MR-guided placement of a temporary vena cava filter: technique and feasibility.

The purpose of this paper was to investigate the feasibility of MR-guided insertion of a temporary vena cava filter on an open low-field imager. In vivo procedures were performed on four anesthetized pigs using a common nonferromagnetic temporary vena cava filter and a special prototype guidewire developed for vascular interventions guided by low-field MRI. Breath-hold spoiled gradient-echo sequences (fast low-angle shot [FLASH]) with flow compensation were used for position monitoring of the passively visualized intravascular devices. Using the described technique and equipment, all steps of the procedure were feasible in the MR unit. Practicability of the procedure seems to be sufficient for clinical purposes but was inferior compared to the conventional technique of filter placement.

HASTE MR imaging: description of technique and preliminary results in the abdomen.

HASTE (Half fourier Single-shot Turbo spin-Echo) is a single-section T2-weighted sequence that acquires images in less than 1 second. Images are breathing independent and possess a variety of other features useful for imaging the abdomen. The design of this technique is described. Clinical studies of 38 consecutive patients were performed using this technique. HASTE images were considered good in 28 and fair in 10 patients, including five patients who could not suspend respiration. Definition of liver and bowel was particularly clear.

Time-of-flight method of MR angiography.

Results of preliminary clinical studies of time-of-flight MR angiography indicate that this technique can provide accurate reproducible flow images in different anatomic regions of the body. A unique advantage of this technique is its capacity to provide multiple projections of complex vascular abnormalities with a single data acquisition. This may increase both the sensitivity and specificity of MR angiography in some patients with cerebrovascular diseases. Preliminary clinical results suggest that time-of-flight MR angiography can screen for identification of normal vasculature as well as stenosis and/or occlusions produced by atherosclerotic disease.

Breath-hold T2-weighted sequences of the liver: a comparison of four techniques at 1.0 and 1.5 T.

T2-weighted images are considered the most sensitive for lesion detection at high field; however, long imaging time is problematic. Accordingly, the authors compared four breath-hold T2 or T2* weighted sequences comprising T2*-weighted FLASH, T2*-weighted PSIF, T2-weighted rapid spin echo (RASE), and T2-weighted Turbo-FLASH (Turbo) in 20 different healthy volunteers, 10 at 1.0 T and 10 at 1.5 T with reference to regular T2-weighted spin echo. Images were evaluated quantitatively by liver signal to noise (S/N) and spleen-liver signal difference to noise (SD/N) ratios and qualitatively for presence of artifacts and image quality. Data were evaluated for 1.0 T and 1.5 T separately and combined. In the combined evaluation, T2*-FLASH had good S/N (23.1 + 5.1) but low SD/N (2.9 + 1.7) and suffered from susceptibility artifacts. T2* PSIF had good S/N (28.1 + 10.0) and moderate SD/N (6.0 + 2.4), but occasionally had heterogeneous signal intensity. Flow signal void was an attractive feature. T2 RASE had very low S/N (4.4 + 1.9) and low SD/N (2.3 + 1.1) and suffered from flow artifacts. T2-Turbo had good S/N (24.6 + 8.6) and SD/N (8.9 + 2.5). Flow signal void was present, but small matrix size decreased image quality. The results of our study suggest that T2*-PSIF and T2-Turbo have good S/N and SD/N and fair image quality which may be clinically useful for breath-hold T2-weighted sequences of the liver.

MR imaging of the pancreas at high field strength: comparison of six sequences.

The authors compared six MR sequences comprising conventional breath-hold [rapid spin echo (RASE) repetition time (TR) 240 ms/echo time (TE) 8 ms/90 degrees, fast low angle shot (FLASH) 130/4.5/80 degrees, TurboFLASH 6.5/3.5/8 degrees], fat suppressed regular spin echo (FS SE 330/15/90 degrees), and two combined fat suppressed breath-hold sequences (FS FLASH 130/8/80 degrees, FS RASE 240/10/90 degrees) for studying the normal pancreas. Sequences were selected on the basis of features desirable for demonstrating the pancreas, particularly absence or decrease in artifacts and improved dynamic range of intraabdominal tissue signal intensities. Ten normal volunteers were studied, six at 1.5 T and four at 1.0 T, and comparison was made to regular short TR/TE SE. Quantitative pancreas signal-to-noise (S/N) and pancreas fat-to-noise (SD/N) measurements and qualitative evaluation of overall resolution and artifacts were determined. Fat suppressed FLASH had the highest S/N (44.1 +/- 10.8, p less than 0.0001) and SD/N (35.0 +/- 11.9, p less than 0.0001), and seven studies were considered good or very good. Fat suppressed SE had good S/N (32.6 +/- 7.7) and SD/N (19.0 +/- 3.6), and eight FS SE studies were considered good or very good. Among the nonsuppressed sequences, FLASH had the best combination of quantitative and qualitative measurements. Our results suggest that fat suppression may be important for studying the pancreas and that nonsuppressed FLASH may be a reasonable alternative.

T1-weighted sequences for MR imaging of the liver: comparison of three techniques for single-breath, whole-volume acquisition at 1.0 and 1.5 T.

The authors compared three T1-weighted magnetic resonance imaging techniques that acquire images encompassing the entire liver in one breath hold. Twenty healthy volunteers were imaged--10 at 1.0 T and 10 at 1.5 T--and the results compared with those of regular short repetition time/echo time spin-echo imaging. Rapid acquisition spin echo was resistant to artifacts and had good image quality but had the lowest liver signal-to-noise (S/N) and spleen-liver signal-difference-to-noise (SD/N) values. Fast low-angle shot (FLASH) had the highest S/N and SD/N, very good image quality, and only mild artifacts. TurboFLASH had good S/N and SD/N, but reduced matrix size decreased image quality. All three sequences had better SD/N than regular spin echo, and FLASH and TurboFLASH had higher S/N. On the basis of this study, the FLASH sequence appears the most attractive for T1-weighted breath-hold imaging.

Carotid-CNS MR flow imaging.

The authors present their 1-year experience with the use of 3DFT, time-of-flight MR angiography for the evaluation of vascular diseases of the head and neck. Their experience with over 150 patients indicates that this examination may be performed in conjunction with standard spin-echo imaging with only a minimal increase in patient examination time. This combined examination is most applicable to atherosclerotic disease of the carotid bifurcation, arterial occlusions of the primary and secondary branches of the intracranial circulation (particularly in pediatric patients such as those following ECMO or with sickle cell anemia), and patients with saccular berry aneurysms. This type of static, angiographic technique adds little to standard spin-echo imaging in patients with arteriovenous fistulae, neoplasms, and giant intracranial aneurysms. Limitations of the present technique include the inability to visualize slow flow lesions (e.g., giant aneurysms) and selected high flow states (arteriovenous fistulae, some severe stenoses).

Intracranial circulation: pulse-sequence considerations in three-dimensional (volume) MR angiography.

The technique and feasibility of magnetic resonance (MR) angiography of intracranial vessels were studied in 35 healthy volunteers. Variations in image orientation, repetition time (TR), and flip angle were evaluated to determine their effects on flow-related enhancement. Gradient modifications--including echo time (TE), motion compensation, bandwidth, and field of view--were also studied in an effort to reduce motion-induced phase shifts. Results indicated that a FISP (fast imaging with steady precession) sequence with a TR of 50 msec, TE of 15 msec, velocity compensation in the read and section-select directions, acceleration compensation in the read direction, anisotropic volume, and a 1.25-mm partition thickness produced three-dimensional angiographic MR images that were accurate and reproducible in the depiction of the major intracranial vessels. Difficulties with field of view, persistent signal void secondary to higher-order motion, and spatial resolution remain major problems requiring additional study.

Intracranial circulation: preliminary clinical results with three-dimensional (volume) MR angiography.

The authors assessed the clinical utility of a magnetic resonance angiography technique in the evaluation of intracranial circulation. Eighteen patients with a low likelihood of cerebrovascular disease (control group) and 40 patients with suspected cerebrovascular disease were imaged with a FISP (fast imaging with steady precession) sequence (repetition time of 50 msec, echo time of 15 msec, velocity compensation in the read and section-select directions with acceleration compensation in the read direction, 15 degrees anisotropic volume, and a 1.25-mm partition thickness). Ninety-four percent of images in the control group and 72% of images in the group with cerebrovascular disease were considered useful for diagnosis. This technique can provide accurate images of intracranial circulation and can be performed in conjunction with two-dimensional spin-echo or gradient-echo imaging. It was most useful in the evaluation of patent intracranial aneurysms, vessel displacement, and large-vessel occlusive disease. Disadvantages included limited field of view, persistent signal voids, limited spatial resolution, and inadequate depiction of lesions with slow flow.

MR angiography with gradient motion refocusing.

In typical spin echo (SE) sequences vascular structures can range from low to high in signal intensity, depending on both velocity distribution and imaging parameters. Control of this contrast spectrum is needed to permit consistent blood vessel evaluation. In this paper, methods for high resolution vascular magnetic resonance imaging which are based on additional gradient pulses to enhance flow and minimize flow artifacts are examined. The gradient motion refocusing technique is applied to both SE and gradient echo sequences. Vascular structures are clearly delineated over long distances by using thick slices or three-dimensional acquisition techniques. Preliminary experience in volunteers and patients indicates that these methods improve visualization of blood vessels, correctly identifying vascular disease.