Vascular Surgery - part 2 pps

106 Part I Imaging Techniques ventional spin echo, segmented gradient-echo sequences, segmented turbo-FLASH, 3-D TOP, spiral acquisitions, and echo planar imaging (13,24-26). MRA visualization of proximal coronary vessels correlates well (>95%) with that of conventional angiography (13,27). Visualization of proximal coronary vessels is far superior to distal vessel imaging and severe stenoses are more accurately identified (27). MR imaging can detect a high proportion of severe stenoses but only a low proportion of moderate stenoses. The sensitivity and specificity of coronary MRA for detecting severe stenosis are 85% and 80% respectively. A moderate decrease in blood flow results in a significant decrease of sensitivity to 38% (26). The advantages of coronary arterial imaging with MR have been mostly noted in the visualization of anom- alous coronary vessels (28). Although conventional an- giography can show anomalous vessels, the position of the vessel relative to the aorta and adjacent organs can be difficult to appreciate. MRA can clearly demonstrate the passage of the anomalous vessels anterior or posterior to the aorta and their spatial relationship to nerves, venous and other parenchymal structures, making it a useful preoperative imaging tool (28,29). Overall, coronary MRA for identification of coro- nary stenoses is not generally accepted with the currently existing technology. Further refinement of imaging techniques is necessary before coronary MRA will achieve widespread acceptance. Aortic Arch and Thoracic and Abdominal Aorta MRA can delineate the aortic arch and its branches with a high degree of resolution (Fig. 7.4). Aortic dissections can be reliably diagnosed and classified as either type A (involving the ascending aorta) or type B (distal to the left subclavian artery) by MRA. MRA accurately demon- strates the relationship of branch arteries to true and false lumen anatomy as well as defining the proximal and distal extents of the dissection flap (Fig. 7.5). Non-nephrotoxic contrast agents such as gadolinium (Gd) have enhanced the accuracy of imaging the aortic arch and aortic branch vessels (renal and visceral abdom- inal arteries). 3-D TOP MRA is used for evaluation prior to thoracoabdominal as well as infrarenal aortic, renal, and visceral reconstructions. The use of contrast enhances the resolution of the signals, improving detection of branch disease. A prospective study of 63 patients with suspected visceral aortic disease showed that using breath-hold ultrafast 3-D Gd-enhanced MRA techniques FIGURE 7.4 Dissection and occlu- sion of left common carotid artery seen by arteriography (A) and MRA (B). Anomalous aortic arch (bovine) shown by contrast arteriography (C) and MRA (D). (Reproduced by per- mission from J vase Surg 1997; 25(1): 147.) Chapter 7 Magnetic Resonance Angiography 107 FICURE7.6 (A) MRA shows a left in- ternal iliac artery aneurysm. (B) In- traoperativeangiogram confirms the finding which is then success- fully treated percutaneously by an endovascular approach that included coiling of the aneurysm and covering the inflow to the aneurysm using a commercially available stent graft. (C) The sizing of the stent graft was designed from the MRA images. combined with 2-D TOP, MR could accurately identify and grade all (n = 51) renal, celiac, superior mesenteric, and inferior mesenteric artery stenosis or occlusions. The combined MRA imaging techniques have 100% sensitivi- ty and specificity when compared with conventional an- giography (30). MRA correctly predicts cross-clamping site in 87%, proximal anastomotic site in 95%, need for renal revascularization in 91 %, and the use of bifurcated graft in 75% of abdominal aortic aneurysm patients. MRA can also be successfully used as the sole imaging modality for aortic or iliac endoprosthetic devices (Figs. 7.6 and 7.7). In a prospective study of 96 consecutive pa- 108 Part I Imaging Techniques FIGURE 7.7 (A-C) infrarenal abdominal aneurysm treated based on preoperative MRA. (D) Intraoperative angiograms confirming the MRA findings. (E) Completion arteriogram after successful endografting. tients, data were collected using Gd-enhanced MRA pre- operatively in place of conventional imaging for patients with renal insufficiency or history of contrast allergy (31). A total of 14 patients had their endograft designed solely on Gd-enhanced MRA. The frequency of intraoperative access failure, the need for proximal or distal extensions, the rate of conversion to open procedures, as well the inci- dence of endoleaks were equal in both the MRA-designed and control groups. Renal Artery stenosis MRA has been advocated for evaluation of renal arteries for the past decade. Initial techniques were limited due to motion artifact and limited spatial resolution. Earlier TOP MRA, when compared with conventional angiogra- phy, had 91% sensitivity, with a 94% negative predictive value. Overall diagnostic accuracy of these techniques was good(81%)(32); however, the detection of accessory renal artery was poor (14). Images and diagnostic accura- cy have improved greatly with the use of Gd-enhanced MRA (Fig. 7.8). Sensitivities of 50% to 70% have been reported in the identification of accessory renal arteries (33). Use of breath-hold ultra-fast 3-D Gd-enhanced tech- niques has increased diagnostic yield of accessory renal arteries to between 89% and 100% (34). This is primarily due to increased spatial resolution and larger field of view with these recent techniques. Re formating the 3-D volume acquisition of the vascular anatomy can provide useful preoperative information about aberrant arteries, degree of stenosis, aneurysms, and associated aortic dis- sections. In contrast, conventional angiography relies on oblique imaging planes to delineate a profile of the steno- sis, making ostial lesions more difficult to be accurately studied, particularly in the setting where the total amount of potentially nephrotoxic contrast volume is restricted. Contrast-enhanced MRA techniques are not associated with contrast nephropathy and can be used safely in patients with renal insufficiency. Peripheral Circulation Lack of filling distal to serial stenoses or occlusions and the presence of bony cortex hinder the ability of conven- tional angiography to detect small and diseased distal runoff vessels. MRA avoids the complications of arterial puncture, eliminates the risk of contrast-induced renal failure, and has been shown to have a greater sensitivity than contrast angiography for identifying distal runoff vessels in patients with severe peripheral arterial occlusive Chapter? Magnetic Resonance Angiography 109 FIGURE 7.8 (A) MRA demonstrating right renal stenosis. (B) cross-sectional view confirms the renal stenosis. (C) Arrows demonstrate celiac stenosis, SMA stenosis, and an aortic ulcer visualized by MRA. (D) MRA demonstrating normal aortoiliac arterial anatomy with normal visceral and renal branches. (E) Superimposed venous, arterial and parenchymal imaging information acquired by MRl/MRA/MRV. disease (35). Recent refinements of magnetic resonance angiography have replaced conventional angiography in some centers. In studies of the aorta, iliac, and femoral inflow, MRA is highly concordant with conventional contrast angiog- raphy. MRA has a sensitivity of 99.6%, a specificity of 100 %, a positive predictive value of 100%, and a negative predictive value of 98.5% in detecting patent segments, occluded segments, and hemodynamically significant stenoses of aortic, pelvic, and proximal femoral inflow vessels (4). The degree of arterial stenosis is measured with high accuracy by MRA compared with conventional angiography (36). Furthermore, MRA provides better in- formation about spatial relationship of blood flow and plaque morphology than conventional angiography (15). This is mostly the result of sophisticated software process- ing of MRI/MRA data, providing enhanced views that may include 3-D reconstructions in multiple longitudinal projections and rotational views in addition to the 2-D cross-sectional and axial views. MRA can be used as the sole preoperative imaging modality for successful open vascular or endovascular in- terventions (Figs. 7.9 to 7.11). In one such study, outpatient MRA of the juxtarenal aorta imaged 80 consecutive pa- tients with ischemic rest pain or tissue loss through the foot (4). Intraoperative pressure measurements of proximal vessels and post-bypass arteriography were performed. Graft patency and limb salvage was evaluated using life table analysis. All patients underwent reconstructive pro- cedures based on MRA alone (11 aortobifemoral and 67 infrainguinal procedures). The intraoperative findings and intraoperative completion arteriography confirmed the ac- curacy of inflow and outflow imaging by preoperative MRA. The limb salvage rate was 84 % with all -month pa- tency rate of 78% for infrainguinal reconstruction based on MRA alone, and was no different from that of a control group whose operations were planned with conventional contrast angiography (37). MRA can detect angiographically occult distal runoff vessels. In studies of lower extremity ischemia patients in which MRA and conventional angiography were com- pared, the detection of distal runoff vessels was superior with MRA. Operative exploration and intraoperative an- giograms confirmed the preoperative evaluation by MRA (4). A subsequent investigation of the adequacy of these occult runoff vessels for use in limb salvage bypass proce- dures showed no significant differences in primary graft patency rate between bypasses planned using convention- al angiography to those done to angiographically occult runoff vessels detected only by MRA (38). 110 Part I Imaging Techniques FIGURE 7.9 (A) MRA showing normal femoral arterial segments. (B) MRA demonstrating a short-segment stenosis and a more distal segmental occlusion of right superficial femoral artery (SFA). The left SFA shows mild diffuse dis- ease. FIGURE 7.10 The use of bolus chase techniques can facilitate rapid imaging of the distal runoff where the (A) popliteal, (B) inf rapopliteal, and (C) foot vessels are accurately visualized. MRA can enhance the clinical accuracy when per- formed in addition to conventional angiography. In a blinded prospective study in six USA hospitals, MRA was compared to contrast angiography to evaluate severe lower limb atherosclerotic occlusive disease in candidates for percutaneous or surgical intervention (39). Sensitivity in distinguishing patent segments from occluded seg- ments was 83% with contrast angiography and 85% in MRA. However, the inclusion of MRA preoperative plan- ning resulted in a change of treatment plan for 13 % of pa- tients and provided superior overall diagnostic accuracy (86%). The improved accuracy related mostly to the in- creased sensitivity of MRA in identifying patent runoff vessels (48%) when compared with conventional angio- graphy (24 %) (40). MRA is most useful in the detection of patent runoff vessels of the distal segments. The detection of patent runoff vessels by MR which are not identified by conventional angiography can lead to improved limb salvage in 13% to 22% of cases (39-40). A meta-analysis of 34 studies indicated that MRA is highly accurate for assessment of lower extremity arteries (41). Techniques using 3-D Gd-enhanced MRA appear to be superior to 2-D methods and to contrast angiography. The superiority of MR techniques over traditional imag- ing techniques is due to characteristics of blood flow in diseased vessels and the sensitivity of MR for detection of slow flow (2cm/s). Images from contrast angiography may not show distal vessels owing to multiple dilutions Chapter 7 Magnetic Resonance Angiography 111 FIGURE 7.11 (A) Distal runoff as vi- sualized by conventional angiog- raphy demonstrating a diseased posteriortibial artery. (B) MRA re- veals that the anterior tibial and peroneal arteries are also patent. (C) intraoperativearteriogram after bypass performed to an an- giographically occult dorsalis pedis artery visualized preopera- tively by MRA, but not by preoper- ative contrast arteriography. (Reproduced by permission from J Vase Surg 1996; 23:483-489.) and reconstitution of the contrast material as the bolus 1. passes distally (Fig. 7.11). MRA can also be used as a sole preoperative imaging modality prior to endovascular procedures (42). A total of 119 consecutive patients underwent MRA for symptomatic leg ischemia. Intraoperative road-map arteriography was performed in patients that underwent 2. endovascular procedures and compared to preoperative MRA images. There were no false positive or negative stud- ies with MRA. A reduction in cost was also noted owing to the elimination of preoperative diagnostic arteriography. New Developments Research in MR techniques continues to improve success- ful clinical applications. Bolus chase techniques involve the movement of the scanner table in a stepwise manner to allow sequential imaging of a bolus during arterial transit 4. (43). Using conventional angiograms as a reference standard, manual bolus chase has been demonstrated to have high sensitivity (93-94%) and specificity (97-98%) (43) for stenosis >50%. 5. Problems There are several well-recognized limitations of the use of MRA: Loss of signal due to presence of metallic objects. Presence of joint prosthesis and surgical clips cause large signal dropout artifacts. Segmental occlusion may be misdiagnosed and correlation with plain films may be necessary to identify metallic clips from previ- ous procedures. MR incompatibility—risk for device displacement. Some recent endovascular devices that use stainless steel in covered stents for aortic aneurysm treatment represent a contraindication for the use of MR imag- ing. MR is also contraindicated in patients with pace- makers or retinal or intracranial metallic objects. Image degradation of horizontal vasculature. Thick slices in coronal reconstructions of 2-D images (that are obtained perpendicular to the long axis of the body) result in a string of diamond appearance of horizontal vessels. Thin slices and better image resolution reduce these artifacts. Lengthy period of data acquisition: Improvements in real-time MRA and bolus chase techniques decrease the length of time required for peripheral MRA studies. Existing MRA techniques have a number of flow- related artifacts, due to signal loss or intravoxel de- phasing, resulting in overestimation of the degree and length of arterial stenosis or signal dropout artifact. Pulsatile arterial flow can also result in ghosting artifacts in peripheral arterial evaluation. Contrast agents reduce these effects. 112 Parti Imaging Techniques Conclusion The time-honored method of contrast angiography is as- sociated with inherent risks and limitations. Develop- ments in noninvasive modalities offer potential benefits in diagnostic accuracy and reduction of costs and morbidity. MRA represents an evolving technology that offers promise as a noninvasive adjunct for vascular imaging. In- dividual centers must validate their MR data and interpre- tation against conventional arteriography techniques. The preoperative workup and eventual therapeutic plan can in many cases be successfully accomplished with the sole or adjunctive use of MR imaging in the treatment of vascular patients. References 1. Hessel SJ, Adams DF, Abrams HL. Complications of an- giography. Radiology 1981; 138:273. 2. Sjejado WJ, Toniolo G. 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This page intentionally left blank PART II Basic Cardiovascular Problems [...]... flow (mL/min) Exercise Rest Exercise After Graft Before Graft After Graft Before Graft 25 0 99 476 98 126 6 95 22 82 91 25 0 80 25 0 24 8 126 6 121 1 25 0 60 18 97 426 32 250 24 6 — 22 8 After Graft Before Graft After Graft 311 75* 645 48 730 42* 25 0 23 5* 645 554* 84 87 25 0 60 157 75 426 32 369 41 426 1155 25 0 23 3 426 545 — 1071 — 76 — 176 Aortic pressure =100 mmHg, graft resistance = 0.004 mmHg/mL/min * Pressure... 4.1(7 .2) 8.6(15.0) 3000 1.1 (2. 7) 4.5(11.1) 8.3 (20 .6) 17.1( 42. 8) 150 0.8(0.9) 1.7(1.8) 4 .2( 4.3) 13 .2( 13.7) 300 1.7(1.8) 3.4(3.7 8.4(9.0) 26 .4 (28 .4) 500 2. 8(3.1) 5.7(6.4) 13.9(15.7) 44.0(49.5) 100 2. 6 (2. 6) 6.9(7.0) 26 .0 (26 .3) 150 3.9(4.0) 10.4(10.5) 39.0(39.7) 20 0 5 .2( 5.4) 13.8(14 .2) 52. 0(53.3) Femorotibial length -8 0 cm' 6-4 5-3 4 -2 50 1.3(1.3) 3.5(3.5) 13.0(13.1) Values are viscous only, equation 8 .2; ... reconstructive vascular surgery Surgery 1969; 66: 994 87 Malan E, Longo T Principles of qualitative hemodynamics in vascular surgery In: Haimovici H Vascular surgery, 2nd edn Norwalk, CT: Appleton-CenturyCrofts, 1984 8 8 Crawshaw HM, Quist WC, et al Flow disturbance at the distal end-to-side anastomosis Effect of patency of the proximal outflow segment and angle of anastomosis Arch Surg 1980; 115: 128 0 89 Klimach... carotid stenosis with continuous-wave (c-w) Doppler ultrasound Stroke 1979:10: 326 23 Russell JB, Miles RD, et al Effect of arterial stenosis on Doppler frequency spectrum Proc 32nd Annu Conf Eng MedBioll979 ;21 :45 24 Keitzer WE Fry WJ, et al Hemodynamic mechanism for pulse changes seen in occlusive vascular disease Surgery 1965; 57:163 25 Strandness DE Jr., Bell JW Peripheral vascular disease Diagnosis and... 4 :22 9 60 Angelides NS, Nicolaides AN, et al The mechanism of calf claudication: studies of simultaneous clearance of 99-Tc from the calf and thigh Br J Surg 1978; 65: 20 4 61 Angelides NS, Nicolaides AN Simultaneous isotope clearance from the muscles of the calf and thigh Br J Surg 1980; 67: 22 0 62 Allwood MJ Redistribution of blood flow in limbs with obstruction of a main artery Clin Sci 19 62; 22 : 27 9... 21 3.7 25 1.6 +84.5 +68.9 +53.4 +37.9 +6.8 -2 4 .2 -8 6.3 -1 48.4 65.7 71.4 77.1 82. 7 94.1 105.5 128 .2 150.9 2. 63 1.43 1.03 0.83 0.63 0.53 0.43 0.38 4.38 2. 38 1.71 1.38 1.05 0.88 0.71 0.63 Based on diagram in Figure 8.11, assuming constant resistances (mmHg/mL/min): true peripheral = 0.6, collateral = 0.35; thigh muscle = 0.475; profundafemoris = 0.017 indicates antegrade; - indicates retrograde collateral flow... resistance of the peripheral vascular bed and the resistance of the collateral arteries At low infusion rates, the pressure developed in the graft does not exceed that at Chapter 8 Hemodynamics of Vascular Disease: Applications to Diagnosis and Treatment 129 FIGURE 8. 12 Resting flow through a 40-cm femoral-(blind) popliteal bypass graft, a 60-cm femorotibial graft, and a 40-cm proximal, 20 -cm distal, sequential... stenosis (%) = (l - ^v0/vs] x 100 122 Part II Basic Cardiovascular Problems 8.7) (24 ) This tends to change the contour of the pulse distal to the stenosis, making it more rounded than that above the stenosis The upslope becomes less steep, the peak becomes more rounded, and the downslope bows away from the baseline (25 ,26 ) Reversed flow components are less evident and often disappear entirely (27 ) Fluctuations... (Suppl): 1 26 Darling RC, Raines JK, et al Quantitative segmental pulse and volume recorder: a clinical tool Surgery 1973; 72: 873 27 Jager KA, Phillips DJ, et al Noninvasive mapping of lower limb arterial lesions Ultrasound Med Biol 1985; 1 1:515 28 Woodcock JE, Gosling RG, Fitzgerald DE A new noninvasive technique for assessment of superficial femoral artery obstruction Br J Surg 19 72: 59 :22 6 29 Johnston... serve to control vascular tone and secrete matrix substances such Theories of Atherosclerosis Monoclonal Hypothesis This theory is borne from the observation by Benditt and Benditt that individual cells from plaques of heterozygote females for the X-linked glucose-6-phosphate dehydrogenase (G-6PD) gene usually only exhibit one G-6PD isotype (Fig 9.7) (22 ) This suggests that the cells of a particular plaque . 1970; 26 :22 1 -2 30. 24 . Meyer CH, Hu BS, et al. Fast spiral coronary artery imaging. Magn Reson Med 19 92; 28 :20 2- 2 13. 25 . Wang Y, Winchester PA, et al. Contrast-enhanced peri- pheral . sex, anatomic varia- tion, and left ventricular hypertrophy or dilation. Circu- lation 19 92; 86 :23 2- 2 46. 22 . Wang Y, Riederer SJ, Ehman RL. Respiratory motion of the heart: kinetics . the evaluation of lower extremity dis- ease: a meta-analysis. JAMA 20 01; 28 5:133 8-1 345. 42. Levy MM, Baum RA, Carpenter JP. Endovascular surgery based solely on noninvasive preprocedural imaging