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.

Inhaled oxygen: a brain MR contrast agent?

Oxygen inhalation led to subtle but readily detectable changes on T2*-weighted images with a conventional MR imager at 1.5 T. We attribute the increase in local signal intensity to changes in blood oxygenation, in particular to a net conversion of deoxyhemoglobin to oxyhemoglobin.

Fast MR angiography of the aortoiliac arteries and arteries of the lower extremity: value of bolus-enhanced, whole-volume subtraction technique.

OBJECTIVE: MR angiography of the abdominal aorta and lower limb arteries has been hampered by the need to depict long segments of the arteries in short imaging times. To overcome these limitations, we combined fast acquisition of coronal sections with bolus-enhancement technique, subtraction, and whole-volume projection display. The purpose of this preliminary study was to determine the value of using this technique in patients with abdominal aortic lesions and atherosclerotic occlusive disease of the lower limbs. SUBJECTS AND METHODS: Ten healthy volunteers and 20 patients with abdominal aortic aneurysms (n = 15), vascular graft (n = 3), arteriovenous fistula (n = 1), and abdominal aortic dissection (n = 1) were examined by segmented K-space ultrafast gradient-echo sequences. Sequential 50-cm coronal sections were acquired before and after an IV bolus injection of gadolinium (0.1 mmol/kg). Subtraction images were created by subtracting precontrast from postcontrast images. In all MR images, signal intensity was measured in the aorta, the inferior vena cava, and the background tissues. The final MR angiogram was a summation image of each subtracted image and was compared with a preoperative conventional arteriogram. Data from each study were collected prospectively and analyzed in a blinded manner. The radiologist evaluated depiction of the following specific areas: the abdominal aorta, common iliac arteries on both sides, superficial femoral arteries and above-knee popliteal arteries, below-knee popliteal arteries, superior mesenteric artery, and renal arteries. The aneurysms were categorized as infrarenal or suprarenal and as involving or not involving the iliac arteries. The graft was considered patent if depicted at both its origin and its periphery. RESULTS: In the healthy volunteers, bolus-enhanced subtraction MR angiography depicted without in-plane saturation the aorta, iliac arteries, and arteries of the lower extremity in less than 2 min. Gadolinium significantly increased the aortic signal-to-noise ratio from 25 +/- 3 to 232 +/- 8 (p < .0001), whereas the signal intensity of the inferior vena cava, muscles, and fat remained relatively stable. Examinations were thought to be interpretable in all but two patients. Clear delineation of aneurysms, intimal flap and arteriovenous fistula, and graft patency (n = 3) or occlusion (n = 2) were similarly diagnosed by conventional and MR angiography. Superior mesenteric artery was depicted in all cases on postcontrast images before subtraction. The number of renal arteries (n = 29) was correctly identified in all 15 patients who had renal areas included in their aortograms on postcontrast images before subtraction. However, renal arteries throughout their full length were usually obscured by renal veins and bowel contrast enhancement on the final MR angiogram. Furthermore, bolus-enhanced subtraction MR angiography was able to depict all lower limb arteries. CONCLUSION: Although further evaluation and technical improvements are required, our preliminary results indicate that bolus-enhanced subtraction MR angiography is a promising new method for fast abdominal and peripheral vascular imaging.

Pulmonary MR angiography at 1.0 T: early results with k-space segmented and post-contrast TurboFLASH two-dimensional time-of-flight sequences.

PURPOSE: To evaluate the combined performance of two time-of-flight methods in imaging the pulmonary arteries. MATERIALS AND METHODS: This study was prospectively conducted in 28 patients suspected for pulmonary embolism (PE). Sixteen patients were free of pulmonary vascular disease, and 12 had pulmonary vascular disease as demonstrated by pulmonary angiography. To reduce artifacts caused by cardiac and respiratory motion, MR images were acquired in all subjects using bi-dimensional (2D), gradient-recalled echo (GRE), breath-hold techniques. Sagittal thin (6-mm) sections obtained with ECG gating, k-space segmentation and incremented flip-angles (TONE), and coronal thick (15-mm) sections obtained after a unique injection of Gadolinium chelate were used. RESULTS: High quality images were obtained in all 16 (100%) subjects free of pulmonary disease with both techniques, and in 10 and 12 (87% and 100%) patients suspected for pulmonary artery disease with sagittal and coronal Gd-enhanced MRA, respectively. In patients free of pulmonary disease, TONE images exhibited distal pulmonary arteries with 2.1 subsegmental divisions on average, whereas Gd-enhanced TurboFLASH images were the most accurate to identify proximal pulmonary arteries within the mediastinum, even if only 0.8 subsegmental divisions were seen on average. A correct diagnosis of pulmonary embolism was obtained in all cases but one, with use of both MRA techniques, with an overall accuracy of 86%. CONCLUSION: The association of segmented sagittal GRE images and coronal first-pass Gd-enhanced GRE images can provide information upon normal and diseased pulmonary arteries within the mediastinum until subsegmental pulmonary branches, even in patients with short-breathing. Further studies of patients with various pulmonary artery diseases will confirm whether this technique makes pulmonary MRA feasible in clinical routine situations.

[Principles of magnetic resonance angiography]. / Principes de l'angiographie par résonance magnétique.

Magnetic resonance angiography is a modality of functional imaging which can only be interpreted on the basis of a good understanding of vascular physiology and the physical principles of image acquisition. Magnetic resonance angiography is based on flow-related "artefacts". The oldest method is based on renewal of the image by proton flow: the flight time effect. Dephasing of the protons travelling in magnetic field gradients results in phase-contrast angiography. These two methods of angiography each have respective advantages and disadvantages: they are not mutually exclusive, but complementary.

[Exploration of vessels of the lower limbs by magnetic resonance angiography (MRA)]. / Exploration des vaisseaux des membres inférieurs en angiographie par résonance magnétique (ARM).

Interesting results are beginning to be published in the literature concerning the capacities of MRA in arterial disease and thrombophlebitis of the lower limbs. The acquisition techniques reported in the limited number of papers published in the literature are unfortunately too disparate to be applied to routine diagnostic activity. Most teams agree on the fact that the axial plane is the most suitable to visualize arteries of the lower limbs in patients with arterial disease, using 2D acquisition with or without intravenous injection of paramagnetic contrast agent. Based on the experience acquired with the use of multiple sequences obtained in 24 subjects (4 healthy volunteers, 12 patients with arterial disease and 8 cases of suspected thrombophlebitis) and a review of the literature, the potential applications and limits of MRA have been defined as a function of the capacities of current machines. MRA is able to detect venous thrombi and can also provide a complete assessment of arterial stenosis and occlusions in arterial disease of the lower limbs. The only drawback of the technique in arterial disease is the number of sections required, as only the axial plane can be used for acquisition. Consequently, MRA is tending to be developed in specific indications such as topography of arteries suitable for bypass procedures, rather than as a substitute to classical angiographic arterial mapping, although it appears to be superior to arteriography of the lower limbs in some cases in terms of the information it provides on the distal blood supply in the case of proximal occlusion. The problem which remain to be resolved are the reduction of the number of sections and the choice of the optimal field of view. Assuming that the technique is accepted by vascular surgeons, examinations composed of non-contiguous sections would appear to constitute a sufficient diagnostic solution. In thrombophlebitis, the problem is not so much the blood flow as extent of the thrombus. The diagnosis is easy in the case of fresh thrombus (which has a high-intensity signal, like subacute haematoma), but appears difficult or even impossible in the case of phlebitic sequelae.

Pulmonary time-of-flight MR angiography at 1.0 T: comparison between 2D and 3D tone acquisitions.

The purpose of this study was to compare the performance of 2D vs. 3D time-of-flight (TOF) methods in imaging the normal pulmonary arteries with commercially available 1.0 T equipment. The study was conducted in 20 volunteers and 7 patients with suspected pulmonary embolism (PE). To reduce artifacts caused by cardiac and respiratory motion, MR images were acquired in volunteers using two-dimensional (2D), gradient-recalled echo (GRE), breath-hold techniques, and three-dimensional (3D) acquisitions. Sagittal thin (6-MM) segmented k-space 2D sections obtained with cardiac gating during systole (turboFLASH, TR/TE9/6 ms, 14 segments of 9 lines) and incremented flip-angles (TONE), and 50-mm 3D volume TONE acquisitions with 32 partitions (FISP, TR/TE34/10ms) were successively performed. In the second phase of the study, patients were examined only with the 3D technique. Images of volunteers were qualitatively and quantitatively analyzed. S/N ratios were statistically compared by means of the paired-sample Wilcoxon ranked-signed test, a value of p < .05 being significant. In volunteers, 3D acquisitions displayed significantly more segment-order pulmonary arteries on average than did 2d acquisitions displayed significantly more segment-order pulmonary arteries on average than did 2D acquisitions (2.95 +/- 0.64 vs. 2.2 +/- 0.85, respectively; p < .01). Moreover, the signal intensity of arteries within the lungs was less homogeneous in the 2D than in the 3D technique, with a signal intensity ratio between peripheral and proximal arteries of 63% +/- 7% and 73% +/- 2%, respectively (p < .05). In patients, no erroneous diagnoses were obtained using the 3D technique. 3D images of normal lungs provide MR angiograms of better quality than do 2D images, and require less contribution from subjects because they are performed in free breathing. Ongoing improvements in MR sequences and further studies are now necessary to assess the value of 3D TONE MRA in the diagnosis of PE.

[Role of MRI in pre-clinical breast lesions. Preliminary results]. / Place de l'IRM dans les lésions mammaires infra-cliniques. Résultats préliminaires.

The purpose of this study is to assess the value of MR imaging in nonpalpable breast tumors. Thirty-two women whose mammogram showed opacity, architectural distorsionor microcalcifications, underwent preoperative dynamic MR studies with quantitative evaluation of contrast enhancement. MR showed early contrast enhancement in 10/12 breast cancers (2 false-negative) and in 6/20 benign lesions (false-positive). Quantitative study of contrast enhancement does not allow clear differentiation between these lesions. Further studies are required to confirm these findings and purpose MR imaging for aid in detection of carcinomas. Multifocal sudden enhancements are possible in benign lesions and incite to limit MR to focal mammographic abnormalities.

Focal nodular hyperplasia of the liver: assessment with contrast-enhanced TurboFLASH MR imaging.

Twenty-two patients with 25 cases of focal nodular hyperplasia (FNH) proved with pathologic study were imaged with a TurboFLASH (fast low angle shot) sequence combined with bolus administration of gadolinium tetraazacyclododecanetetraacetic acid (DOTA), spin-echo (SE) T2-weighted sequences, and postcontrast T1-weighted sequences. FNH-liver signal-difference-to-noise ratios were quantified; the features of the central scar were qualitatively analyzed. On SE T2-weighted images, all FNHs were hyperintense; in two cases the central scar exhibited a high signal intensity associated with hypointense areas corresponding to fibrous tissue within the branches of the scar. Unenhanced TurboFLASH images always demonstrated the FNHs as hypointense and always depicted the central scar as a hypointense area within the lesion. After bolus injection, arterial enhancement of FNH was clearly seen, and in 10 of 25 lesions, enhancement within the scar was seen 40-80 seconds after injection. Both unenhanced and enhanced TurboFLASH sequences produced the best signal-difference-to-noise ratios in comparison with T2-weighted images.