Cervical spine: MR imaging with a partial flip angle, gradient- refocused pulse sequence. Part II. Spinal cord disease

A magnetic resonance imaging pulse sequence (GRASS) with a short repetition time (TR), short echo time (TE), partial flip angle, and gradient refocused echo was prospectively evaluated for the detection of cervical cord disease that caused minimal or no cord enlargement in eight patients. Sagittal T2-weighted, cerebrospinal fluid (CSF)-gated images and sagittal and axial GRASS images were obtained in all patients. The following GRASS parameters were manipulated to determine their effect on signal-to-noise ratio (S/N) and contrast: flip angle (4 degrees-18 degrees), TR (22-50 msec), and TE (12.5-25 msec). Flip angle had the greatest effect on S/N and contrast. There were no differences between axial and sagittal imaging for the spinal cord or lesion. However, because the signal intensity of CSF did differ on sagittal and axial images and because this influenced the conspicuity of lesions, there was a difference in the useful flip angle range for axial and sagittal imaging. No one set of imaging parameters was clearly superior, and in all patients, the gated image was superior to the sagittal GRASS image in lesion detection. GRASS images should be used in the axial plane primarily to confirm spinal cord disease detected on sagittal CSF-gated images. For this, a balanced approach is suggested (TR = 40 msec, TE = 20 msec, with flip angles of 4 degrees-6 degrees for sagittal and 6 degrees-8 degrees for axial imaging).     

Fast MR imaging of liver metastasis using FLASH and FISP--optimal sequences for T1- and T2*-weighted images

This paper deals with a study to obtain the optimal sequence of gradient echo (GE) for T1- and T2*-weighted images similar to T1- and T2-weighted images of spin echo (SE). Two GE sequences, fast low angle shot (FLASH) and fast imaging with steady-state precession (FISP), were performed in 15 cases of liver metastasis in various combination of flip angle (FA), repetition time (TR), and echo time (TE). The optimal combinations were summarized as follows: 1) T1-weighted FLASH image with FA of 40 degrees, TR of 22 msec and TE of 10 msec, 2) T1-weighted FISP image with FA of 70 degrees, TR of 100 msec, TE of 10 msec, 3) both T2*-weighted FLASH and FISP images with FA of 10 degrees, TR of 100 msec and TE of 30 msec. Not only to provide the adequate T1- and T2*-weighted images but also to enable breath-holding MR imaging, GE sequences can optionally take place SE in cases of deteriorated images caused by moving artifacts. Other applications support the re-examination and further detailing when required, conveniently rather in short time.     

Low flip angle spin-echo MR imaging to obtain better Gd-DTPA enhanced imaging with ECG gating]

ECG-gated spin-echo imaging (ECG-SE) can reduce physiological motion artifacts. However, ECG-SE does not provide strong T1-weighted images because repetition time (TR) depends on heart rate (HR). We investigated the usefulness of low flip angle spin-echo imaging (LFSE) in obtaining more T1-dependent contrast with ECG gating. in computer simulation, the predicted image contrast and signal-to-noise ratio (SNR) obtained for each flip angle (0-180 degrees) and each TR (300 msec-1200 msec) were compared with those obtained by conventional T1-weighted spin-echo imaging (CSE: TR = 500 msec, TE = 20 msec). In clinical evaluation, tissue contrast [contrast index (CI): (SI of lesion-SI of muscle)2*100/SI of muscle] obtained by CSE and LFSE were compared in 17 patients. At a TR of 1,000 msec, T1-dependent contrast increased with decreasing flip angle and that at 38 degrees was identical to that with T1-weighted spin-echo. SNR increased with the flip angle until 100 degrees, and that at 53 degrees was identical to that with T1-weighted spin-echo. CI on LFSE (74.0 +/- 52.0) was significantly higher than CI on CSE (40.9 +/- 35.9). ECG-gated LFSE imaging provides better T1-dependent contrast than conventional ECG-SE. This method was especially useful for Gd-DTPA enhanced MR imaging.     

Lymph nodes of the neck. Diagnosis using magnetic resonance with gradient-echo technique

As for the pathologic conditions of neck lymph nodes, the clinician needs to know if the involved node is reactive, phlogistic, or neoplastic in nature. If accurate tumor staging is required, imaging techniques play a fundamental role. Our study was aimed at assessing the actual role of MR imaging in the evaluation of neck lymph node involvement. The study was performed using an MR Max Plus by General Electrics operating with an 0.5 T superconductive magnet. We employed gradient-echo (GE) pulse sequences with TR 500, TE 15 ms and 90 degrees flip angle for T1-weighted images, and with TR 500, TE 30 ms and 25-30 degrees flip angles for T2-weighted images; for Pd-T2-weighted images, TR was 520, TE 30 ms, and flip angles were 40-45 degrees. The results were correlated with histopathologic findings obtained at biopsy. The advantages of GE sequences were: 1) whole neck imaging--thus saving time, and reducing radiation dose and contrast media; 2) optimal anatomical and topographic evaluation of the lesion; 3) imaging of the longitudinal diameter of the node; 4) higher sensitivity for lymph node tissue modifications; 5) imaging of necrosis, hemorrhage, and/or fibrosis. GE sequences were especially useful for accurate tumor staging, in the follow-up, and to verify response to therapy. However, even though MR imaging has proven to have high sensitivity, its specificity was similar to that of contrast-enhanced CT. Further studies with the use of paramagnetic contrast media are needed to solve these problems.     

Cervical spine: MR imaging with a partial flip angle, gradient- refocused pulse sequence. Part I. General considerations and disk disease

A magnetic resonance imaging pulse sequence with a short repetition time (TR), short echo time (TE), partial flip angle, and gradient refocused echo was evaluated for the detection of cervical disk disease in a prospective study of 90 patients. These parameters were manipulated to adjust signal-to-noise ratio (S/N) and contrast: flip angle (3 degrees-18 degrees), TR (22-60 msec), and TE (12.5-25 msec). Flip angle had the greatest effect on S/N and contrast; its effect differed between axial and sagittal imaging. Cerebrospinal fluid S/N reached a peak at a smaller flip angle in sagittal imaging than in axial imaging. The useful range of flip angles depended on TR. Increasing TR had minimal direct effect on S/N or contrast, but because a longer TR allowed the use of larger flip angles for both axial and sagittal imaging, higher S/N could be achieved with similar contrast. This effect of increasing TR had to be balanced against increased imaging time and increased probability of motion artifact. Increasing TE decreased S/N, increased contrast, and increased magnetic susceptibility artifacts. For the diagnosis of cervical disk disease, the best sequence appears to be one with a very short TR, short TE, and small flip angles within a narrow range.     

Thin-section, three-dimensional Fourier transform, steady-state free precession MR imaging of the brain.

The authors evaluated a three-dimensional Fourier transform implementation of a very short repetition time (TR) (24 msec), steady-state free precession (SSFP) pulse sequence for clinical imaging of the brain and compared it with a conventional two-dimensional Fourier transform long TR/echo time (TE) spin-echo sequence. First, the optimal flip angle of 10 degrees for generating images with contrast similar to that of long TR/TE spin-echo images was determined. Then, 29 patients with suspected brain lesions were studied with both techniques. Although the SSFP images did not exhibit the magnetic susceptibility artifacts that plague other rapid-imaging techniques, the conspicuity of most parenchymal lesions was often less than that on the spin-echo images. Also, the visibility of paramagnetic effects, such as the low signal intensity of brain iron, was less obvious at SSFP imaging. These substantial limitations may relegate the SSFP sequence to an adjunctive role, perhaps mainly demonstration of the cystic nature of mass lesions, because of its extreme sensitivity to slow flow.     

Partial flip angle MR imaging

Theoretical analysis predicts that performing magnetic resonance (MR) imaging with partial (less than 90 degrees) flip angles can reduce imaging times two- to fourfold when lesions with elevated T1 values are being examined. This time savings occurs because repetition time (TR) is reduced when imaging is performed with partial flips. Partial flip MR imaging can also improve signal-to-noise ratio (S/N) in fast body imaging. For this study, analytical tools were used to predict image contrast and S/N for short TR, partial flip sequences. Experimental implementation of the short TR, partial flip sequences that analytical work had predicted would be optimal supported the analytical predictions and demonstrated their validity. Partial flip MR imaging is applicable to reducing imaging time only when the ratio of signal differences to noise exceeds threshold values in conventional MR images. Partial flip sequences can be used to advantage in MR imaging of both the head and the body, and the observed effects are predictable through theoretical analysis.     

Musculoskeletal MR imaging: turbo (fast) spin-echo versus conventional spin-echo and gradient-echo imaging at 0.5 tesla

The value of T2-weighted fast spin-echo imaging of the musculoskeletal system was assessed in 22 patients with various neoplastic, inflammatory, and traumatic disorders. Images were acquired with high echo number (i.e., echo train length) fast spin-echo (FSE; TR 2000 ms, effective TE 100 ms, echo number 13, lineark-space ordering), conventional spin-echo (SE; TR 2000 ms, TE 100 ms) and gradient-echo (GRE) sequences (TR 600 ms, TE 34 ms, flip angle 25°). Signal intensities, signal-to-noise ratios, contrast, contrast-to-noise ratios, lesion conspicuousness, detail perceptibility, and sensitivity towards image artifacts were compared. The high signal intensity of fat on FSE images resulted in a slightly inferior lesion-to-fat contrast on FSE images. However, on the basis of lesion conspicuity, FSE is able to replace time-consuming conventional T2-weighted SE imaging in musculoskeletal MRI. In contrast, GRE images frequently showed superior lesion conspicuity. One minor disadvantage of FSE in our study was the frequent deterioration of image quality by blurring, black band, and rippling artifacts. Some of these artifacts, however, can be prevented using short echo trains and/or short echo spacings.     

MR imaging of joints: analytic optimization of GRE techniques at 1.5 T.

To clarify the choice of imaging parameters for optimal gradient-recalled echo MR scanning of joints, we analyzed the behavior of contrast-to-noise and signal-to-noise ratios for spoiled (i.e., fast low-angle shot [FLASH] or spoiled GRASS) and steady-state (i.e., gradient-recalled acquisition in the steady state [GRASS] or fast imaging with steady precession) techniques at 1.5 T. The analysis is based on tissue characteristics derived from spin-echo measurements of hyaline cartilage and synovial fluid signal in the patellofemoral joints of 11 volunteers. Separate analysis of contrast-to-noise and signal-to-noise ratios for multiplanar (long TR) acquisitions shows that these parameters are each improved compared with single-slice methods. At TRs greater than 250 msec, there is no significant difference in the contrast behavior of FLASH and GRASS. For optimal contrast-to-noise ratio (synovial fluid-cartilage), the best multiplanar sequence (for TE less than 23 msec) is with a short TE and a large flip angle (e.g., 400/9/73 degrees [TR/TE/flip angle]). If a single-scan or three-dimensional technique is desired, than a GRASS sequence at minimal TR and TE and intermediate flip angle (18/9/32 degrees) is best. For optimal signal-to-noise ratio (for both synovial fluid and hyaline cartilage), the best multiplanar sequence uses a short TE and an intermediate flip angle (e.g., 400/9/30 degrees). If a short TR, high signal-to-noise technique is desired, then GRASS (18/9/13 degrees) is superior to FLASH.     

MR imaging of the brain: preliminary results with a gradient echo sequence after paramagnetic contrast enhancement

In a series of 15 patients with contrast enhancing lesions of the brain, a gradient echo sequence (TR = 400 ms; TE = 10 ms; flip angle 90 degrees, one excitation) after IV administration of a paramagnetic contrast agent provided equal diagnostic information and image quality to conventional T1 weighted spin-echo images. Imaging time for one sequence could be reduced by two thirds to 1 min 45 sec. Susceptibility artifacts caused only slight image degradation in the bitemporal region and the region of the sella turcica. This gradient echo sequence can be useful in combination with paramagnetic contrast enhancement, as a complement to conventional T1 and T2 weighted spin-echo sequences.     

Time-reduced T2-weighted celebral MRI with optimized flip angles

This study was set up to see whether lowering the flip angle in proton density- and T2-weighted double-spin echo sequences allows for shortening of repetition time (TR) and imaging time without significant change of image quality. Ten patients with celebral white matter lesions were investigated with an 1.5 T MR scanner using a conventional long- TR double-spin echo sequence (TR = 2500 ms, TE = 15 and 70 ms) and reduced-TR double-spin echo sequences (TR = 1900 ms, TE = 15 and 70 ms) at flip angles of 90°, 80°, 70°, 60°, and 50°. Lowering the flip angle resulted in less T1-contrast and a relative increase of T2-contrast. At a flip angle of 70°, contrast-to noise ratios (NNRs) between lesions and brain, as well as image artifacts of the reduced-TR sequence (CNR: 22.4) were similar to the conventional long-TR sequence (CNR:21.1), while imaging time was shortened by about 25%. Offprint requests to: Peter Schubeus     

Contrast in rapid MR imaging: T1- and T2-weighted imaging

Partial saturation (PS) is an imaging technique that is useful in applications that require rapid image acquisitions (imaging time less than 1 min). Image contrast in PS imaging, as in other magnetic resonance methods, depends on the often conflicting effects of differences in proton density, T1, and T2. Previous analyses of pulse sequence optimization to maximize image contrast have assumed 90 degrees pulses and examined the effects of varying repetition times (TR) and echo times (TE). In this paper we present theoretical calculations and images made with a 0.6 T imager to show that the radiofrequency pulse tip angle alpha, and not the pulse sequence timing parameters, is the most important parameter for producing image contrast. For large tip angles (alpha greater than or equal to 60 degrees), contrast is primarily determined by differences in T1, but for small tip angles (alpha approximately equal to 25 degrees), contrast is primarily due to differences in T2. The T2-weighted images can be produced as quickly as T1-weighted images by using a small pulse angle and a long TE; it is not necessary to use a long TR to reduce the effects of T1 differences. Optimum pulse angles are calculated, and the potential advantages and disadvantages of T2-weighted and T1-weighted PS imaging are discussed.     

Low flip angle gradient echo magnetic resonance imaging of the cervical spine at 0.3 tesla

The influence of flip angle and TR on signal to noise ratio and contrast between cerebrospinal fluid (CSF) and cord was evaluated in cervical spine imaging in 5 volunteers, using gradient echo technique. All experiments were performed on a 0.3 tesla Fonar beta-3000 M scanner using solenoidal surface coils. The most useful sequence was considered to be TR/TE = 300/12 ms and 10 degrees flip angle. This sequence provided images with a 'myelographic appearance' with good delineation of cord, CSF and epidural space. The grey and white matter was also regularly visualized. The acquisition time was considerably shorter than would have been necessary if a long TR/TE spin echo sequence had been used to obtain the same contrast pattern and the sequence was not as sensitive to motion as was the spin echo sequence. The sequence was also evaluated in 10 patients with degenerative disease and in 5 with lesions in the cord. The gradient echo sequence was found to be equal to or better than short and long TR/TE spin echo sequences in demonstrating narrowing of the spinal canal and cord lesion. The drawback is the limited signal to noise ratio.     

Assessment of myocardial viability using delayed enhancement magnetic resonance imaging at 3.0 Tesla

OBJECTIVE: Cardiac magnetic resonance imaging (MRI) at 3.0 T has recently become available and potentially provides a significant improvement of tissue contrast in T1-weighted imaging techniques relying on Gd-based contrast enhancement. Imaging at high-field strength may be especially advantageous for methods relying on strong T1-weighting and imaging after contrast material administration. The aim of this study was to compare cardiac delayed enhancement (DE) MRI at 3.0 T and 1.5 T with respect to image quality, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) between infarcted and normal myocardium. MATERIALS AND METHODS: Forty consecutive patients with history of myocardial infarction were examined at 3.0 T (n = 20) or at 1.5 T (n = 20). Myocardial function was assessed using cine steady-state-free-precession (SSFP) sequences (TR 3.1 milliseconds, TE 1.6 milliseconds, flip angle 70 degrees , and a matrix of 168 x 256 at 1.5 T and TR 3.4 milliseconds, TE 1.7 milliseconds, flip angle 50 degrees and a matrix of 168 x 256 at 3.0 T), acquired in long- and short-axes views. DE images were obtained 15 minutes after the administration of 0.15 mmol of Gd-DTPA/kg body weight using a segmented inversion recovery prepared gradient echo sequence at 1.5 T (TR 9.6 milliseconds, TE 4.4 milliseconds, flip angle 25 degrees , matrix 160 x 256, bandwidth 140 Hertz/pixel) and at 3.0 T (TR 9.8 milliseconds, TE 4.3 milliseconds, flip angle 30 degrees , matrix 150 x 256, bandwidth 140 Hertz/pixel). For image analysis, standardized SNR and CNR measurements were performed in infarcted and remote myocardial regions. Two independent observers rated image quality on a 4-point scale (0 = poor image quality, 1 = sufficient image quality, 2 = good image quality, 3 = excellent image quality). RESULTS: High diagnostic image quality was obtained in all patients. Rating of mean image quality was 2.2 +/- 0.8 at 1.5 T and 2.5 +/- 0.6 at 3.0 T (P = 0.012) for observer 1 and 2.2 +/- 0.7 at 1.5 T and 2.6 +/- 0.6 at 3.0 T (P = 0.003) for observer 2, respectively. Interobserver agreement was good (kappa = 0.68 at 1.5 T and 0.78 at 3.0 T). SNR measurements yielded a mean SNR of 37.8 +/- 13.9/22.9 +/- 6.0 in infarcted myocardium (P < 0.001) and 5.6 +/- 2.2/5.9 +/- 2.4 in normal myocardium (P = 0.45) at 3.0 T/1.5 T, respectively. CNR measurements revealed mean values of 32.4 +/- 13.0/16.7 +/- 5.4 (P< 0.001) at 3.0 T/1.5 T, respectively. CONCLUSIONS: Delayed enhancement MRI at 3.0 T is feasible and provides superior image quality compared with 1.5 T. Furthermore, using identical contrast doses, increased SNR and CNR values were recorded at 3.0 T.     

Sodium imaging optimization under specific absorption rate constraint.

The concept of sodium imaging RF pulse parameter optimization for signal-to-noise ratio (SNR) under specific absorption rate (SAR) constraints is introduced. This optimization concept is unique to sodium imaging, as sodium exhibits ultrarapid T(2) relaxation in vivo, and involves minimizing echo time (TE). For 3D radial k-space acquisition, minimizing TE (and T(2) loss) requires minimizing the RF pulse length. SNR optimization also involves exploiting rapid T(1) relaxation with shortened repetition time (TR) values. However, especially at higher fields, both RF pulse length and TR are constrained by SAR, which is also dependent on the flip angle. Quantum mechanical simulations were performed for SAR equivalent sets of RF pulse length, TR, and flip angle. It was determined that an SNR advantage is associated with a spoiled steady-state approach to sodium imaging with radial acquisition even though significantly longer RF pulses (and TE) are required to implement this approach under the SAR constraint at 4.7T. This advantage, compared to RF pulse sequences implementing ultrashort echo times, 90 degrees flip angles, and longer repetition times, was confirmed in healthy volunteers (measured SNR increase of approximately 38%) and used to produce excellent quality sodium images of the human brain.     

Low flip-angle spin-echo imaging of the liver. Basic study and its application to hepatic space-occupying lesions]

Dependence on T1 contrast can be reduced by changing the excitation flip angle. Low flip-angle spin-echo imaging can reduce imaging time because repetition time (TR) is reduced. The authors assessed the efficacy of low flip-angle spin-echo images in phantoms and in liver. MR phantoms made from polyvinyl alcohol gel to model the properties of normal liver, HCC, and hemangioma were scanned with various flip angles at TR 2400 and 1200 msec. Measured signal intensities fitted well with theoretical values. The T1 contrast of signal intensity decreased as the flip angle was reduced, accompanied by a decrease in signal-to-noise ratio (S/N). Thirty patients with hepatic space-occupying lesions (23 with HCC, three with metastases and four with hemangioma) were studied by conventional SE (CSE) at 2400/60/2 (TR/TE/NEX [number of excitations]) (10 min 46 sec imaging time) and low flip-angle SE (LFSE) at 1200/60/30 degrees/2 (TR/TE/FA/NEX) (5:20) and/or 1200/60/30 degrees/4 (10:18). The sensitivity of CSE in detecting lesions was 93% (44/47). It was 92% (35/38) for LFSE with two NEX and 94% (34/36) for LFSE with four NEX pulse sequences. The contrast-to-noise ratio (C/N) for images (HCC/liver, hemangioma/liver) obtained by LFSE with four NEX was significantly higher than that for those obtained by CSE (4.8 vs 3.5, p less than 0.01; 13.4 vs 9.7, p less than 0.01, respectively). Although the C/N (lesion/liver) for LFSE with two NEX sequences was lower than that of CSE for any type of lesion (3.0 vs 3.5 for HCC; 5.1 vs 6.3 for metastases; 8.3 vs 9.7 for hemangioma), the difference was not significant. Although reducing the flip angle from 90 degrees to 30 degrees with two NEX resulted in a decrease in S/N (10.7 to 8.9 for HCC; 15.3 to 11.9 for metastases; 20.0 to 18.1 for hemangioma; 7.4 to 6.3 for normal liver; 10.7 to 10.1 for spleen), the difference was not significant. For hepatic space-occupying lesions, low flip-angle spin-echo imaging is useful to obtain T2-weighted images in a shorter imaging time without sacrificing lesion detectability.     

Cardiac MRI: comparison between single-shot fast spin echo and conventional spin echo sequences in the morphological evaluation of the ventricles

PURPOSE: Black blood single shot FSE sequences (Nffse) employ 180 degrees RF refocalisation pulses preceded by an inversion RF double pulse associated to presaturation pulses. The latter produce signal void of the external volume, and possible reduction of the field of view without wrap-around artifacts along the phase coding direction. The aim of our study was to compare the diagnostic possibilities of the Nffse sequences with those of conventional SE study of cardiac morphology. MATERIAL AND METHODS: Twenty-five patients (19 males and 9 females with age ranging from 20 to 54 years) presented findings suggesting right ventricular arrhythmogenic dysplasia. MR examinations were performed with a 1,5 T unit (GE Signa Horizon Echospeed 8.3, Milwaukee, USA) and Torso Phased Array coil positioned at thoracic level. The morphologic study was performed with SE multiphase-multislice ECG-gated sequences (TR: R-R, TE: 30 ms, FOV 320X250, matrix 160X256, slice thickness 10 mm, acquisition time about 5 minutes) and Single-Shot FSE Half Fourier sequences (TR: R-R, TE: 30 ms, flip angle 120 degrees, ETL 30-40, FOV 360X180, Phase FOV 0,5, VBW 64 MHz, slice tickness 10 mm, acquisition time about 10-12 seconds), by imaging along the long and short axis. The study was completed with Fast Gradient Echo sequences (TR: 9ms, TE: 8,2ms, flip angle 25 degrees, VBW 15,63 MHz, FOV 320X250, 10 mm slice thickness, matrix 128X256), subsequently assessed by cine-MR. In order to compare both sequences, two experienced radiologists performed an analysis of quantitative parameters (signal intensity ratio between fat and muscular interventricular septum) and qualitative parameters (double blind evaluation for the presence of cardiac and respiratory artifacts). RESULTS: The signal intensity ratio for the Nffse sequence images was 4.63 +/- 1.56 on the long axis and 7.69 +/- 2.46 on the short axis, whereas it was 3.17 +/- 0.64 on the long axis and 3,50 +/- 0,75 on the axis one for SE images, with a statistically significant difference (p<0,001 and p<0.002 for the long and short axis, respectively). The two radiologists evaluation of the magnitude of artifacts on the SE and Nffse images was similar only as regards the images with significant artefacts alone. Nffse images consistently afforded a detailed evaluation of the right ventricular wall, although blurring artifacts were more common than with good quality SE images. Presence of fatty infiltration of the right ventricle wall was observed in 5 out of 25 patients. In the remaining 20 patients no fatty substitution of the muscular wall of the right ventricle was observed. DISCUSSION AND CONCLUSIONS: The Nffse sequences provide a number of gated multiphase-multislice images, similar to that obtained by conventional SE sequences, in one breath-hold time interval. Due to high intrinsic contrast and reduction of motion artifacts, the Nffse sequences allow a good evaluation of the ventricular morphology and subepicardial and paracardiac adipose tissue. Image quality can be suboptimal due to blurring artifacts. Therefore Nffse sequences can be advantageously employed to image patients with suspected right ventricular arrhythmogenic dysplasia, whenever conventional SE images exhibit substandard quality.     

Initial experience with fast low-angle multiecho (FLAME) imaging of the central nervous system

Fast low-angle multiecho (FLAME) imaging uses partial flip angles of less than 90 degrees with 180 degrees radiofrequency refocusing pulses. The partial flip angle permits imaging with shorter repetition time (TR) values on the order of 750-1,000 ms for 30 degrees angles with image contrast characteristics identical to those obtained with conventional 90-180 degrees schemes and TRs on the order of 2,500 ms. The approximately threefold reduction in imaging time is accompanied by a decrease in signal-to-noise ratio. In many circumstances, however, this trade-off may produce entirely acceptable images of the CNS at a significant reduction in imaging time.     

Magnetic resonance angiography of the neck vessels: imaging optimization

The neck vessels of 60 patients were studied by means of magnetic resonance angiography, with gradient-echo FISP sequences with short TR and TE and with a 25 degrees flip angle. To study arterial neck vessels, sequences were acquired on the coronal and on the sagittal planes, centered on the cricoid. The intracranial tract of the vertebral arteries required axial sequences centered under the floor of the sella turcica. Post-processing was obtained with the maximum intensity projection technique. The coronal and sagittal sequences were rotated on the axial plane from 0 degrees to 180 degrees with 15 degrees interval, while axial sequences were rotated on the sagittal plane from 0 degrees to 180 degrees with the same interval. TR, TE and flip angle values were very important for image quality: the thinner the volumes the more effective resolution power and vessel visualization. These volumes should not exceed 1.5 mm. Axial rotations of coronal sequences from -45 degrees to +45 degrees and of sagittal sequences from 60 degrees to 120 degrees were useful for diagnosis. The intracranial tract of the vertebral arteries was clearly depicted after axial sequences and after 75 degrees and 135 degrees sagittal rotations.     

Fast spin-echo imaging of the knee: Factors influencing contrast

Conventional T2-weighted spin-echo magnetic resonance imaging of the knee requires a long TR. Fast spin-echo (FSE) imaging can improve acquisition efficiency severalfold by collecting multiple lines of k space for each TR. Compromises in resolution, section coverage, and contrast inevitably result. The authors examined the compromises encountered in FSE imaging of the knee and discuss the variations in image contrast and resolution due to choices of sequence parameters. For short TR/TE knee imaging, FSE does not appear to offer any advantages, since the increased collection efficiency for one section reduces the available number of sections, so that the total imaging time for a given number of sections remains constant relative to conventional spin-echo imaging. For T2-weighted images, considerable time can be saved and comparable quality images can be obtained. This saved time can be usefully spent on increasing both the resolution of the image and its signal-to-noise ratio, while still reducing total acquisition time by a factor of two. The preferred FSE T2-weighted images were acquired with a TR of 4,500 msec, TE of 120 msec, and eight echoes. The available number of sections is compromised, and the sequence remains sensitive to flow artifacts; however, the FSE sequence appears to be promising for knee imaging.