MR imaging of blood vessels using three-dimensional reconstruction: methodology

Multisection, dual-echo magnetic resonance (MR) transaxial images of blood vessels contain both anatomic and qualitative information about flow. Even so, the images are produced as a series of two-dimensional tomographic sections from which full visualization of connected structures is difficult. A computer algorithm was developed that automatically detects flowing blood based on pixel intensity and calculated T2 and provides reconstructed views of vessels while analyzing and displaying flow characteristics. Images of abdominal vessels, aortic aneurysms, and the heart were encoded by flow and color to demonstrate depth. In addition, these data were reconstructed to derive a more accurate assessment of patency. With this technique, transaxial images can be used to analyze flow patterns, determine patent areas, and visualize all levels of vessels in a single image.     

Velocity-coded color MR angiography.

We developed a method of velocity-coded color MR angiography using a color code from the data obtained from velocity-phase images of phase-contrast MR angiography in order to add flow direction information to MR angiograms. Phase-contrast MR angiography with reconstruction of velocity-phase images was performed in 30 patients. Two projection images from velocity-phase images of each phase-contrast MR angiogram were obtained and assigned color according to flow direction. We then superimposed the two color images onto the maximum intensity projection image of the MR angiogram. The velocity-coded color MR angiogram clearly showed flow direction from the data on the phase-contrast MR angiogram of the neck. Veins were readily distinguishable from arteries, and flow changes, such as a subclavian steal, were also identified.     

MR vascular imaging with a fast gradient refocusing pulse sequence and reformatted images from transaxial sections

The authors present a method for obtaining magnetic resonance (MR) images of intra- and extracranial vessels from thin contiguous transaxial sections. A section-selective gradient refocusing pulse sequence with a short repetition time caused flow-related enhancement from spins that flowed perpendicular to the transaxial sections. The signal was further enhanced by means of flow compensation gradients to rephase any phase shifts resulting from moving spins in the presence of the imaging gradients. Coronal and sagittal sections, reformatted from multiple transaxial sections, are shown to have excellent vessel contrast without the use of contrast material. These images were obtained in 12 minutes of acquisition time from as many as 60 sections of 3-mm thickness. Such a technique shows significant promise for MR angiography.     

MR angiography and dynamic flow evaluation of the portal venous system

We studied the value of MR angiographic techniques in imaging the portal venous system. Projection angiograms were created by postprocessing a series of two-dimensional, flow-compensated gradient-echo images. Flow velocity was determined by a bolus-tracking method with radiofrequency tagging and multiple data readout periods. Each image was acquired during a breath-hold. MR angiography was applied to six normal subjects and four patients with abnormal hemodynamics in the portal venous system. Flow velocity determined by MR was correlated with the results of duplex sonography. The main portal vein and intrahepatic branches were shown in all cases. Portosystemic collaterals were identified in all patients with portal hypertension. In normal subjects, peak flow velocities (17.9 +/- 2.8 cm/sec) on MR correlated well with values determined by duplex sonography (17.5 +/- 2.2 cm/sec) (r = .846, p less than .04). Reversed portal blood flow was shown in two patients. One patient with portal vein thrombosis had no evidence of flow by MR angiography. Our results indicate that MR angiography can provide a three-dimensional display of normal and abnormal vascular anatomy as well as functional information in the portal venous system.     

Carotid arteries: maximizing arterial to venous contrast in fluoroscopically triggered contrast-enhanced MR angiography with elliptic centric view ordering

PURPOSE: To obtain high-spatial-resolution, venous-suppressed, contrast material-enhanced, three-dimensional (3D) magnetic resonance (MR) angiograms of the carotid arteries and aortic arch by using an elliptic centric view ordering with MR fluoroscopic triggering. MATERIALS AND METHODS: Forty consecutive patients with cerebrovascular disease in the differential diagnosis were evaluated with fluoroscopically triggered 3D MR angiography (gadoteridol dose range, 0.1-0.3 mmol per kilogram of body weight; mean acquisition time, 40 second +/- 8 [SD]). The contrast-enhanced 3D MR angiograms were evaluated for overall quality, vascular signal intensity, venous suppression, and motion artifact. Twenty patients also underwent two-dimensional (2D) time-of-flight (TOF) MR angiography. The overall quality of the 2D TOF MR angiograms and comparative quality between the 2D TOF and contrast-enhanced 3D MR angiograms were determined. RESULTS: The contrast-enhanced 3D MR angiograms were of excellent or more than adequate quality for diagnosis in 36 of the 40 studies (90%). In 35 of the 38 contrast-enhanced 3D studies in which the contrast material bolus was detected fluoroscopically, the internal jugular vein signal intensity was either not detectable or barely visible. In 18 of the 20 patients who also underwent 2D TOF MR angiography, the quality of the contrast-enhanced 3D MR angiograms was graded as markedly superior or superior. CONCLUSION: Contrast-enhanced, elliptic centric 3D MR angiography with real-time MR fluoroscopic triggering offers high-spatial-resolution images of the carotid arteries and aortic arch with reliable venous suppression.     

Computed tomography of the carotid space and related cervical spaces. Part II: Neurogenic tumors

In order to determine the value of coronal magnetic resonance (MR) in diagnosing thoracic abnormalities, the multisection coronal spin echo MR images were reviewed of 10 normal subjects and 20 patients with thoracic abnormalities. In the abnormal patients, coronal images were compared with transaxial MR images obtained with TR values of 0.5 and 2.0 sec. In general, coronal imaging was of value in several situations. It allowed structures oriented in the coronal plane to be imaged along their longitudinal axes, provided an additional perspective and increased the confidence of diagnosis, and helped clarify anatomic relationships difficult or impossible to resolve on transaxial images because of volume averaging. Specifically, coronal images were sometimes superior to transaxial images in evaluating the aorticopulmonary window and masses at the lung apex or base. Transaxial images were often superior in evaluating the pretracheal space, subcarinal space, and hili. Within the pulmonary hili, lateral hilar masses were better defined on coronal images than were anterior or posterior hilar masses. Coronal images obtained with a TR of 1.0 sec (10 sections) allow evaluation of most node-bearing mediastinal compartments and provide adequate mass/fat contrast.     

MR angiography of the head and neck: value of two-dimensional phase-contrast projection technique.

MR angiography is commonly performed by using two- and three-dimensional time-of-flight and three-dimensional phase-contrast techniques. These procedures require long examination times and processing of imaging data by computing maximum intensity projections. Two-dimensional phase-contrast projection angiography has neither of these disadvantages. We analyzed the value of this technique for head and neck MR angiography in 84 patients and 15 control subjects. Patients were examined to resolve specific clinical questions such as the presence of arteriovenous malformations (20 cases), patency of carotid and vertebral arteries (35 cases), patency of the superior sagittal sinus (14 cases), patency of saphenous vein bypass grafts (11 cases), and vascularity of masses (four cases). Conventional angiograms were available for correlation in 22 patients. Two-dimensional phase-contrast projection angiograms were generated by using a gradient-recalled-echo sequence sensitized to flow with the use of flow-encoding gradients. Projection MR angiograms were obtained in approximately 3.5 min by combining images obtained with flow-encoding gradients applied along the axes defining the image plane. MR angiograms were subjectively evaluated by three observers without reference to routine MR images or conventional angiograms. High-quality studies were obtained in 93% of control subjects and 90% of patients examined. The findings based on phase-contrast angiography were confirmed with conventional angiography in 21 of 22 patients for whom conventional angiograms were available. Evaluation of vascular grafts and of the patency of major cranial vessels was easily done with two-dimensional phase-contrast angiography and was useful in postoperative follow-up examinations. We also advocate its use for superior sagittal sinus thrombosis and follow-up examinations in patients with arteriovenous malformations.     

Visualization of intraaneurysmal flow patterns with transluminal flow images of 3D MR angiograms in conjunction with aneurysmal configurations

BACKGROUND AND PURPOSE: How the complex flow phenomena generated within unruptured cerebral aneurysms relate to the corresponding aneurysmal geometry is unknown. To estimate the interaction between flow patterns and morphologic features of unruptured cerebral aneurysms, we developed a method to visualize intraanuerysmal flow patterns with transluminal flow imaging of 3D MR angiograms in conjunction with aneurysmal configurations. METHODS: Transluminal images of the vessel lumen were reconstructed with use of a parallel volume-rendering algorithm by selecting information on the margin of lumina from the volume data sets of 3D time-of-flight MR angiograms. Transluminal flow images were then created by superimposing flow-related intraluminal information onto transluminal images. Intraaneurysmal flow patterns were evaluated in three cases of unruptured cerebral aneurysms, based on the animated display of transluminal flow images with stepwise extracted intraluminal volume data of signal intensity, in conjunction with the corresponding aneurysmal configurations depicted on 3D MR angiograms. RESULTS: Transluminal flow images showed 3D visualization of flow-related signal intensity distribution obtained from volume data of MR angiograms, so that qualitative information regarding intraaneurysmal flow patterns could be estimated with respect to morphologic features of cerebral aneurysms. CONCLUSION: Transluminal flow images of 3D MR angiograms allowed feasible visualization of intraaneurysmal flow patterns that were studied. More work is required to validate the technique and clarify the significance of being able to visualize intraaneurysmal flow patterns.     

Blood flow imaging through detection of temporal variations in magnetization.

Two hypotheses were tested: (a) that view-to-view variations in bulk phase and modulus of magnetization in vascular volume elements can indicate the presence of disordered blood flow, and (b) that a substantial loss of signal intensity on magnetic resonance (MR) angiograms of poststenotic regions is due to view-to-view changes in magnetization. To test these hypotheses, a technique was developed in which view-to-view variations in transverse magnetization were used to create angiographic projection images, which showed only disordered flow (disordered flow maps) in vitro and in vivo. In phantom studies, this technique recovered signal intensity downstream from stenoses. A combination of disordered flow maps with morphologic images improved visualization of stenotic regions and provided information on characteristics of local flow. These results show that view-to-view variations in transverse magnetization occur in regions of disordered flow and are an important cause of loss of signal intensity. This technique can provide information about dynamic blood flow and improve depiction of anatomic structures on MR angiograms.     

Evaluation of small pulmonary arteries by 16-slice multidetector computed tomography: Optimum slab thickness in condensing transaxial images converted into maximum intensity projection images

OBJECTIVE: The purpose of this study was to determine the optimal slab thickness for condensing transaxial images into maximum intensity projection (MIP) images in the evaluation of small pulmonary arteries using 16-slice multidetector-row computed tomography (MDCT). METHODS: Helical computed tomography (CT) scans were obtained from lung apices to bases using 16-slice MDCT [120 kV(peak), 180 mA, beam width of 10 mm, beam pitch of 1.375, and reconstruction thickness of 1.25 mm] in 29 patients suspected of having a pulmonary embolism. Four kinds of image series (1.25-mm thick original transaxial source images and 3 kinds of reconstructed images using the MIP technique with slab thicknesses of 2.5 mm, 5 mm, and 10 mm) were obtained from each patient and forwarded to monitors of a picture archiving and communication system for analysis by 2 independent observers. The observers recorded the name of the segmental (20 total; 10 in each lung) and subsegmental (40 total; 20 in each lung) arteries that were traceable in each image series. Image quality of the 4 image types were graded into 5 scales based on their degree of vascular opacification, the sharpness of the vascular margins of the contrast-enhanced CT angiograms, and the visibility of lung parenchyma (excellent [5] to nondiagnostic [1]) and compared. RESULTS: In both the 1.25-mm thick original transaxial and 2.5-mm thick MIP images, a higher percentage of subsegmental arteries was traceable (91.3% [2119/2320 observations] and 87.2% [2023/2320 observations], respectively; P <0.05) than in the 5-mm and 10-mm thick MIP images (66.4% [1540/2320] and 40.5% [940/2320], respectively). No statistically significant difference was observed between the 1.25-mm thick transaxial and 2.5-mm thick MIP images in this respect. Image quality of 2.5-mm thick MIP images was superior to that of the 5-mm and 10-mm thick MIP images (P < 0.0001). No statistically significant difference was found between the scores of the image quality of the 1.25-mm thick original transaxial images and the 2.5-mm thick MIP images. CONCLUSION: After reducing the image number by one half, 2.5-mm thick MIP images using 16-slice MDCT are found to provide satisfactory images, which are comparable to 1.25-mm thick transaxial images for the analysis of subsegmental pulmonary arteries in patients suspected of pulmonary embolism.     

Motion artifact simulating aortic dissection on CT

We recently imaged two patients clinically suspected of having aortic dissection whose contrast-enhanced CT examinations, obtained on a new scanner with a 1-sec scanning time, showed findings suggesting an ascending aortic dissection. The subsequent clinical course and evaluation implied that the CT findings were predominantly artifactual. We identified identical artifacts in 18% of 50 consecutive contrast-enhanced CT examinations performed for a variety of indications on the same scanner. The double-lumen artifact, simulating an intimal flap, occurs in the proximal ascending aorta and is limited to one or two contiguous transaxial images. The artifact was not detected on two other CT units. We believe the artifact arises from motion of the aortic wall and the surrounding pericardial recesses during image acquisition.     

Peripheral vascular disease: correlation of MR imaging and angiography

The capability of magnetic resonance (MR) imaging for detecting aortic, iliac, and femoral stenoses and occlusions was evaluated. Multisection spin-echo studies at 0.35 tesla were obtained of the infrarenal aorta to the femoral bifurcation in 24 patients, all of whom had undergone intraarterial angiography within 14 days of imaging. Transaxial MR images were compared with the angiograms. Arterial stenoses and occlusions in these vessels detected by MR imaging correlated with angiographic findings in 91% of the instances. Protrusional atherosclerotic plaques and occlusions and stenoses in the aortoiliac region were demonstrated accurately on MR images; complications of previous vascular surgery, such as aneurysms at sites of previous anastomoses or endarterectomy, were also identified. Due to the limited spatial resolution, MR images failed to demonstrate some femoral stenoses. MR imaging may be used for evaluation of aortoiliac vascular disease and for follow-up study after surgical revascularization. However, the limited spatial resolution, noncomposite display of the aortoiliofemoral circulation, and lack of evaluation of peripheral runoff provided by current MR imaging techniques militate against its replacing angiography prior to vascular intervention.     

MR imaging with remote control: feasibility study in cardiovascular disease

The institutional review board approved this HIPAA-compliant study and waived informed consent. The purpose was to retrospectively evaluate remote control magnetic resonance (MR) imaging in complex cardiovascular procedures, whereby operational expertise was made available locally from a remote location. Thirty patients underwent cardiac (12 patients) and/or vascular (30 patients) 1.5-T MR imaging with a remote operator by using a personal computer. All patient studies were compared with 30 control studies obtained with conventional local imaging. Cardiac cine, myocardial delayed enhancement, and MR angiograms were assessed for overall image quality and motion artifact. MR angiograms were evaluated for vascular definition. Image quality was excellent in 90% (38 of 42) of remote images versus 60% (25 of 42) of control group images (P < .01). Scores for motion artifact were not significantly different (P = .11). Interactive MR imaging was successfully implemented with remote control in complex cardiovascular cases; diagnostic quality of images was superior to that of images obtained locally.     

MR virtual angioscopy of thoracic aortic atherosclerosis in homozygous familial hypercholesterolemia

PURPOSE: The thoracic aorta is an important site of atherosclerotic disease in patients with homozygous familial hypercholesterolemia (HFH). Thoracic aortic atherosclerosis in patients with HFH was assessed with contrast-enhanced MR angiograms using exoscopic and endoscopic virtual angioscopy reconstructions and maximum intensity projections (MIPs). METHOD: Contrast-enhanced MR angiograms of the thoracic aorta of 15 patients with HFH and 8 normal volunteers were obtained. Perspective surface reconstructions of the MR angiograms including virtual angioscopy views were evaluated by three radiologists blinded to the diagnosis. RESULTS: Thoracic wall irregularity was depicted on 8 of 15 (53%) patient scans and only 1 of 8 (13%) normal subject scans using surface reconstructions. Wall irregularity scores of patients with HFH were significantly increased compared with controls (2.0 +/- 0.9 vs. 1.0 +/- 0.6; p = 0.008). There was excellent interobserver agreement (weighted kappa = 0.82 +/- 0.12). Virtual endoscopy views added diagnostic confidence compared with exoscopic surface renderings alone. MIP reconstructions were unable to depict wall irregularity. CONCLUSION: MR angiography with virtual angioscopy of the thoracic aorta depicts nonstenotic wall irregularity of thoracic aortic atherosclerosis in patients with HFH. This may be important for assessing disease progression and response to treatment and may be generalizable to routine (non-HFH) atherosclerosis.     

Comparison of pre- and postcontrast 3D time-of-flight MR angiography for the evaluation of distal intracranial branch occlusions in acute ischemic stroke

BACKGROUND AND PURPOSE: Three-dimensional time-of-flight (TOF) MR angiography is used routinely in stroke workup to detect arterial occlusions, but a major drawback is its inadequate depiction of vessels with slow or in-plane flow. We hypothesized that the use of contrast-enhanced MR angiography improves delineation of vessels with diminished or absent flow on precontrast MR angiograms. METHODS: Pre- and postcontrast 3D TOF MR angiograms were acquired in 55 consecutive patients with acute stroke. Patency of 480 intracranial vessels was assessed on both the pre- and postcontrast angiograms. Diffusion-weighted (DW) and perfusion-weighted (PW) imaging data were also obtained and results correlated with those of pre- and postcontrast MR angiography. RESULTS: For 50 abnormal vessel segments seen on precontrast MR angiograms, postcontrast MR angiograms resulted in change in the vascular signal intensity in 70% (35 vessel segments); 94% of these changes showed a greater extent of vessel patency. Venous and soft-tissue contrast enhancement had no effect on assessment in 95% of all 480 vessels examined. Interobserver reliability was moderate, with postcontrast interpretation (kappa = 0.48) showing a slight improvement over precontrast interpretation (kappa = 0.41). Good agreement was found between the TOF results and the pooled DW and PW imaging results. CONCLUSIONS: Compared with precontrast 3D TOF MR angiograms, postcontrast 3D TOF angiograms improve assessment of intracranial vessel patency in acutely ischemic vascular territories. In some patients, an improved understanding of acute ischemic stroke was obtained by viewing the pre- and postcontrast images. Postcontrast MR angiography should be included in the MR evaluation of acute stroke.     

Time-resolved 3D MR velocity mapping at 3T: improved navigator-gated assessment of vascular anatomy and blood flow

PURPOSE: To evaluate an improved image acquisition and data-processing strategy for assessing aortic vascular geometry and 3D blood flow at 3T. MATERIALS AND METHODS: In a study with five normal volunteers and seven patients with known aortic pathology, prospectively ECG-gated cine three-dimensional (3D) MR velocity mapping with improved navigator gating, real-time adaptive k-space ordering and dynamic adjustment of the navigator acceptance criteria was performed. In addition to morphological information and three-directional blood flow velocities, phase-contrast (PC)-MRA images were derived from the same data set, which permitted 3D isosurface rendering of vascular boundaries in combination with visualization of blood-flow patterns. RESULTS: Analysis of navigator performance and image quality revealed improved scan efficiencies of 63.6%+/-10.5% and temporal resolution (<50 msec) compared to previous implementations. Semiquantitative evaluation of image quality by three independent observers demonstrated excellent general image appearance with moderate blurring and minor ghosting artifacts. Results from volunteer and patient examinations illustrate the potential of the improved image acquisition and data-processing strategy for identifying normal and pathological blood-flow characteristics. CONCLUSION: Navigator-gated time-resolved 3D MR velocity mapping at 3T in combination with advanced data processing is a powerful tool for performing detailed assessments of global and local blood-flow characteristics in the aorta to describe or exclude vascular alterations.     

MR evaluation of chronic aortic dissection

Thirty patients with suspected or known chronic aortic dissection were imaged with magnetic resonance (MR), CT, and angiography. Five of these patients had previously undergone surgical repair of the ascending aorta for a type A dissection. Magnetic resonance demonstrated an intimal flap and a double lumen in 25 cases. In four cases with a thrombosed false lumen, proved angiographically, an intimal flap and double channel were not seen. In two of four aortic dissections with a thrombosed false lumen, CT made the diagnosis by showing displaced intimal calcifications not visualized on MR. In one case the aortic dissection was made on CT and angiography but was not supported by MR which showed an aortic aneurysm, subsequently confirmed at surgery. Magnetic resonance, CT, and aortography differentiated between type A (nine patients) or B (20 patients) dissection in all cases and demonstrated extension into the abdominal aorta. Extension into the iliac arteries was seen on MR in three patients but missed in nine patients. Magnetic resonance differentiated the true and false lumen in all but one case. Thrombosis of the false channel was identified in four cases by a decrease in signal intensity on the second echo image. Cardiac gating and longitudinal contiguous sections seemed to be more suitable for appreciation of the relationships with arch vessels. Transverse contiguous slices allowed determination of the origin of celiac, mesenteric, and renal arteries from either the true or the false lumen. This study confirms that MR is an accurate and noninvasive method for the evaluation and follow-up of chronic aortic dissection, obviating the need for iodinated contrast media.     

Pitfalls in the diagnosis of thoracic aortic dissection at CT angiography.

Two hundred seventy-five computed tomographic (CT) angiograms of the thoracic aorta were obtained over a period of approximately 4 years in patients with suspected or known aortic dissection. In all cases, unenhanced images were initially obtained, followed by contrast material-enhanced images. A variety of pitfalls were encountered that mimicked aortic dissection. These pitfalls were attributable to technical factors (eg, improper timing of contrast material administration relative to image acquisition); streak artifacts generated by high-attenuation material, high-contrast interfaces, or cardiac motion; periaortic structures (eg, aortic arch branches, mediastinal veins, pericardial recess, thymus, atelectasis, pleural thickening or effusion adjacent to the aorta); aortic wall motion and normal aortic sinuses; aortic variations such as congenital ductus diverticulum and acquired aortic aneurysm with thrombus; and penetrating atherosclerotic ulcer. Although several of these pitfalls are easy to recognize and therefore unlikely to present a diagnostic problem, others are potentially confusing. Familiarity with these common pitfalls, coupled with a knowledge of normal intrathoracic anatomy, will facilitate recognition of true aortic dissection and help avoid misdiagnosis at thoracic aortic CT angiography.     

Thoracic aortic dissections: magnetic resonance imaging

Six patients with documented dissections of the thoracic aorta (two Type A, four Type B) were examined by magnetic resonance (MR) imaging using a 0.6-Tesla superconductive magnet. Cardiac gating was applied in five cases. Correlation was made with CT and angiography. MR imaging demonstrated the dissection in all six cases and accurately differentiated Type A from Type B dissections. Coronal and sagittal MR sections were advantageous in establishing the relationship of the three arch vessels to the dissection. In addition, cardiac-gated MR was useful in demonstrating mural thrombus and in distinguishing the true from the false lumen based on differences in signal intensity resulting from different flow rates. In five cases, the information obtained by MR was equal to or surpassed that obtained by CT. In the one case of a completely thrombosed dissection, the CT scan was more helpful. MR should become an important imaging modality in the evaluation of aortic dissections.     

Value of image subtraction in 3D gadolinium-enhanced MR angiography of the renal arteries

The objective of this study was to evaluate quantitatively and qualitatively the effect of image subtraction on the image quality of three-dimensional (3D) gadolinium-enhanced MR angiograms of the renal arteries. Breath-hold 3D gadolinium MR angiography (MRA) as well as conventional contrast angiography of the renal arteries was performed on 20 patients with suspected renovascular hypertension. MR angiograms were acquired before and during dynamic infusion of gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA). Contrast-enhanced images were compared with images that had undergone voxel-by-voxel signal intensity subtraction of contrast-enhanced data from precontrast data. One false positive finding for significant renal artery stenosis was recorded with MRA using conventional angiography as the gold standard. Image subtraction did not alter the diagnosis at MRA in any case. The mean contrast-to-noise ratio (CNR) was significantly higher (P < .05) on the subtraction MR angiograms compared to the nonsubtracted MR angiograms. There was no significant difference in the signal-to-noise ratio (SNR). Qualitative analysis revealed a significant improvement in image quality after image subtraction with respect to visualization of the distal renal arteries. In conclusion, image subtraction improves the quality of renal MRA in terms of both CNR and visualization of the distal renal arteries.