Reduce Imaging Time Research Articles

131. 頭部 T2 強調画像の撮像時間短縮について(第 46 回総会学術大会会員研究発表予稿)

131. 頭部 T2 強調画像の撮像時間短縮について(第 46 回総会学術大会会員研究発表予稿)

Improved contrast at 1.5 Tesla using half-Fourier imaging: Application to spin-echo and angiographic imaging

Half-Fourier imaging is useful for reducing imaging time by requiring less than the usual number of phase-encoding steps. This increase in speed can be traded off for longer repeat times, TR, for improved contrast-to-noise in the same imaging time or to collect short asymmetric echoes. Consequently, it is shown to be especially useful for long TR spin-echo imaging where at 1.5 T a repeat time of 4 sec is recommended for a double-echo TE = 30 90 sequence or 3 sec for a double-echo TE = 15 90 sequence. Short TR FLASH imaging also benefits from a longer TR since there is more time to spoil the signal. In both cases, there is the advantage when a multislice acquisition mode is used that more slices (and hence, a larger volume) can be taken. Another application is to apply half-Fourier imaging in the read direction to avoid spin dephasing and motion artifacts. This is particularly useful in angiographic imaging where smaller pixel sizes and shorter echo times both reduce pixel dephasing. Again, even though taking less than the usual number of data points leads to a reduction in S N , the improved signal and resolution for blood vessels can more than compensate this loss.

The use of an improved inversion pulse with the spin-echo/inversion-recovery sequence to give increased accuracy and reduced imaging time for T1 measurements.

The hyperbolic secant inversion pulse has been incorporated into the interleaved saturation-recovery/inversion-recovery sequence to provide T1 measurements combining high precision with a significantly reduced imaging time (a mean accuracy of 2% over a T1 range of 300 to 1650 ms for standard gel phantoms, with Tr1 = Tr2 = 2.0 s, Ti = 0.67 s).

Whole-body MR imaging and spectroscopy with a 4-T system.

The clinical potential and limitations of magnetic resonance imaging and spectroscopy at 4 T were investigated with the use of a newly constructed system, which has been in use since January 1987. The magnet has a warm bore that measures 1.25 m in diameter, and its homogeneity in a sphere with a diameter of 50 cm is better than +/- 2.5 ppm. It was hypothesized that the improvement in the signal-to-noise ratio (S/N) afforded by the higher field strength would be useful in reducing imaging time and in improving spatial resolution. In experiments in human volunteers, believed to be the first in which an entire human body was exposed to a magnetic flux density of that magnitude, the subjects were exposed to 4 T for 10-30 minutes. They showed no changes in well-being or heart activity. The expected gain in spectral resolution due to chemical-shift scaling was achieved with the 4-T system, and an improvement in S/N was verified for phosphorus at 34 and 68 MHz. In sodium imaging, the high flux density appears to be useful in reducing imaging time, which should increase the usefulness of sodium imaging in evaluating brain tumors and strokes. In spectroscopy, the increase in flux density improves the quality of the spectra.

A simple approach to T2 imaging in MRI

A simple method for obtaining images whose contrast depends only on T 2 is described and tested both on phantoms and in vivo. The method works reliably and effectively under clinically realistic operating conditions using standard imaging protocols. It can result in a substantial reduction in imaging times for T 2 weighted images.

Reduced MR acquisition time with the CROCUS technique.

A technique to reduce magnetic resonance (MR) image acquisition time by approximately 50% is introduced. The conjugate reconstruction by off-center under-sampling (CROCUS) method samples only every other phase-encoded line in raw data space and uses the conjugate symmetry of the data to reconstruct a real image. If acquisition and image reconstruction were done in the simplest manner, this under-sampling would produce unacceptable image aliasing. However, if the phase-encoded lines are offset from the origin, the assumption of conjugate symmetry allows the image to be calculated without aliasing. Unfortunately, information from all practical MR systems contain phase-shift errors that produce deviations from conjugate symmetry. The CROCUS method incorporates a correction technique, similar to that used in other half-Fourier methods, which uses low-resolution phase-shift information obtained from a few extra lines of phase-encoded data. This paper provides a theoretical derivation of the reconstruction algorithm and correction technique and illustrates the results. Excellent image quality is obtained with no loss of spatial resolution. Image signal-to-noise ratio is reduced by a factor of approximately 1.4 because of the reduced acquisition time. When the imaging circumstances are such that a high signal level is available, CROCUS imaging can be an effective means of reducing imaging time.

Prenatal Diagnosis of Fetal Anomalies Using Ultrasound and MRI

In conclusion, it is unlikely that MRI will replace ultrasound as the primary obstetric imaging modality in the near future. Ultrasound has a proven record of accuracy and safety in addition to its easy access and low cost. MRI has promise, however, in providing crucial information in patients with underlying medical or surgical conditions that would ordinarily require ionizing radiation for evaluation. Currently, MRI is helpful in evaluating gross fetal anomalies and disturbances of fetal growth and development when ultrasound is limited by oligohydramnios or maternal obesity. Further experience is required to determine the value of spectroscopy and quantitative relaxation times regarding fetal metabolism and fetal well-being. Technologic improvements and software updates will reduce imaging time and increase spatial resolution, thus rendering MRI more competitive with existing imaging techniques. The ultimate role, however, that MRI will have in the evaluation of the fetus is currently undefined.

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.

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.

Halving MR imaging time by conjugation: demonstration at 3.5 kG.

Conjugation can be used to synthesize half of the data acquired during a conventional two-dimensional Fourier transform imaging procedure, thus reducing imaging time by nearly half. The images acquired by this process have the same object contrast and spatial resolution as conventional images do, but with a 40% reduction in the signal-to-noise ratio (S/N). Conjugation can be used to advantage in magnetic resonance imaging units in which S/N levels are higher than needed to permit imaging with a single acquisition of each projection.

Inner volume MR imaging: technical concepts and their application.

Although cross-sectional magnetic resonance examination of the head and body is useful for screening large regions of tissue, subsectional regions of the head and body often need to be examined. Orthogonally directed, selectively irradiated planes with different flip angles produce a spatially limited signal region from which two- or three-dimensional volume images can be reconstructed. Images with limited fields-of-view can be acquired in reduced imaging time. We present a general description of this technique. These subsectional or "inner volume" images eliminate respiratory motion artifacts by excluding moving tissues from the imaged volume. A result of this technique is a high signal from rapid pulsatile blood flow, produced without cardiac gating the pulse sequence.

Whole-body tomographic imaging by NMR.

Since producing our first whole-body nuclear magnetic resonance (NMR) line-scan images some two years ago (Mansfield et al., 1978), we have made a number of changes to our imaging machine which have made possible a considerable improvement in picture quality together with a substantial reduction in imaging time. Other approaches to NMR imaging are extensively discussed and reviewed elsewhere (Mansfield, 1976; Brunner and Ernst, 1979; Mansfield and Morris, 1981). The present technique utilizes a selective radio-frequency pulse and switched magnetic field selection gradient (Mansfield et al., 1976) used initially to define a thin slice through the specimen. The excited plane of nuclear spins arising from the distribution of protons in the tissues produces a transient signal in a second “read” gradient. This signal is Fourier transformed to give the projection of the total spin distribution in a direction orthogonal to the read gradient. Successive projections produced by mechanically rotating the read gradi...