Magnetic resonance angiography of abdominal vessels: early experience using the three-dimensional phase-contrast technique

MR Angiography of in Situ and Transplanted Renal Arteries

Three-dimensional (3D) time-of-flight (TOF) MR angiography (MRA) was performed in 34 patients with suspected renal artery disease. In situ (i.e., nontransplanted) renal arteries were studied with MRA in 14 patients. Of these, 12 had conventional angiography for comparison. Twenty-four MRAs of transplanted renal arteries were obtained in 20 patients; 8 of these had angiography as well. Significant stenoses of in situ renal arteries were diagnosed with a sensitivity of 100% and a specificity of 95%. The stenoses were all proximal; 3D TOF MRA proved inadequate for depiction of peripheral renal arteries. MRA and angiography showed good agreement between findings in 7 of 8 patients with renal transplants. In one patient with a renal transplant, MRA showed a significant stenosis of the arterial anastomosis which appeared completely normal at i.a. DSA, indicating that findings at MRA still need to be confirmed by more established alternative methods.

MR angiography after renal revascularization: spectrum of expected anatomic results and postintervention complications

The use of magnetic resonance (MR) angiography in screening for renal artery stenosis has been extensively evaluated. However, the MR angiographic findings after renal artery revascularization are not as well characterized. The renal artery and parenchyma can be evaluated after revascularization with a comprehensive MR imaging protocol that includes T1- and T2-weighted spin-echo sequences, three-dimensional (3D) gadolinium-enhanced MR angiography, and 3D phase-contrast MR angiography. Because surgical techniques for revascularization vary, knowledge of the surgical procedure is necessary to ensure inclusion of the pertinent anatomy at 3D gadolinium-enhanced MR angiography and to define appropriate 3D phase-contrast MR angiography volumes. The 3D gadolinium-enhanced MR angiography volume can be manipulated to view relevant vascular anatomy at the optimal obliquity and section thickness. This protocol allows robust, noninvasive evaluation of the expected arterial anatomy after revascularization, including renal artery endarterectomy, aortorenal bypass grafts, and extraanatomic reconstructions. In cases of suspected postrevascularization complications, gadolinium-enhanced MR angiography is useful because of its lack of nephrotoxicity and radiation exposure. Immediate complications of renal revascularization include renal artery thrombosis, renal infarction, and perinephric hemorrhage. Long-term complications include aneurysms of bypass grafts and recurrent stenosis of the renal artery.

Cerebrovascular biomodelling: a technical note

BACKGROUND

Recently computed tomographic angiography (CTA) and MR angiography (MRA) have been used to image cerebrovascular structures. Although CTA and MRA are accurate and sensitive imaging modalities, limitations have been identified in relation to image interpretation. Stereolithographic (SL) biomodelling is a new technology that allows three-dimensional (3D) CT and MR data to be used to accurately manufacture solid plastic replicas of anatomical structures. A prospective trial of SL biomodelling in cerebrovascular surgery has been performed to investigate the feasibility and clinical utility of this new display medium.

METHODS

Fifteen patients with cerebral aneurysms and 1 patient with a cerebral arteriovenous malformation (AVM) were selected. 3D CT and/or MR angiograms were acquired and 19 solid anatomical biomodels manufactured using the rapid prototyping technology of stereolithography. The biomodels were used for patient education, diagnosis, operative planning and surgical navigation.

RESULTS

The biomodels replicated the CTA and MRA source data. The accuracy of one biomodel was verified by comparison with a post mortem specimen, which corresponded exactly in the x and y planes but differed by 2 mm in the z plane. The ability to closely study an overview of complex cerebrovascular anatomy from any perspective on a solid biomodel was reported to enhance the surgeon's understanding, particularly when conventional images were equivocal. Cerebrovascular biomodels were found to be useful when positioning the patient's head for surgery, for selecting the best aneurysm clip and for the simulation of clipping. Patient informed consent was anecdotally improved. Disadvantages of the technology were the cost and manufacturing time.

CONCLUSIONS

Cerebrovascular biomodelling may have utility in complex cases or when the standard imaging is felt to be equivocal.

Gd-enhanced 3D phase-contrast MR angiography and dynamic perfusion imaging in the diagnosis of renal artery stenosis

The objective of this study was to investigate the role of contrast enhancement using a three-dimensional (3D) phase-contrast (PC) magnetic resonance (MR) sequence (3D PC-MRA) and to assess the value of a dynamic MR perfusion study of the kidneys to determine the hemodynamic relevance of unilateral renal artery stenosis (RAS). Seventeen patients with unilateral RAS were examined on a standard 1.0 T imaging system using a phase shift and magnitude sensitive 3D PC sequence (TR=160 ms, TE=9 ms, venc. 30 cm/s). Following the initial pre-contrast 3D PC-MRA a dynamic first pass perfusion study was performed using a Turbo-FLASH 2D sequence (TR=4.5 ms, TE=2.2 ms, TI=400 ms) after bolus injection of 0.15 mmol gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA)/kg body weight. The 3D PC-MRA was then repeated during infusion of 0.15 mmol Gd-DTPA/kg body weight. Evaluation by three independent readers was based on maximum intensity projection images. Source images were rendered on request. Signal intensity (SI) over time curves of the renal cortex were obtained from the dynamic perfusion study and analyzed for maximum signal enhancement as well as temporal relationship to the aortic SI curve. Results from 3D PC-MRA revealed a sensitivity (pre-/post-contrast) of 100%/89%, specificity of 76%/63%, positive predictive value of 80%/69 %, negative predictive value of 90%/78%, and accuracy of 85%/75% (p=0.07). Interobserver agreement was kappa=0.61/kappa=0.47 (pre/post Gd-DTPA), respectively. Increased signal-to-noise was present in all segments of the renal arteries post contrast (p=0.0003). This came along with image degradation due to aliasing and elevated SI of venous flow that partially obscured the renal arteries. Dynamic SI curves showed a significantly decreased maximum SI in RAS (p=0.01-0.001). A temporal delay of cortical signal intensity enhancement could not be confirmed in this setting. Gd-enhanced 3D PC-MRA did not yield a superior diagnostic value in the diagnosis of RAS compared to pre-contrast measurements. Dynamic perfusion imaging of the kidneys, in combination with 3D PC-MRA, can contribute additional information in suspected unilateral RAS.

Renal anatomic changes on magnetic resonance imaging and gadolinium-enhanced magnetic resonance angiography after renal revascularization. Original investigation

RATIONALE AND OBJECTIVES

The anatomic and hemodynamic renal changes after renal arterial revascularization (RAR) were investigated.

METHODS

Thirty-seven kidneys and 40 renal arteries were evaluated in 20 patients by using magnetic resonance imaging/magnetic resonance angiography (MRI/MRA) to assess pre- and post-RAR renal length and mass, parenchymal thickness, renal enhancement, renal artery caliber, poststenotic dilation, and signal dephasing on 3D phase contrast (PC). The kidneys and renal arteries were segregated into three groups. Group 1 included 16 patients who benefited from RAR (defined as clinical improvement based on decreased serum creatinine or fewer number of antihypertensive medications) in whom 26 renal arteries in 25 kidneys were studied. Intervention included renal artery endarterectomy (n = 20); aortorenal bypass (n = 3); renal artery reimplantation (n = 3); and percutaneous transluminal angioplasty (PTA; n = 1). A total of 27 interventions was performed, as PTA failed for one patient who subsequently underwent aortorenal bypass before reimaging. Group 2 included four patients who did not clinically benefit. A total of eight revascularized arteries were studied in seven kidneys. In group 3, six renal arteries in five kidneys from groups 1 and 2 without RAS/RAR were analyzed as an internal control.

RESULTS

Technical success (defined as increased vessel caliber after intervention) was achieved in 33 of the 34 revascularized arteries. A statistically significant increase in renal length occurred regardless of clinical outcome (pre-RAR, 9.5 cm; post-RAR, 10.5 cm; P < 0.0001). Parenchymal thickness and renal mass, however, improved only in patients who benefited clinically from RAR. Parenchymal enhancement was unchanged in any of the groups studied. No significant morphologic changes were detected in the control group.

CONCLUSIONS

Magnetic resonance imaging and Gd-MRA detect anatomic and hemodynamic changes that occur with renal revascularization.

Variations in blood flow waveforms in stenotic renal arteries by 2D phase-contrast cine MRI

Waveform variations in blood flow measurements through stenotic renal arteries have been reported already with echo Doppler studies. We studied these variations with MRI in 14 patients (mean age, 60 years) with suspected renal arterial stenosis (24 patent arteries, four occluded). Flow measurements were successful in 15 arteries and unsuccessful in nine, due to practical limitations. Seven healthy younger volunteers (mean age, 28 years) and five healthy older volunteers (mean age, 58 years) were recruited for comparison purposes. In patients, the severity of stenoses was also assessed by digital subtraction angiography and intraarterially measured transstenotic pressure drops. We found flow patterns to be statistically significantly (P < 0.01) age-related. Younger healthy subjects showed shorter wave duration, higher diastolic flow, and total blood flow per minute. Also, with increasing stenosis severity, the systolic wave became more damped and the systolic wave duration became statistically significantly (P = .03) longer.

Use of three-dimensional segmented FLASH sequence with magnetization transfer contrast to improve Gd-DTPA-enhanced intrahepatic MR portography

Twenty healthy volunteers underwent gadopentetate dimeglumine (gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA))-enhanced MR angiography (MRA) using three-dimensional-segmented fast low angle shot images (FLASH) with magnetization transfer contrast (MTC) pulses. MRA was obtained at 75 seconds (early phase) and 135 seconds (late phase) after bolus injection of Gd-DTPA (MTC+ group) during one period of breath-holding. Within 1 week. MRA without MTC was performed under the same scanning conditions. Visualization of intrahepatic portal branches with these methods was compared in both phases. Portal vein-liver contrast-to-noise ratios were significantly higher in the MTC+ group in both phases. For third- and fourth-order portal branches, visualization was significantly better in the MTC+ group in both phases. Use of three-dimensional-segmented FLASH shortened acquisition time and facilitated imaging during breath-holding and also reduced whole-body average specific absorption rate values. Visualization of intrahepatic portal vein branches was improved by MTC pulses, and effective imaging time was prolonged.

Two-dimensional time-of-flight MR venography: assessment with detection of chronic deep venous thrombosis in combination with magnetization transfer contrast

PURPOSE

The purpose of this study was to determine the optimum imaging parameters for 2D-TOF MR venography (MRV) of the pelvis and suprapopliteal deep venous system in combination with off-resonance magnetization transfer constant (MTC) and to evaluate the use of MRV in patients suspected of having chronic deep venous thrombosis (DVT) by comparing its accuracy with that of conventional venography (CV).

METHOD

MRV was performed in 10 normal subjects to determine the optimum imaging parameters. Systematically varied scan parameters included flip angles ranging from 30 to 120 degrees by 10 degrees increments. Best TR was calculated by using the theoretical relation between the optimal TR and flow velocity. We then evaluated the use of MRV in 26 patients (total 52 deep venous systems) suspected of having chronic DVT by comparing its accuracy in evaluating the intrapelvic and suprapopliteal deep venous system with that of CV.

RESULTS

The optimal imaging parameters for intrapelvic and suprapopliteal venous system MRV were found to be a flip angle of 60 degrees and a pulse sequence of 35/6.9 (TR ms/TE ms) combined with an off-resonance MTC technique. All the main veins of the intrapelvic and suprapopliteal venous systems were clearly demonstrated in each subject. The sensitivity of MRV was 100% and its specificity was 100% for diagnosis of pelvic venous stenosis. The collateral vessels were demonstrated with MRV in all patients.

CONCLUSION

2D-TOF off-resonance MTC MRV of the pelvis and the suprapopliteal lower extremities is suitable for evaluation of intrapelvic branches and suprapopliteal deep venous system and chronic DVT.

Use of magnetization transfer contrast to improve single breath-hold three-dimensional MR-portography with bolus injection of gadopentetate dimeglumine: a preliminary report

We investigated whether better visualization of the intrahepatic portal system could be obtained by adding magnetization transfer contrast (MTC) to three-dimensional MR portography (3D-MRP). In 15 healthy volunteers, 3D fast low-angle shot images combined with an off-resonance pulse were obtained after bolus injection of gadopentetate dimeglumine during one period of breath-holding. Within 1 week after injection, MR images without MTC were obtained using the same scanning condition. MRPs with and without the MTC pulse were compared quantitatively and qualitatively. The contrast-to-noise ratio of the portal vein-liver was significantly higher in the group imaged with the addition of MTC. Qualitatively, the use of MTC improved peripheral portal branch depiction on maximum-intensity projection images. Even when the MTC pulse was added to the conventional method of 3D data collection, it was possible to obtain images within one period of breath-holding. In conclusion, the use of MTC can substantially enhance the quality of 3D-MRPs.

Screening of renal artery stenosis: assessment with magnetic resonance angiography at 1.0 T

The results of MR angiography at 1.0 T with digital intraarterial angiography in the screening of patients with suspected renal hypertension were compared. In this first phase of the study, 10 volunteers underwent examination with both two-dimensional (2D) with traveling saturation time-of-flight (TOF) magnetic resonance angiography (MRA) with various parameters to develop a protocol for evaluation of the renal arteries. In the second phase, 36 patients with suspected renovascular hypertension underwent both 2D TOF MRA and intraarterial digital angiography to evaluate the clinical value of MRA. The degree of stenosis was graded with a two-point scale. In volunteers, using 2D acquisitions C/N ratios indicated the best flip angle as being 55 degrees (p = .02). MRA showed 100% (70/70) of all main arteries and 86% (6/7) of all accessory renal arteries seen on angiography. MRA had a sensitivity of 94% (15/16) and a specificity of 98% (60/61) for detection of stenoses of greater than 50% present in 14 patients. 2D-TOF MRA at 1.0 T shows promise in the noninvasive diagnosis of patients with suspected renovascular hypertension.

MR angiography of abdominal and peripheral arteries. Techniques and clinical applications

This review article deals with MR angiography (MRA) of abdominal and peripheral arteries. Pulsatile flow, respiratory motion and peristalsis impose difficulties in imaging the vascular structures in the abdomen and the lower extremities. Development of new techniques, such as segmentation of the data acquisition, using specific acquisition windows in relation to a cardiac trigger, magnetization preparation of the tissue and phase-encoding re-ordering or sorting, have reduced the artifacts associated with abdominal and peripheral MRA. Clinical MR investigations of the arteries branching from the abdominal aorta such as the renal and mesenteric arteries and arteries in the lower extremities have revealed that severe stenoses or occlusions can be diagnosed accurately while the grading of less severe stenosis is more difficult. The phase-contrast method has been used to quantify blood flow and study the hemodynamics in abdominal and peripheral vessels. Quantitative flow information can be used to diagnose vascular disease and provides important physiological information. More prospective clinical studies, in which recently developed MRA techniques are compared with conventional angiography, are necessary before conclusive decisions can be made as to whether MRA may replace these methods.

Assessment of abdominal aortic aneurysm with magnetic resonance imaging

Pre-operative imaging of abdominal aortic aneurysms (AAA) is important in determining suitability for operation and operative approach. Ultrasound imaging is an excellent screening modality but is relatively poor at identifying renal arteries and the extent of iliac involvement. Computed tomography scanning with intravenous contrast and arteriography are invasive modalities that are associated with a small risk. Magnetic resonance imaging (MRI) offers the potential of accurate anatomical definition without use of contrast agents and passage of an intra-arterial catheter. Eight patients who had their AAA evaluated with MRI are reported. All had renal arteries accurately defined, intra-aneurysmal thrombosis was well delineated, and iliac extension was correctly identified in four cases. The initial experience has been most encouraging and the authors consider that MRI may become the investigation of choice for pre-operative AAA assessment.

Case report: renal transplant artery stenosis--three cases where magnetic resonance angiography was superior to conventional arteriography

Stenosis of renal transplant arteries is a common cause of graft dysfunction and hypertension. Conventionally it is investigated by intraarterial digital subtraction angiography (IA DSA). Recently three-dimensional phase contrast magnetic resonance angiography (MRA) has been used successfully as a non-invasive method of assessing renal transplant arteries. We report three cases where MRA was superior to IA DSA in the detection of renal transplant artery stenosis.

Single breath-holding three-dimensional magnetic resonance portography with bolus injection of Gd-DTPA in subjects with normal liver: a comparison with two-dimensional time-of-flight technique

Two-dimensional time-of-flight (2D-TOF) magnetic resonance (MR) portography tends to produce general image degradation owing to the phase discrepancy caused by multiple breath-holding. To solve this problem, three-dimensional (3D)-MR portographies were reconstructed from image data acquired with a bolus injection of Gd-DTPA with single breath-holding in 20 subjects with normal liver function. 2D-TOF MR portographies were obtained beforehand for the comparison. All the images were displayed in a cine mode for the two MR angiographic techniques. Visualization was assessed as positive when the vessel was visualized without disruption. 3D-MR portography produced a positive rate of over 80% for all but the left first-order branch. With the 2D-TOF techniques, however, only the portal trunk and left second-order branches were visualized at this rate. The positive rate for most portal branches was significantly higher with 3D-MR portography than with the 2D-TOF technique. In conclusion, 3D-MR portography makes it possible to depict the portal venous system without disruption using a short measurement time with single breath-holding and has the potential to become a powerful method for abdominal MR angiography.

MR angiography in the preoperative evaluation of abdominal aortic aneurysms: a preliminary study

PURPOSE

The ability of magnetic resonance (MR) angiography to depict visceral and renal vessels was evaluated in patients with abdominal aortic aneurysms (AAAs).

PATIENTS AND METHODS

MR sequences (sagittal T1-weighted, two-dimensional coronal, and three-dimensional axial time-of-flight) were compared in a prospective blinded fashion with conventional angiograms obtained preoperatively in 23 patients with AAAs. Results were correlated with surgical findings when available.

RESULTS

Operative aortic clamp site was correctly predicted with conventional angiography in 95% of patients and with MR angiography in 86% (P > .1). Aneurysm neck measurements obtained with the two modalities were within 1 cm in 91% of cases. With conventional angiography as the standard of reference, 96% of all renal arteries were identified on MR angiograms but 36% of accessory arteries were missed. MR angiography enabled identification of patients who had at least one renal artery stenosis greater than 50% with a sensitivity of 100% and specificity of 89%. For identifying individual renal artery, celiac artery, and superior mesenteric artery stenoses of similar severity, the sensitivity and specificity were 67% and in excess of 96%, respectively. The celiac artery could not be evaluated in one case.

CONCLUSION

The results of this small study suggest that the role of MR angiography in the preoperative evaluation of AAA warrants further investigation.

Three-dimensional time-of-flight MR angiography using selective inversion recovery RAGE with fat saturation and ECG-triggering: application to renal arteries

A three-dimensional (3D), ECG-triggered, selective inversion recovery (SIR) rapid gradient-echo (RAGE) technique is proposed to obtain MR angiograms of the main renal arteries. By using the selective inversion recovery and fat saturation, the background is significantly suppressed while blood maintains a high signal intensity as compared with conventional 3D time-of-flight (TOF) MR angiography. The sequence is ECG-triggered so that blood in-flow is maximized during systole, and intravoxel dephasing and pulsatile flow artifacts are minimized by collecting data during diastole. As a result, vessel boundary blurring and ghosting artifacts due to background motion are dramatically reduced, and the conspicuity and lumen definition of the arteries are significantly improved. High-quality MR angiograms of the main renal arteries with excellent blood/tissue contrast and suppression of motion artifacts have been consistently obtained for normal volunteers, with the length of visualization being 51 +/- 07 mm for the left, and 57 +/- 06 mm for the right renal arteries, significantly greater than using conventional 3D TOF pulse sequences. Statistical analysis was performed by using a one-sided Student's t test.

Intrapelvic two-dimensional time-of-flight magnetic resonance angiography in healthy and diseased subjects

We evaluated the use of magnetic resonance angiography (MRA) based on two-dimensional (2D) time-of-flight (TOF) technique in the pelvic vasculature. In the initial phase of this study, MRA was performed in 10 normal subjects for determination of the optimum imaging parameters. Systemically varied scan parameters included flip angle ranging from 20 degrees to 90 degrees by 10 degrees increments and repetition time (TR) of 30, 40, and 50 ms. The optimum imaging parameters for intrapelvic MR angiography were found to be flip angle of 90 degrees, pulse sequence of 30/4.9 (TR ms/TE ms). Using these imaging parameters, 70-100% of the second division branches of the internal iliac artery could be visualized in each subject. In the second phase of this study, we evaluated the use of MR angiography in patients with intrapelvic vascular disease by comparing its accuracy in evaluating the iliac arteries with that of conventional angiography. In patients with atheromatous disease, the MR angiographic determination was consistent with that of conventional angiography in 23 of 28 visualized arterial segments (82%). In eight lesions in patients with non-stenotic vascular disease or hypervascular pelvic tumours, all lesions were demonstrated clearly with MR angiography. 2D TOF technique with high flip angle excitation is suitable for the evaluation of main iliac branches and diseased vessels.

Magnetic resonance angiography and detection of renal artery stenosis in a patient with impaired renal function

Diagnosing renovascular disease in patients with renal insufficiency has challenged physicians for many years. Although contrast angiography is the "gold standard," it is associated with major risks in patients with preexisting renal failure. Other noninvasive tests have not proved to have sufficient sensitivity and specificity to supplant angiography. Developments in magnetic resonance (MR) angiographic technology, however, now enable physicians to assess the vasculature noninvasively and without use of potentially nephrotoxic agents. Herein we describe a patient with hypertension and renal failure in whom MR angiography proved to be the only effective noninvasive test for diagnosing renal artery stenosis. In addition, we review the current literature on MR angiography for renovascular disease. In the setting of renal impairment, MR angiography may be useful in screening patients for renovascular disease. More studies are needed in order to refine MR angiographic techniques and, ultimately, to determine specific situations in which MR angiography may be useful.

Rapid three-dimensional surface reconstruction of magnetic resonance images of large arteries and veins: a preliminary evaluation of clinical utility

We have implemented a computerized system for relatively rapid (< 1 h) three-dimensional (3D) surface rendering of flow-sensitive (gradient refocused) magnetic resonance images. The method has been applied in 8 patients with six different clinical problems and was found to enhance understanding of normal and abnormal aortic, caval, and portosystemic venous anatomy. 3D images are useful for communicating complex anatomic information and may help with difficult diagnoses. Advantages and limitations of the present system are discussed.

MR angiography of lower-extremity arterial disease: preliminary experience

Nineteen patients underwent magnetic resonance (MR) angiography for evaluation of lower-extremity arterial disease. The underlying conditions included atherosclerotic occlusive disease in 12 patients, femoral or popliteal aneurysms in four, and bypass graft stenoses or occlusion in four. In the patients with occlusive disease, the iliac and femoropopliteal vessels were classified as patent, moderately stenotic, severely stenotic, or occluded. Fifteen of 16 occlusions (accuracy = 94%) were correctly classified. In the one missed case, there was a long delay between MR angiography and x-ray angiography and it is likely that the occlusion occurred during the interval. Three of five severe stenoses were correctly classified with MR angiography. In two cases of iliac artery stenosis, there was a signal void at the point of maximal stenosis, which on the basis of anatomic features could be recognized as severe stenosis rather than an occlusion. Three of four moderate stenoses were correctly classified. Correlation with x-ray angiography or surgery demonstrated the ability of MR angiography to accurately depict the status of runoff vessels.

The application of total vertical projections for the unbiased estimation of the length of blood vessels and other structures by magnetic resonance imaging

A new stereological method has recently been developed to estimate the total length of a bounded curve in 3D from a sample of projections about a vertical axis. Unlike other methods based on serial section reconstructions, the new method is unbiased (i.e., it has zero systematic error). A basic requirement, not difficult to fulfill in many cases, is that the masking of one structure by another is not appreciable. The application of the new method to real curvilinear structures using a clinical magnetic resonance (MR) imager is illustrated. The first structure measured was a twisted water-filled glass tube of known length. The accuracy of the method was assessed: With six vertical projections, the tube length was measured to within 2% of the true value. The second example was a living bonsai tree, and the third was a clinical application of MR angiography. The possibility of applying the method to other scientific disciplines, for example, the monitoring of plant root growth, is discussed.