Two-dimensional Time-of-flight Research Articles

The AAPM/RSNA physics tutorial for residents. An introduction to MR angiography.

This article provides an overview of the basic principles of magnetic resonance (MR) angiography. The parameters in MR imaging manipulated to generate high contrast between flowing nuclei and stationary tissue are discussed. Two primary strategies are used: time-of-flight (TOF) MR angiography, which creates differences in magnetization magnitude between flowing and stationary nuclei, and phase-contrast MR angiography, which induces changes in the spatial orientation, or phase, of flowing nuclei relative to stationary nuclei. The end result of an MR angiographic study is typically a three-dimensional data set composed of either sequential two-dimensional sections or true three-dimensional data. Two-dimensional TOF methods are sensitive to slow flow and are valuable for differentiating between slow flow and occlusion. Three-dimensional TOF methods have better resolution and are more useful in imaging tortuous vessels. Phase-contrast MR angiography can be effectively used to avoid problems of magnetization saturation that occur in three-dimensional TOF studies and to eliminate signal from high-intensity stationary material such as blood products, which may appear bright and mimic flow signal in TOF studies. Careful use of postprocessing tools aids in the assessment of vascular abnormalities once the MR angiographic data have been acquired.

Evaluation of renal artery stenosis by magnetic resonance angiography

Accurate identification of patients with surgically correctable renovascular hypertension has been difficult by noninvasive means. Advances in the technique of magnetic resonance angiography (MRA) have begun to provide detailed, accurate imaging of the vascular system. This study reports our recent experience in the evaluation of the renal arteries by this technique. MRA and contrast arteriography were performed in 32 arteries (16 adult patients) for evaluation of hypertension, abdominal aortic aneurysm, mesenteric vascular disease, and aorto-iliac occlusive disease. Luminal diameter reduction (%) was determined from two-dimensional time-of-flight (TOF) axial images. Contrast arteriography served as the gold standard for comparison. Contrast arteriography revealed a 50% or greater stenosis in 11 of 32 vessels studied (34%). As a screening test for detection of greater than 50% diameter reduction, MRA had a sensitivity of 91%, a negative predictive value of 94%, and an overall accuracy of 81%. Linear regression analysis demonstrated significant correlation between MRA and arteriographic measurements (r = 0.8; P < 0.001). This study demonstrates the ability of MRA to accurately assess the main renal arteries for the presence of critical stenosis. This noninvasive evaluation compares well with conventional angiography and may have increasing application in the screening of patients with suspected renovascular disease.

Dynamic contrast-enhanced MR imaging assessment of vascularized free fibular grafts.

Magnetic resonance (MR) imaging may be a noninvasive method for assessing perfusion of vascularized bone grafts placed for treatment of avascular necrosis. One proximal femur of seven beagles was devascularized, with insertion of a vascularized fibular graft. MR imaging at 1 week (seven dogs) and 6 weeks (five dogs) after surgery included pre- and postcontrast spin-echo sequences, unenhanced two-dimensional time-of-flight (TOF) vascular imaging, and dynamic gradient-echo imaging during infusion of gadolinium. Relative signal intensity values of selected regions obtained from the dynamic gradient-echo images were plotted as percent enhancement versus time. In the operated hip, MR imaging did not show enhancement in six of seven femoral heads and greater trochanters at 1 week after surgery, with similar results after 6 weeks. MR imaging of fibular grafts 6 weeks after surgery showed an initial rapid increase in enhancement and a subsequent slower increase in five of five dogs, although no enhancement was seen in six of seven dogs at 1 week. These findings contrasted with a rapid initial increase in enhancement followed by slow decline in non-operated hips. Two-dimensional TOF imaging did not show the vascular pedicle of the graft in any dog. Findings of radionuclide bone scanning performed 1 week after surgery were consistent with devascularization of the operated femur and fibular graft. However, tetracycline distribution and histologic findings confirmed the viability of five of five grafts within the devascularized femurs 6 weeks after surgery.(ABSTRACT TRUNCATED AT 250 WORDS)

Pulmonary MR Angiography

To compare two-dimensional (2D) time-of-flight (TOF) MR angiography and three-dimensional (3D) TOF MR angiography for visualization of pulmonary arteries and to determine whether pulmonary veins can be viewed selectively. Imaging was performed using a 1.5 T superconducting MR system with a body coil. Three kinds of 2D TOF MR angiography were compared in four normal volunteers: using a 2D fast low angle shot (FLASH) without cardiac gating and both with and without breath-holding and using a T2-weighted TurboFLASH with cardiac gating and breath-holding. Two-dimensional TOF MR angiography using a T2-weighted TurboFLASH with cardiac gating and breath-holding and 3D TOF MR angiography using fast imaging with steady precession with neither cardiac gating nor breath-holding were compared in four other normal volunteers. Two-dimensional TOF MR venography with a presaturation pulse to suppress artery signals was evaluated in another four normal volunteers. Three-dimensional TOF MR angiography appeared to be better than 2D TOF MR angiography for delineating pulmonary arteries in the volunteers. The third order branches of peripheral pulmonary veins were distinguishable on 2D TOF MR venography using a presaturation pulse. Three-dimensional pulmonary TOF MR angiography is useful in demonstrating pulmonary arteries whereas 2D TOF MR venography with a presaturation pulse is useful in showing pulmonary veins.

MR angiography in the diagnosis of extralobar pulmonary sequestration: a case report

We report a case of extralobar pulmonary sequestration in which magnetic resonance (MR) angiography was performed. Two-dimensional time-of-flight (TOF) MR angiography with three-dimensional (3D) rendering techniques allowed very accurate anatomical depiction of venous structures. 3D TOF MR angiography was also performed but provided disappointing results. The MR angiographic techniques are described and compared with contrast-enhanced CT and MR imaging.

Atherosclerotic occlusive disease of the lower extremity: prospective evaluation with two-dimensional time-of-flight MR angiography.

A prospective, blinded comparison of two-dimensional time-of-flight (TOF) magnetic resonance (MR) angiography and conventional arteriography was performed in 25 patients who underwent routine arteriography for symptomatic atherosclerotic occlusive disease of the lower extremity. MR angiography was performed from the distal abdominal aorta through the popliteal trifurcation. The native arterial tree was divided into nine segments; each segment was assessed for patency (defined as stenosis < 50% of arterial diameter), moderate stenosis (50%-69%), severe stenosis (70%-99%), or occlusion (100%). In all 206 segments examined, the sensitivity of MR angiography in diagnosis of occlusion was 100%; the specificity, 98%. All long occlusions were correctly classified. In specific categories of occlusive disease, the sensitivity and specificity were as follows: in all segments with 70% or greater stenosis, including occlusions, 90% and 97%; in all segments with 50% or greater stenosis, including occlusions, 92% and 88%; in femoropopliteal segments with 50% or greater stenosis, 92% and 93%; and in iliac segments, 93% and 83%. Long occlusions were distinguished from short occlusions and stenoses; hence, MR angiography enabled identification of patients with lesions suitable for angioplasty.

Combined use of duplex imaging and magnetic resonance angiography for evaluation of patients with symptomatic ipsilateral high-grade carotid stenosis

Purpose: Advances in cerebral vascular imaging suggest that patients with critical levels of carotid artery stenosis (>70%) who have symptoms can be identified accurately and necessary information about the intracranial and extracranial circulation obtained before surgery without conventional angiography. We have used carotid duplex imaging in combination with magnetic resonance angiography (MRA) to evaluate 20 patients with symptomatic ipsilateral high-grade carotid stenosis.Methods: All patients underwent CT and magnetic resonance imaging brain scans, as well as MRA and conventional arteriography of the cerebral circulation. Magnetic resonance angiograms were obtained with two-dimensional phase contrast and time-of-flight techniques. Phase contrast was used for intracranial vascular imaging and for determining qualitative flow velocities and the direction of blood flow in the circle of Willis. Two-dimensional time of flight was used to assess the carotid bifurcations.Results: Twenty patients with symptoms (six with strokes, 11 with transient ischemic attacks, and three with amaurosis fugax) had duplex evidence of high-grade carotid stenoses. Computed tomographic and magnetic resonance brain scans were positive for cerebral infarction in six patients with clinical strokes. Comparison of MRA with conventional angiography was 91% accurate for high-grade stenoses and occlusions (sensitivity 100% and specificity 90% for stenosis; sensitivity/specificity was 100% for complete occlusion). Comparison of duplex imaging with conventional angiography demonstrated 86% accuracy for detection of severe stenosis or occlusion (sensitivity 94% and specificity 89% for stenosis; sensitivity and specificity were 100% for complete occlusion).Conclusions: This study suggests that combined use of MRA and duplex imaging is accurate for detection and evaluation of high-grade carotid stenoses in patients with symptoms.

Magnetic resonance angiography and measurement of blood flow in the peripheral vessels.

Peripheral vascular disease has been evaluated using conventional imaging techniques such as contrast angiography to demonstrate the morphology and duplex sonography to evaluate the hemodynamic significance of a stenosis. Two-dimensional time-of-flight (TOF) magnetic resonance angiography (MRA) provides useful anatomic information in both normal and diseased popliteal and tibioperoneal vascular segments, whereas corresponding velocity-encoded cine MR determined velocities and waveforms correlate well with those determined using color-coded sonography and showed excellent interstudy reproducibility. Velocity-encoded cine MR may be useful in assessing the hemodynamic significance of a peripheral vascular stenosis whose severity might be overestimated by using MRA alone. When two-dimensional TOF MRA is combined in a complementary manner with velocity-encoded cine MR, the anatomic information from contrast angiography and the physiologic information from duplex sonography can be obtained during the course of one MR examination.

Magnetic resonance angiography of the pelvis and lower extremities. Works in progress.

Two-dimensional time-of-flight (TOF) magnetic resonance angiography (MRA) of the lower extremities has recently been rediscovered as a technique that can be used to find small vessels in the foot and leg for surgical revascularization in patients with severe peripheral vascular disease. The purpose of this article is to acquaint the reader with some of the common problems that may be encountered in this imaging technique and to show their derivations and solutions were applicable. In addition, the authors show receiver operator curve analysis of one radiologist's ability to determine the location and patency of arteries using this technique to be outstanding (AZ, 0.9737). This represents a promising new technique for imaging the arterial system in patients with severe peripheral vascular disease.

Occurrence of aldehydes in garden and field soils

Idiopathic Parkinson's disease (IPD) remains one of those neurodegenerative diseases for which the cause remains unknown. Many clinically diagnosed cases of IPD are associated with cerebrovascular disease and white matter hyperintensities (WMHs). The purpose of this study was to investigate the presence of transverse sinus and extracranial venous abnormalities in IPD patients and their relationship with brain WMHs.Twenty-three IPD patients and 23 age-matched normal controls were recruited in this study. They had conventional neurologic magnetic resonance structural and angiographic scans and, for blood flow, quantification of the extracranial vessels. Venous structures were evaluated with two-dimensional time of flight; flow was evaluated with two-dimensional phase contrast; and WMH volume was quantified with T2-weighted fluid-attenuated inversion recovery. The IPD and normal subjects were classified by both the magnetic resonance time-of-flight and phase contrast images into four categories: (1) complete or local missing transverse sinus and internal jugular veins on the time-of-flight images; (2) low flow in the transverse sinus and stenotic internal jugular veins; (3) reduced flow in the internal jugular veins; and (4) normal flow and no stenosis.Broken into the four categories with categories 1 to 3 combined, a significant difference in the distribution of the IPD patients and normal controls (χ2 = 7.7; P < .01) was observed. Venous abnormalities (categories 1, 2, and 3) were seen in 57% of IPD subjects and in only 30% of controls. In IPD subjects, category type correlated with both flow abnormalities and WMHs.From this preliminary study, we conclude that a major fraction of IPD patients appear to have abnormal venous anatomy and flow on the left side of the brain and neck and that the flow abnormalities appear to correlate with WMH volume. Studies with a larger sample size are still needed to confirm these findings.