res Med 1996; 36:345-351. 31. Prince MR, Arnoldus C, Frisoli JF. Nephrotoxicity of high-dose gadolinium compared to iodinated contrast.] Magn Reson Imaging 1996; 6:162-166. 32. Masui T, Takehara Y, Igarashi T. et al. MR angiography of the renal artery: comparison of breath-hold two-dimensional phase-contrast cine technique with the phased-array coil and breath-hold two-dimensional tin1e-of-flight technique with the body coil. Eur J Radiol 1997; 25:62-66 33. Schoenberg SO, Knopp MV, Bock M, et al. Renal artery stenosis: grading of hemodynamic changes with CINE phase-contrast MR blood flow measurements. Radiology 1997; 203:45-53. 34. Prince MR. Renal MR angiography: a comprehensive approach. J Magn Reson Imaging 1998; 8:511-516. 35. Maki JH, Chenevert TL, Prince MR. The effects of incomplete breath-holding on 3D MR imaging quality. J Magn Reson Imaging 1997; 7:1132-1139. 36. Marks B, Mitchell DG, SimeJaroJP. Breath-holding in healthy and pulmonary-compromised populations: effects of hyperventilation and oxygen inspiration. J Magn Reson Imaging 1997; 7:595-597. 37. Earls], Rofsky NM. DeCorato DR, Krinsky GA, Weinreb Jc. Hepatic arterial-phase dynamic gadoliniumenhanced MR imaging: optimization with a test examination and a power injector. Radiology 1997; 202:268-273. 38. Prince MR, Chenevert TL, Foo TK, Londy F], Ward JS, Maki JH. Contrast-enhanced abdominal MR angiography: optimization of imaging delay time by automating the detection of contrast material arrival in the aorta. Radiology 1997; 203:109-114. 39. Wilman AH, Riederer SJ, King BF, Debbins]P, Rossman PJ, Ehman Ri. Fluoroscopically triggered contrast-enhanced three-dimensional MR angiography with elliptical centric view order: application to the renal arteries. Radiology 1997; 205:137-146. 40. Maki JH, Prince MR, Londy FJ, Chenevert TI. The effects of time valying intravascular signal intensity and k-space acquisition order on three-dimensional

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MR angiography image quality. ] Magn Reson Imaging 1996; 6:642-651. 41. Walsh P, Rofsky NM, Krinsky GA, Weinreb ]c. Asymmetric signal intensity of the renal collecting systems as a sign of unilateral renal artery stenosis following administration of gadopentetate dimeglumine. ] Comput Assist Tomogr 1996; 20:812-814. 42. LUscher TF,Lie ]T, Stanson AW, Houser OW, Hollier LH, Sheps SG. A.t1erial fibromuscular dysplasia. Mayo Clin Proc 1987; 62:931-952. 43. Kincaid OW. Renal Angiography. Chicago, IL: Year Book Medical Publishers, Inc, 1996:124-125. 44. Williams DM, Lee DY, Hamilton BH, et al. The dissected aOl1a. Part III: Anatomy and radiologic diagnosis of branch-vessel compromise. Radiology 1997; 203:37-44. 45. Kallman DA, King BF, Hattery RR, et al. Renal vein and inferior vena cava tumor thrombus in renal cell carcinoma: cr, US, MRI, and venacavography. ] Comput Assist Tomogr 1992; 16:240-247. 46. Kadir S. Atlas of Normal and Variant Angiographic Anatomy. Philadelphia, PA: WE Saunders Co, 1991: 388. Renal MR Angiography: A Comprehensive

Approach Martin R. Prince, MD, PhD

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Renal artery MR angiography has now emerged as a safe, accurate approach to renal arteriography. A comprehensive examination, including both three-dimensional (3D) dynamic gadolinium-enhanced and 3D phase contrast MRA techniques, allows evaluation of both the aortarenal and splanchnic arterial anatomy as well as the hemodynamic significance of any stenoses identified. The 3D gadolinium-enhanced MRA technique produces a contrast arteriogram but without risks of iodinated contrast or ionizing radiation. The 3D phase contrast technique is a flow-based technique, which may show dephasing in the presence of hemodynamically significant stenoses. A comprehensive examination should also include Tl- and T2-weighted imaging for the assessment of potential neoplastic masses and the ubiquitous renal cysts. Through trial and error over the course of over 1000 examinations, this comprehensive approach to the MR evaluation of renal vascular pathology has emerged. This comprehensive approach to performing renal MR angiography (MRA) uses a l.5-T imaging system witt high-performance gradients capable of performing three-dimensional (3D) spoiled gradient-echo imaging with TRs less than 10 msec and TIs less than 3 msec. This method also requires access to a computer workstation that performs refOimations and subvolume maximum intensity projections.

Clinical Issues High-resolution renal artery imaging is used in several clinical settings. Patients with hypertension may require renal arteriography because renal artery stenosis is the cause in 2-5% of patients who are difficult to control with medical therapy (1). The incidence is higher in patients with hypertension and suggestive clinical features such as an abdominal bruit, accelerated hypertension, new-onset hypertension before 25 years of age or after 55 years of age, hypokalemia, or a positive captopril test. A second group of patients requiring renal artery imaging are those with worsening renal function. Bilateral renal artery stenoses may represent a reversible cause of renal failure that, when corrected, could avert or at least postpone dialysis. A third group of patients are those who have an abdominal aortic aneurysm. It is important for surgical planning to know the number of renal arteries and their location relative to the aneurysm and whether significant atherosclerotic disease is present. The final group of patients are those who have undergone renal revascularization; they may need to be evaluated for complications of the revascularization. Imaging Issues An effective renal artery imaging study must address all of the imaging needs of the patient, which are as follows: (a) determining the number and location of the renal arteries; (b) identifying and categorizing any renal artery stenoses as orificial, proximal, midrenal artery, or distal; (c) evaluating the hemodynamic and functional significance of each stenosis; (d) determining possible bypass sites by assessing atherosclerotic involvement of the aorta as well as the celiac and superior mesenteric artery origins; and (e) identifying any potentially neoplastic renal or perirenal masses which may supersede the clinical importance of renal revascularization. MRI Protocol

Before the onset of imaging, an intravenous line is stal1ed, typically in the forearm, antecubital fossa, or wrist. For hand injections, it is acceptable to use an existing central line, peripherally inserted central catheter line, or subcutaneous pOl1 if they are accessed with sterile technique and subsequently flushed with heparinized saline. It is useful to have a standard tubing set which has a known priming volume and allows simultaneous attachment of two syringes: one for gadolinium contrast and another for saline flush (Fig. 1). This way the operator can become comfortable with a single IV tubing set that always has the same priming volume and flow resistance. The patient is positioned feet first in the magnet with a respiratoly monitoring device (Fig. 1). It is acceptable to use the body coil on adults. Infants and small children can often be placed in a head coil for better image quality provided by the higher signal-to-noise ratio afforded by this coil. It is also possible to use any of a variety of phased array torso coils, although they may

Figure 1. Positioning for renal MRA. The patient is placed feet first in the magnet with a respiratory monitor, an intravenous line, and oxygen tubing. The intravenous tubing has two syringes attached: one large, 60-mL syringe for gadolinium and another 30-mL syringe for saline flush. Landmarking is performed on the lower margin of the rib cage Carrow) to center on the kidneys.

have field inhomogeneity that can degrade the images. A centering landmark is placed at the middle of the kidneys. This level is easily identified by palpating for the lower margin of the rib cage. A large field-of-view Tl-weighted sagittal sequence is acquired initially. This can be a spin-echo pulse sequence with interleaved acquisition so images are available halfway through the scan to begin proscribing the next sequence. A TR of 350 msec with 8-1O-rnm thick sections from LIOO to RIOO will encompass both kidneys in most patients and resolves the aorta sufficiently for planning the subsequent pulse sequences. Alternatively, gradient-echo imaging can be used to acquire similar images in one or two breath-holds. By avoiding gradient moment nulling and using a full echo with relatively low bandwidth (16 kHz), the aorta appears as a black flow void surrounded by Tl bright retroperitoneal fat. In the next step, axial T2-weighted images with fat saturation are acquired, encompassing the length of the kidneys (Fig. 2a). While the magnet is acquiring this data, the 3D dynamic gadolinium-enhanced acquisition can be planned (Fig. 2b). The 3D spoiled gradient-echo imaging is performed in the coronal plane precontrast, during the arterial phase, and during venous or equilibrium phases produced by bolus injection of a gadolinium chelate (2). To select the imaging parameters, the imaging time should be chosen first by estimating the length of time the patient can suspend breathing. Typically, older patients (70 years of age or older), smokers, and patients with cardiopulmonary disease can suspend breathing for only 20-25 seconds or less. Younger patients (younger than 60 years of age) who do not smoke and have no cardiac or pulmonary disease can usually breath-hold for 30-40 seconds easily. Additional information about a patient's breath-holding capacity can be obtained by observing the respiratory pattern. Patients who pause between breaths and have a respiratory rate of less than 20 breaths per minute can easily suspend breathing for 30-40 seconds. Patients with a respiratory rate exceeding 25 breaths per minute have difficulty holding their breath at all. With oxygen and hyperventilation, however, they may suspend breathing for up to 20 seconds (3)

After the breath-hold duration has been estimated, the slice thickness, number of slices, and number of phase-encoding steps are adjusted to encompass the

aorta and kidneys within this time constraint. It is generally necessary to use the shortest possible TR and TE, but this should not be done at the expense of increasing bandwidth excessively. A typical bandwidth is 32 kHz. In theory, the flip angle should be adjusted to optimize Tl contrast for the gadolinium-enhanced blood. However, because image contrast is only mildly dependent on the flip angle with this technique, it is acceptable to use a flip angle of 45° for most situations (4). The field of view is set to the width of the patient's torso to avoid excessive wraparound artifact. Typically, 28-34 cm is suitable. It may be useful to place cushions on either side of obese patients to make them narrower so that a smaller field of view can be used. To deal with poor breath-holders who require thick, large field-of-view imaging volumes and high resolution, partial Fourier imaging can be used. With partial Fourier imaging, the magnet collects the full echo at each phaseencoding step (instead of a fractional echo) but samples only 60% or 75% of the phase-encoding steps required to fill in k space. The missing portion of k space can then be calculated using a modified homodyne reconstruction or other techniques. This reduces scan time by 25-40% with only a minimal reduction in signal-to-noise ratio, assuming the TE is still much less than T2. For this coronal 3D sequence, arms are elevated over the head (Fig. 3) to minimize aliasing and to ensure a downhill path for the gadolinium, thus maintaining a compact contrast bolus. A 3D volume acquisition before injection of contrast media is performed to practice breath-holding and to ensure that the aorta and kidneys are adequately visualized without excessive aliasing artifact. The precontrast scan may also be used subsequently as a mask for digital image subtraction. Bolus Timing For the next scan, the arterial phase, timing of the gadolinium bolus is critical. A preferred approach to bolus timing is to use a pulse sequence that can detect gadolinium arriving in the arteries and automatically synchronize acquisition of central k-space data with the peak arterial gadolinium concentration (2,5). One software package that can do this is called FASTRACK (General Electric Medical Systems, Milwaukee, WI). An alternative, albeit more cumbersome, approach is to measure the delay time using a test bolus (6,7). Another alternative, although less reliable, approach is to start injecting

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a.

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b.

c.

d.

Pulse sequence positioning. Ca) Axial T2-weighted images encompassing both kidneys; (b) 3D contrast-enhanced MRA extends from the back edge of the kidneys to in front of the abdominal aorta, with the top positioned just below the diaphragm; Cc) 3D phase-contrast MRA is centered high on the renal arteries, as seen on the 3D contrast-enhanced MRA, anticipating that they will move superiorly when the patient is no longer breath-holding; and Cd) 2D cine phase-contrast flow measurements are performed perpendicular to each renal artery approximately .5-1 cm from the aorta.

Figure 2.

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and then start scanning 10 seconds later. If there is no MR-compatible clock conveniently within view, one can take advantage of the natural pace of hyperventilations to time the bolus. Each hyperventilation respiratory cycle is approximately 4-5 seconds: hyperventilate the patient twice (deep breathe in, breathe out, deep breathe in, breathe out); begin injecting; hyperventilate 2lf2 more times (deep breathe in, breathe out, deep breathe in, breathe out, deep breathe in, hold your breath); and then start scanning. If a patient is 70 years of age or

older, has congestive heart failure, or has aortic aneurysmal disease, it will be necessary to have a longer delay, perhaps three or four hyperventilation breaths. It may also be necessary to increase the delay by a few seconds if the intravenous line is in the wrist instead of the antecubital fossa. On the other hand, in young patients (younger than 30 years of age) with hypertension who are otherwise healthy, a shorter delay by one Jess breath may be optimal. One final approach to bolus timing is to repeatedly acquire 3D image data (with

b.

a.

Figure 3.

3D contrast-enhanced MRA. (A) Co(Onal MIP and (B) sagittal MIP.

oversampling of central k space) so rapidly that at least one central k-space dataset will, by chance, line up with the peak arterial phase of the bolus (8). The dose of the gadolinium chelate is an impOltant consideration. It is clifficult to specify the dose by weight alone. This is because dosage determined by weight results in variability in volume of contrast among patients. It is simpler to give every patient the same volume of contrast. A dose of 40 rnL is given for most patients. For unusually large (>200 pounds) or small «100 pounds) patients, it may be necessary to increase or decrease the dose. Contrast medium is injected at a rate of approximately 2 rnL/sec. For a 40-rnL dose of contrast medium, this injection will continue for approximately one half to two thirds of the time period of a 30-40-second breath-hold acquisition. This is sufficient to avoid excessive artifact caused by variation in gadolinium concentration during image data acquisition (9). It is important for a physician to be in the scanner room to coordinate the injection, breath-holding instructions, and initiation of scanning (Fig. 4). After the acquisition in the arterial phase, the venous phase scan is started after three or four additional hyperventilations. It is often useful to perform one more delayed scan to capture images during equilibrium enhancement. Finally, an axial 3D phase-contrast volume is acquired immediately after the dynamic contrast-enhanced acquisition (0). The following parameters are suitable: TR = 25 msec, TE = 8 msec, flip angle = 45°, bandwidth = 16 kHz, field of view = 26 to 32 cm, 28 slices with 2.5-3-mm slice thickness, flow compensation, no

phase wrap, matrix = 256 X 128, right-to-left frequency encoding, 2 NEX. The 3D phase-contrast imaging volume can be prescribed from the 3D contrast-enhanced MRA images. It should be centered on the renal arteries anticipating the upward shifting of kidneys resulting from free breathing (Fig. 2c). The image acquisition time is generally at least 8 minutes. Images should be reconstructed with the phase-difference method illustrating the maximum velocity in all flow directions. It may be useful to reconstruct right-to-left flow images as well to better evaluate the retrocaval course of the right renal artery. Image quality on 3D phase-contrast MRA is directly related to matching the velocity encoding (V en) value to the mean renal artery flow velocity, which may vary from approximately 20-60 cm/sec. In patients 70 years of age or older, patients with aortic aneurysm, patients with a history of severe cardiac disease (ie, congestive heart failure), and patients with serum creatinine >2.0 mg/dL, the Venc should be set at approXimately 30 crn/sec. An even lower v enc should be used if more than one of these factors are present. All other patients should be imaged with a Venc of 40-60 crn/sec. The fastest Venc (60 ern/sec) should be used for children and young, athletic adult patients who are expected to have the fastest flow. An optional sequence for measuring renal artery blood flow is two-dimensional cine phase contrast. Sagittal images are acquired perpendicular to each renal artery close to the aorta (Fig. 2d). The foJlowing imaging parameters can be used: TR = 9.1 msec (as short as possible), TE = 4,3 msec (as short as possible), bandwidth = 32 kHz, field of view = 24 X 18 em, slice

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on a computer workstation that can perform reformations and subvolume maximum intensity projections (MIPs) in multiple planes. First, a coronal MIP is constructed using the entire imaging volume (Fig. 3a). Then, a sagittal subvolume MIP is used to assess the celiac axis and superior mesenteric artery origins (Fig. 3b). Subvolume MIPs are performed encompassing each renal artery in coronal oblique and axial oblique planes to evaluate each renal artery in two perpendicular views. If any stenosis is identified, it is further evaluated on 3D phase contrast for evidence of de phasing. Severe signal dephasing is a highly reliable sign of hemodynamic significance and a good predictor of benefit after revascularization (11). Other indicators of Significant stenosis include poststenotic dilatation, loss of corticomedullary differentiation, and the delayed renal enhancement. The flow curves can also help predict hemodynamic significance (12). Accuracy of renal MRA is now well established (13-24).

Figure 4. Positioning for 3D contrast-enhanced MRA. Arms are placed over the head. The radiologist coordinates instructing the patient in breath-holding, timing the contrast injection, and activating scanning while standing in the scanner room. The bolus is timed for maximum arterial gadolinium concentration to occur during acquisition of central k-space data (the middle of the scan).

thickness = 6 mm, matrix = 256 X 128 with .75-phase field of view. The Venc is set to 80-100 ern/sec to avoid aliasing. Total imaging time is approximately 30 seconds, depending on the heart rate and the number of images per cardiac cycle (one R-to-R interval). The acquisition period is short enough to allow breath-holding. Images are reconstructed shoWing right-to-left flow and analyzed with a flow quantification program to obtain plots for renal artery flow during the cardiac cycle.

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Image Analysis The four pulse sequences are analyzed systematically beginning with the sagittal Tl-weighted pulse sequence to determine kidney length and parenchymal thickness. If the renal axis is unusual, then its length can be evaluated on an equilibrium phase of the 3D dynamic gadolinium-enhanced sequence. Next, the axial T2weighted images are examined for renal or other masses. Renal cysts are noted on the T2-weighted sequence to be simple or complex. The complex cysts and any renal masses seen are further evaluated on the dynamic contrast-enhanced scans for evidence of abnormal enhancement and other neoplastic characteristics. The aorta is evaluated on the 3D dynamic gadolinium-enhanced scan for evidence of atherosclerotic disease. In particular, ulcerated plaques are noted because they can be a source of emboli if disturbed during a revascularization procedure. This is ideally performed

Synopsis Renal artery MRA ideally should be performed on a magnet equipped with high-performance gradients. Careful attention must be paid to contrast injection timing to capture central k-space data during the arterial phase of the bolus. Combining information from multiple pulse sequences allows a complete assessment of the anatomy and physiologic effects of renal artery pathology. Acknowledgments: The author thanks Drs. Tom Chenevert, Stefan Schoenberg, Tom Grist, Jorg Debatin, Neil Rofski, Jeff Weinreb, John Kaufman, Don Mitchell, James Stanley, Mike Knopp, Mike Bock, and George Holland for many helpful discussions; Evelyn Mohalski for artwork; Bob Combs for photography; and Frank Landy and Jennifer Ward for technical assistance.

References 1. Badr KF, Brenner BM. Vascular injury to the kidney. In: Isselbacher KJ, Braunwald E, Wilson JD, Martin JB, Fauci AS, Kasper DL, eds. Harrison's Principles of Internal Medicine, vol 2. New York, NY: McGrawHill, 1994: 1320. 2. Prince MR, Grist TM, Debatin JF. 3D contrast MRA. Berlin: Springer, 1997. 3. Marks B, Mitchell DG, SimelaroJP. Breath-holding in healthy and pulmonary-compromised populations: effects of hyperventilation and oxygen inspiration. J Magn :Reson Imaging 1997; 7:595-597. 4. Prince MR, Narasimham DL, Stanley JC, et al. Breathhold gadolinium-enhanced MR angiography of the abdominal aorta and its major branches. Radiology 1995; 197:785-792. 5. Wilman AH, Debbins JP, Rossman PJ, et al. Fluoroscopically-triggered contrast-enhanced three dimensional MR angiography. In: Proceedings of the 4th annual scientific meeting of the International Society

for Magnetic Resonance in Medicine. New York: International Society for Magnetic Resonance in Medicine, 1996:202. 6. Earls]P, Rofsky NM, Decoroto D, KIinsky GA, Weinreb]C. Hepatic arterial-phase dynamic gadoliniumenhanced MR imaging: optimization with a test examination and a power injector. Radiology 1997; 202:268-273. 7. Grist TM, Sproat IA, Kennel TW, Korosec FR, Swan ]S. MR angiography of the renal arteries during a breath-hold using gadolinium-enhanced 3D TOF with k-space zero-filling and a contrast timing scan. In: Proceedings of the 4th annual scientific meeting of the International Society for Magnetic Resonance in Medicine. New York: International Society for Magnetic Resonance in Medicine, 1996:63. 8. Korosec FR, Frayne R, Grist TM, Mistretta CA. Timeresolved contrast-enhanced 3D MR angiography. Magn Reson Med 1996; 36:345-351. 9. Maki]H, Prince MR, Londy F], Chenevert TL. The effects of time varying intravascular signal intensity and k-space acquisition order on the three-dimensional MR angiography image quality.] Magn Reson Imaging 1996; 6:642-651. 10. Bass ]C, Prince MR, Londy F], Chenevert TL. Effect of gadolinium on phase-contrast MR angiography of the renal arteries. Am] Roentgenol 1997; 168:261266. 11. Prince MR, Schoenberg SO, Ward ]S, Londy F], Wakefield TW, Stanley]C. Hemodynamically significant atherosclerotic renal artery stenoses: MR angiographic features. Radiology 1997; 205:128-136. 12. Schoenberg SO, Knopp MV, Bock M, et al. Renal artery stenosis: grading of hemodynamic changes with cine phase-contrast MR blood flow measurements. Radiology 1997; 203:45-53. 13. Ros PR, Gauger), Stoupis C, et al. Diagnosis of renal artery stenosis: feasibility of combining MR angiography, MR renography, and gadopentetate-based measurements of glomerular filtration rate. A]R Am] Roentgenol 1995; 1651447-1451. 14. Vosshenrich R, Kallerhoff M, Grone H], et al. Detection of renal ischemic lesions using Gd-DTPA-enhanced turbo FLASH MR1: experimental and clinical results. ] Comput Assist Tomogr 1996; 20:236-243. 15. Grist TM. Magnetic resonance angiography of the aorta and renal arteries. Magn Reson Imaging Clin North Am 1993; 1:253-269. 16. de Haan MW, Kouwenhoven M, Thelissen GR, et al. Renovascular disease in patients with hypertension: detection with systolic and diastolic gating in threedimensional, phase-contrast MR angiography. Radiology 1996; 198:449-456 17. Duda SH, Schick F, Teufl F, et al. Phase-contrast MR

angiography for detection of arteriosclerotic renal artery stenosis. Acta Radiol 1997; 38:287-291. 18. Loubeyre P, Trolliet P, Cahen R, Grozel F, Labeeuw M, Minh VA. MR angiography of renal artery stenosis: value of the combination of ttu'ee-dimensional time-of-flight and three-dimensional phase-contrast MR angiography sequences. A]R Am ] Roentgenol 1996; 167:489-494. 19. Steffens ]C, Link ], Grassner ], et al. Contrast-enhanced, K-space-centered, breath-hold MR angiography of the renal arteries and the abdominal aorta. ] Magn Reson Imaging 1997; 7:617-622. 20. Wilman AH, Riederer S], Grimm RC, et al. Multiple breathhold 3D time-of-flight MR angiography of the renal arteries. Magn Reson Med 1996; 35:426-434. 21. Snidow]], Johnson MS, Harris V], et al. Three-dimensional gadolinium-enhanced MR angiography for aortoiliac inflow assessment plus renal artery screening in a single breath hold. Radiology 1996; 198:725-732 22. Wasser MN, Westenberg], van der Hulst VP, et al. Hemodynamic significance of renal artery stenosis: digital subtraction angiography versus systolically gated three-dimensional phase-contrast MR angiography. Radiology 1997; 202:333-338. 23. Hany TF, Debatin ]F, Leung DA, Pfammatter T. Evaluation of the aortoiliac and renal arteries: comparison of breath-hold, contrast-enhanced, three-dimensional MR angiography with conventional catheter angiography. Radiology 1997; 204:357-362 24. De Cobelli F, Vanzulli A, Sironi S, et al. Renal artery stenosis: evaluation with breath-hold, three-dimensional, dynamic, gadolinium-enhanced versus threedimensional, phase-contrast MR angiography. Radiology 1997; 205:689-695.

Sunday, March 21, 1999 3:30 pm-5:30 pm Categorical Course: Surgical Principles Part (1) Venous Access (C103) Moderator: Matthew A. Mauro, MD 3:30 pm Tools of the Trade Warren L. Gamer, MD University of Michigan Ann Arbor, Michigan Instruments The follOWing are the instruments that I would use to do small surgical procedures in the subcutaneous space, such as create a pocket for the implantation of a access device. Develop a tray of instruments that can be used for all planned procedures. Have two of each critical

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