Michael A. Zatina

Magnetic resonance imaging of acute occlusive intestinal ischemia

The mortality of acute superior mesenteric artery occlusion and mesenteric infarction remains high, in part because of the failure to identify the patients with the disorder. A reliable noninvasive diagnostic study may facilitate earlier definitive diagnosis and therapy. Proton magnetic resonance imaging may offer a noninvasive diagnostic alternative. We examined this possibility by using an experimental rabbit model of acute superior mesenteric artery occlusion in this study. Animals were scanned 1 hour after the ischemic insult. Relative intestinal wall signal intensity was increased in experimental (ischemic) animals when compared to sham operated controls on T2-weighted (4.35 +/- 0.5 vs 2.57 +/- 0.31, p less than 0.02) and proton spin density-weighted images (2.1 +/- 0.2 vs 1.4 +/- 0.21, p less than 0.05). Significant increases in image intensity were found on T2-weighted and proton spin density images when compared to control animals. Differences between groups could be further highlighted after the administration of a paramagnetic contrast agent gadolinium diethyltriamine pentacetic acid on T1-weighted images. The data from this preliminary study demonstrate that proton magnetic resonance imaging may be used to discriminate between ischemic and nonischemic small intestine. This noninvasive tool may someday become clinically useful to enhance our diagnostic capabilities when a diagnosis of acute superior mesenteric artery occlusion is being entertained.

In-vitro assessment of the behavior of magnetic resonance angiography in the presence of constrictions.

Sebok NK, Sebok DA, Wilkerson D, Mezrich RS, Zatina M. In-vitro assessment of the behavior of magnetic resonance angiography in the presence of constrictions. Invest Radiol 1993; 28:604-610. RATIONALE AND OBJECTIVES.Timc-of-flight magnetic resonance angiography (TOF/MRA) is increasingly used to assess the nature and severity of stenotic blood vessels. Flow artifacts associated with high flows and/or narrow constrictions may confuse the interpretation of these studies. Accurate TOF/MRA evaluations demand an understanding of the nature of these flow effects. METHODS.A two-dimensional TOF pulse sequence was used to acquire images of five smoothly constricted phantoms at various flows. Analysis included, assessment of phantom appearance and quantification of apparent constriction diameter and signal variations. RESULTS.Most notable flow phenomena were a cone-shaped region of high signal, a region of signal void, and signal preservation along the wall. When visible, constriction diameter was accurately measured. CONCLUSIONS.The behavior observed in TOF/MRA images can be understood by considering the contributing mechanisms of phase dispersion, turbulence, poststenotic flow eddies, flow reversal, and flow separation.

Catheter occlusion of a mycotic renal artery aneurysm with cure of associated renovascular hypertension

We describe successful transcatheter occlusion of a mycotic renal artery aneurysm. The patients hypertension resolved following occlusion of the aneurysm and infarction of the renal parenchyma in the distribution of the aneurysmal vessel.

Increased oxygen extraction as adaptation to acute renal ischemia.

Acute renal ischemia is an infrequently encountered clinical entity with occasionally devastating consequences. The renal compensation to acute ischemia is unknown and is the purpose of this report. Eight pigs were anesthetized and ventilated. Left atrial, aortic, CVP, left renal venous, and ureteral catheters were inserted. Renal blood flow (RBF) reduction was accomplished by the graded constriction of the left renal artery using a balloon occluder. Renal oxygen extraction ratio (RER, %), renal oxygen delivery (RO2D, cc/min per 100 gm), renal oxygen consumption (RVO2, cc/min/100 gm), creatinine clearance (CrCl, ml/min), and renal lactate production (delta [L], mg/min per hgm) were measured at baseline and following sequential 90-minute intervals of moderate and then severe left renal flow reduction. Significant increases in renal oxygen extraction were observed when RBF was severely limited (.30 +/- .05 vs .64 +/- .06, p < .01). CrCl decreased precipitously (16.5 +/- 4.6 vs 0.2 +/- 0.07, p < .05). Lactate production by the ischemic organs correlated with blood flow reduction (r = .546, p = .0034). In severe ischemia, healthy kidneys increase oxygen extraction to preserve oxygen consumption.

Interleaved magnetic resonance and ultrasound by electronic synchronization.

Increasing attention has been directed toward using magnetic resonance imaging (MRI) to assess blood flow velocity. Complete acceptance of this application requires validation of MRI-derived flow measurements against an accepted flow measurement technique such as Doppler ultrasound in an in vivo situation. To provide an accurate correlation in the presence of rapid changes in blood flow, the MR acquisition should be made nearly simultaneously with the ultrasonic measurements. Unfortunately, standard ultrasound equipment generates radio frequency signal which interferes with MRI. Near-simultaneous acquisition of MR data and ultrasonic blood flow data should be possible if the two measurements are properly synchronized. In the technique presented, ultrasound is made to peacefully coexist with MRI by gating the ultrasound so that it is disabled during the time of MR data acquisition. Phantom and animal experiments confirm the use of this procedure. Although we did not specifically test new fast-scan MR techniques, our technique is completely general and should work equally well with spin-echo as well as newer fast scanning MRI techniques.

Can intraoperative prebypass arteriography substitute for the preoperative arteriogram

Preoperative contrast arteriography is presently considered a requirement when planning infrainguinal arterial reconstructions in patients with limb-threatening ischemia. We reviewed the clinical data from 22 infrainguinal bypasses done in 20 patients to see if appropriate decisions concerning operability and the nature of the operation could be made from the physical examination and noninvasive data supplemented by a limited intraoperative on-table prebypass arteriogram. The presence of a normal femoral pulse with either a normal thigh pulse volume tracing or a normal high-thigh index assured adequate inflow to the groin level. The presence of an arterial Doppler signal at the ankle level, heard with a hand held Doppler, confirmed the presence of patent outflow vessels for the distal anastomosis. The exact site of the distal anastomosis could be determined with the on-table prebypass arteriogram. In patients with limb-threatening ischemia due to occlusive disease limited to the infrainguinal arterial tree, an appropriate operative bypass may be performed without the aid of the preoperative arteriogram.

Infrarenal rupture of an abdominal aortic aneurysm, previously repaired using an endoaneurysmorrhaphy technique

Unusual as well as well-known complications can occur after aortic reconstruction. In an effort to heighten awareness of these possibilities, a case is presented of a 71-year-old male who was brought to the emergency department with severe back pain of 2 days duration and hypotension. He had undergone repair of an infrarenal abdominal aortic aneurysm 6 years earlier. An emergency computed tomography scan demonstrated a 10-cm abdominal aortic aneurysm extending from just above the celiac axis, through the aortic bifurcation, with retroperitoneal and intraperitoneal hematoma. He was found at operation to have extension of his aneurysmal disease proximally, with complete separation of the proximal suture line, and rupture of the distal aortic wall. Since the aneurysm had been closed around the graft at the time of the original operation, his aneurysm had essentially been restored, and the diseased wall was again exposed to the tensile stresses from the pulsatile column of blood. Emergency repair was successful, despite postoperative complications including myocardial infarction, and later rupture of an iliac artery aneurysm. Patients presenting with signs and symptoms consistent with a ruptured abdominal aortic aneurysm after previous repair should be addressed aggressively with computed tomography if it is immediately available and the diagnosis is in doubt. The patient should then undergo an immediate operation. Such recurrence, although rare, must always be considered a possibility. Similar scenarios may be encountered secondary to endoleaks occurring after endoluminal aortic repairs.