Fast Low-angle Shot MR Imaging Research Articles

Selective water excitation for faster MR imaging of articular cartilage defects: initial clinical results.

Our objective was to compare a water-excitation (WE) 3D fast low-angle shot (FLASH) MR sequence for faster imaging of articular cartilage defects of the knee to a conventional fat-saturated (FS) 3D FLASH MR sequence. This prospective study included 16 knees of 16 patients with suspected cartilage lesions. The MR imaging in transverse and sagittal planes included (a) FS 3D FLASH (TR/TE: 45 ms/11 ms, scan time 8 min, flip angle 50 degrees), and (b) WE 3D FLASH (TR/TE: 28 ms/11 ms, scan time 4 min 58 s, flip angle 40 degrees). For each sequence signal-to-noise ratios (SNR) and contrast-to-noise ratios (CNR) were quantified. The detected cartilage lesions were evaluated using a semi-quantitative four-scale scoring system (grades 0-III). The data were compared between the sequences using the paired Student's t-test. No statistically significant differences between the sequences were found for SNR, CNR, and cartilage defect grading (p=0.14-0.8). The WE 3D FLASH MR imaging seems to be promising for fast imaging of articular cartilage lesions of the knee.

Breath-hold FLASH and FISP Cardiovascular MR Imaging: Left Ventricular Volume Differences and Reproducibility

To compare fast imaging with steady-state precession (FISP) and fast low-angle shot (FLASH) magnetic resonance acquisitions to quantify left ventricular volumes, mass, and function and to determine if the two techniques are comparable. Left ventricular volume studies were performed in 10 patients with heart failure and in 10 healthy subjects by using FISP and FLASH imaging. Identical section positions were used for section-by-section contour comparisons. Manual analysis was performed by two experienced observers. The study was repeated on a different day and interobserver and interstudy reproducibility assessed. With FISP, end-diastolic volume was larger (healthy subjects: +18 mL [13%], P <.001; patients: +6 mL [3%], not significant), end-systolic volume larger (healthy subjects: +9 mL [17%], P =.001; patients: +8 mL [6%], P =.001) and left ventricular mass smaller (healthy subjects: -25 g (19%), P <.001; patients: -21 g (11%), P <.001). There were no significant differences in ejection fraction. Both sequences had excellent interstudy and interobserver reproducibility, with statistically better reproducibility for interstudy healthy-subject ejection fraction on FISP images (P =.05). Section-by-section analysis determined that at FISP, endocardial contours were drawn larger and the epicardial contours smaller than on FLASH images. FISP enabled better delineation of epicardial fat from myocardium, of blood-myocardium interface in areas of trabeculation or papillary muscles, and of the atrioventricular ring. FISP produces small but significantly higher left ventricular volume measurements, as compared with FLASH imaging. FLASH imaging and FISP have similar reproducibility.

Single breath-hold left ventricular volume measurement by 0.3-sec turbo fast low-angle shot MR imaging.

In this study, we measured global cardiac parameters during a single breath-hold using T1-weighted single-shot turbo fast low-angle shot (turboFLASH) MR imaging. Ten cardiac patients who had undergone cardiac catheterization were examined with MR imaging to evaluate cardiac function. On average, 10 slices, which covered enough of the heart to measure the left ventricular volume, were acquired in a single breath-hold. During the acquisition of each slice, T1-weighted turboFLASH MR imaging with inversion recovery of both end diastole and end systole was sequentially accomplished twice in the same R-R interval. The end-diastolic volume, end-systolic volume, and ejection fraction calculated from MR images were compared with data obtained from left ventriculography. In these 10 cardiac patients, the ejection fraction calculated from turboFLASH MR images showed a good correlation with the measurements from radiographic left ventriculography (r = .87). The inter- and intraobserver variabilities in calculating the ejection fraction were 7.8% and 4.7%, respectively. Single-shot turboFLASH MR imaging with inversion recovery can provide data for the quantification of the ejection fraction during a single breath-hold.

Reconstruction of three-dimensional images of the pharynx in patients with sleep apnea using three-dimensional fast low-angle shot MR imaging.

turbo-FLASH sequences, image slices are 20 mm at largest. To analyze the entire pharynx dynamically, imaging has to be performed from slightly different positions, and then each region has to be displayed in cine mode. Also, by imaging from one direction on 2D turbo-FLASH, obtaining a stereoscopic image of the obstructed region is impossible. For example, imaging in the sagittal plane will not show movement toward the lateral

Effect of radiation on blood volume in low-grade astrocytomas and normal brain tissue: quantification with dynamic susceptibility contrast MR imaging.

The purpose of this study was to determine whether it is possible to measure radiation-induced changes in blood volume in low-grade astrocytomas and in normal brain tissue. The passage of a bolus of gadopentetate dimeglumine was monitored on a series of 55 T2*-weighted simultaneous dual fast low-angle shot MR images with a standard 1.5-T MR imaging system. Absolute blood volumes were calculated as the area under the tissue concentration-time curve in regions of interest and normalized to the arterial input function. We performed 41 examinations on 19 patients with grade II astrocytomas. For comparison, 13 patients were studied after whole-brain irradiation. A reduction in blood volume (mean +/- SD in milliliters per 100 g) within the tumors from 12.2 +/- 8.7 to 6.5 +/- 5.3 after fractionated conformation radiotherapy was detected, although there was no consistent pattern in different patients. An insignificant reduction was noted in normal gray (9.2 +/- 2.8 to 7.4 +/- 3.2) and white (4.4 +/- 1.9 to 4.1 +/- 2.3) matter outside the target volume. Conversely, we observed a significantly lower blood volume in gray (6.3 +/- 1.2) and white (3.1 +/- 1.0) matter after whole-brain radiotherapy. Our results show that a reduction of blood volume in astrocytomas and normal brain tissue after radiotherapy can be quantified by use of dynamic susceptibility contrast MR imaging. Thus, functional monitoring of tumor response and of normal tissue effects becomes possible.

Benign and malignant musculoskeletal lesions: dynamic contrast-enhanced MR imaging--parametric "first-pass" images depict tissue vascularization and perfusion.

To assess the diagnostic value of parametric magnetic resonance images that display the first pass (FP) of gadopentetate dimeglumine. Dynamic, contrast material-enhanced, fast low-angle shot MR imaging was performed of 100 musculoskeletal lesions in 36 female and 64 male patients (aged 9-81 years [mean, 34 years]). The highest slope value of the time-intensity curve was calculated pixel by pixel and displayed on the FP image, with a gray-scale value equal to the slope value. Tissue vascularization and perfusion were evaluated at pathologic and angiographic examinations in 33 lesions. A significant difference (P < .001) was found between the FP slope values of benign (mean, 36.2% per second) and malignant (mean, 67.4% per second) lesions. FP images depicted tissue vascularization and perfusion rather than benignity or malignancy, because there is an overlap in the slope values of highly vascular benign lesions and malignant lesions. FP images provide diagnostically useful information by depicting tissue vascularization and perfusion.

Findings of focal hepatic lymphoma on Gd-DOTA enhanced dynamic FLASH MR imaging

Hepatic involvement of lymphoma can be either primary or secondary. Primary hepatic lymphoma is a rare tumor and tends to produce focal liver lesions, whereas secondary lymphomatous infiltration is a diffuse disease which occasionally presents with a focal mass. Although some reports regarding ultrasonic and computed tomography (CT) features exist in the literature [l-4] very few focal hepatic lymphomas have been investigated by magnetic resonance imaging (MRI) [5-71, and no reports were found concerning lymphoma examined by contrast enhanced dynamic FLASH (fast low angle shot) pulse sequences MRI. Radiological features of a rare manifestation of secondary hepatic lymphoma are presented, with emphasis on Gd-DOTA (gadolinium-diethylenetriamine tetra acetate) enhanced dynamic FLASH MRI.

Adrenal masses: differentiation with chemical shift, fast low-angle shot MR imaging.

The purpose of this study was to investigate the efficacy of chemical shift, fast low-angle shot (FLASH) imaging at 1.5 T to differentiate adrenal masses. The materials included patients with adrenocortical adenomas (n = 32), metastatic tumors (n = 17), and pheochromocytomas (n = 4). FLASH images were obtained with breath holding at 100/11 (repetition time msec/echo time msec) (out-of-phase images) and 100/13 (in-phase images) and a flip angle of 20 degrees to differentiate the lipid contents in the adrenal tumors. The signal-intensity (SI) indexes of adrenal masses ([SI on IP - SI on OP]/[SI on IP x 100]), where IP = in-phase image and OP = out-of-phase image, were calculated. All adenomas had SI indexes of more than 5%, while SI indexes of metastatic tumors and pheochromocytomas were less than 5%, with accuracy of 100% in the differentiation between adenoma and nonadenoma. Hyperfunctioning adenoma, however, could not be differentiated from nonhyperfunctioning adenoma. Chemical shift FLASH imaging was superior to the calculated T2 in the characterization of adrenal masses.

Incremental flip angle snapshot FLASH MRI of hepatic lesions: improvement of signal-to-noise and contrast.

Incremental flip angle (IFA) snapshot fast low angle shot (FLASH) is a new modification of inversion recovery snapshot FLASH MR imaging. The method changes the flip angle incrementally from low to high during data acquisition and was applied in the evaluation of 16 focal hepatic lesions in 10 patients. Sequence comparisons were performed with a fixed flip angle inversion recovery snapshot FLASH sequence (standard), a T1- and T2-weighted spin-echo (SE) sequence, and a T1-weighted breath-hold FLASH sequence. Whereas snapshot FLASH images in both pulse sequences were free from physiological motion artifacts, SE and FLASH images showed respiratory artifacts in some patients. Quantitative analysis of IFA snapshot FLASH images at low hepatic and low lesion signal revealed both superior lesion-liver signal-difference-to-noise ratio (SD/N) and superior contrast compared with standard snapshot FLASH without additional artifacts. Unless motion artifacts were evident, SE and FLASH images showed a higher anatomic resolution but lower SD/N and lower contrast than IFA snapshot images. Because of its superior SD/N and contrast, IFA snapshot FLASH will likely widen the application of fast MR imaging techniques.

Differentiation between hepatoma and hemangioma with inversion-recovery snapshot FLASH MRI and Gd-DTPA.

Thirty-eight patients with focal liver tumors (20 hepatomas, 18 hemangiomas) were studied by dynamic sequential inversion recovery (IR) snapshot fast low angle shot (FLASH) MR imaging with Gd-DTPA. Immediately after 0.05 mmol/kg Gd-DTPA was administered intravenously for 2-3 s followed by flushing with normal saline for 4-5 s, 10 images were obtained in the first 20 s (time zero is the end of flush, early phase). Then, one image every 30 s from 1 to 3 min (late phase) and images at 5 min and 7 min (delayed phase) were obtained serially. Hepatomas showed total enhancement in 18 of 20 patients in the early phase, and isointense or low intensity enhancement with respect to the surrounding liver parenchyma in 18 patients in the late to delayed phases. Hemangiomas showed peripheral enhancement in 14 patients in the early phase, but did not show total enhancement (except for two hemangiomas less than 3 cm in size) in the early phase, and showed high intensity enhancement in 15 patients in the late phase. Ninety percent of hepatomas and 82% of hemangiomas showed their characteristic enhancement patterns in the early to delayed phases. We conclude that dynamic sequential IR snapshot FLASH MR images enhanced with Gd-DTPA can facilitate differentiation between hepatomas and hemangiomas.

Musculoskeletal neoplasms: fast low-angle shot MR imaging with and without Gd-DTPA.

In 121 patients, image contrast and contrast-to-noise ratios (C/Ns) obtained with gadolinium diethylene-triaminepentaacetic acid (DTPA)-enhanced and nonenhanced fast low-angle shot (FLASH) sequences were compared with those achieved with spin-echo (SE) sequences. Among FLASH sequences, contrast between neoplasms and muscle was sufficient with a flip angle of 90 degrees following administration of Gd-DTPA but was 61% lower than that with the T2-weighted SE sequence. High contrast levels were obtained between tumors and bone marrow or fat in sclerotic, calcified, and fibrotic lesions with the use of a flip angle of 90 degrees and in lytic lesions with a flip angle of 10 degrees, reaching 66% of the contrast level obtained with the T1-weighted SE sequence. C/N between tumor and surrounding tissue was always significantly lower with FLASH sequences than with the ideal SE sequences usually used for tumor delineation. Thus, a replacement of the T2-weighted SE sequences by FLASH sequences cannot be recommended. A replacement of the T1-weighted SE sequences by FLASH sequences seems possible but does not significantly reduce examination time.

Temporomandibular joint motion: efficacy of fast low-angle shot MR imaging.

One hundred temporomandibular joints (TMJs) in 62 symptomatic patients and two healthy volunteers were prospectively examined with T1-weighted spin-echo and fast low-angle shot (FLASH) gradient-echo sequences. FLASH sequences were performed during opening of the mouth and provided a pseudodynamic depiction of TMJ motion. In 49 joints, FLASH sequences provided information that potentially influenced the therapeutic approach to the patient. This information was not available from standard T1-weighted images. The additional information fell into four general categories: (a) determination of the exact time of disk recapture, (b) distinction of normal variants from pathologically displaced disks, (c) clarification of discrepancies found between clinical examination results and T1-weighted images, and (d) elimination of motion degradation of images.

SYS-FLASH. Systemic saturation in FLASH MR imaging.

A simple modification of FLASH (Fast Low Angle SHot) MR imaging, which results in a variable reduction of the intensity of flowing blood and flow artifacts in transaxial tomograms, is reported. Here a nonselective radiofrequency pulse of variable flip angle is used before the acquisition of each projection in FLASH imaging to saturate flowing blood within the whole volume (SYstemic Saturation = SYS-FLASH).

Three-dimensional FLASH MR imaging of thorax and abdomen without triggering or gating

FLASH (fast low-angle shot) imaging is a new technique for rapid two- and three-dimensional (2D and 3D) MRI with high signal-to-noise, high spatial resolution and low sensitivity to motions. In particular, transverse magnetizations are strongly dephased by rapid motions with time constants of several milliseconds. They are not refocused by the gradient echo signal used for FLASH imaging and therefore do not contribute to the image. Artifacts due to motions with longer time constants are averaged out by multiple data acquisitions either because of a statistical nature of the dynamic process itself or, in the case of periodic motions, because of the incoherent data sampling with respect to the time period. In general, the large number of signal acquisitions necessary for a true 3D data set turns out to be sufficient for this purpose. Since 128 3 3D FLASH MR images require measuring times of only 4 min, even two or four accumulations of entire 3D data sets remain within acceptable clinical measuring times. The method for the first time depicts high-quality three-dimensional views of the thoracic and abdominal anatomy in enddiastole and endexspiratory rest without the need of gating or triggering. 3D FLASH images will be of great medical utility, especially in cases where topographic information is needed as, for example, for radiation therapy and angiography planning or preoperative diagnosis.