Parameter optimization and calibration of 19F magnetic resonance imaging at 1.5 Tesla

Fluorine-19 magnetic resonance imaging is limited by the fact that acquisition times are long and that high concentrations must be used in order to obtain good signal to noise. A significant improvement in signal to noise ratio may be brought about by the addition of Gd-DTPA, a paramagnetic agent which shortens T1. Images of phantoms containing trifluoroacetic acid (TFA) doped with Gd-DTPA were obtained using a standard spin echo sequence in a 1.5 T field. Interpulse times (TR and TE) and Gd-DTPA concentrations were optimized to yield maximum signal to noise ratios. The use of fast-field-echo scans to image fluorine is also demonstrated. Signal averaging successive FFE scans yields good signal to noise and resolution and may find clinical applicability in imaging areas subject to motion.     

Doubly tuned solenoidal resonators for small animal imaging and spectroscopy at 1.5 Tesla

The design and construction of solenoidal resonators for use with small animals in a 1.5-Tesla clinical imaging system are described. The coils have been designed to exploit the B1 distributions of two resonant modes of a four-turn solenoid whose windings are in parallel. Both singly and doubly tuned versions have been constructed. 1H images of normal and pathologic anatomy in mice and rats as well as a 31P spectrum of a Walker 256 rat sarcoma are presented. A primary advantage of this design is that the coils are easy to build and implement while providing the necessary sensitivity to allow high quality images to be obtained with no changes to the hardware or software of the clinical unit.     

High signal-to-noise FLASH imaging at 8 Tesla.

A radio frequency (RF) and gradient spoiled fast low angle shot technique was used to acquire images from the human brain at 8 Tesla. The resulting FLASH images, obtained with a 17 degrees nutation, a 70 ms repetition time, and a 17 ms echo time, displayed an average signal-to-noise ratio (SNR) of 220:1 (slice thickness 2.2 mm, field-of-view 24 cm, matrix 256 x 128). These images were compared with images obtained at 1.5 Tesla using identical parameters yielding a signal-to-noise of less than 10:1. As such, the 8 Tesla images display a remarkable improvement in SNR with increasing field strength. The images also show little evidence of susceptibility distortion, chemical shift, or RF penetration limitations.     

Gradient-echo line scan imaging using 2D-selective RF excitation

A gradient-echo line scan imaging technique was developed which employs two-dimensional spatially selective radiofrequency (2DRF) pulses for consecutively exciting individual columns of transverse magnetization, i.e., image lines. Although a variety of trajectories are possible for 2DRF excitation, the current implementation involved a blipped-planar trajectory in conjunction with additional saturation RF pulses to suppress side excitations above and below the desired image section, i.e., along the blip direction of the 2DRF pulse. Human brain imaging at 2.0 T (Siemens Vision, Erlangen, Germany) resulted in measuring times of 5.2 s for a 5-mm section at 1.0 x 1.0 mm in-plane resolution. Functional neuroimaging of the motor cortex at 1.2 s temporal resolution and 0.78 x 1.56 mm in-plane resolution exploited the capability of imaging inner volumes (here a 25-mm strip) without signal aliasing.     

Reduced susceptibility effects in perfusion fMRI with single-shot spin-echo EPI acquisitions at 1.5 Tesla

Arterial spin labeling (ASL) perfusion contrast is not based on susceptibility effects and can therefore be used to study brain function in regions of high static inhomogeneity. As a proof of concept, single-shot spin-echo echo-planar imaging (EPI) acquisition was carried out with a multislice continuous ASL (CASL) method at 1.5T. A bilateral finger tapping paradigm was used in the presence of an exogenously induced susceptibility artifact over left motor cortex. The spin-echo CASL technique was compared with a regular gradient-echo EPI sequence with the same slice thickness, as well as other imaging methods using thin slices and spin-echo acquisitions. The results demonstrate improved functional sensitivity and efficiency of the spin-echo CASL approach as compared with gradient-echo EPI techniques, and a trend of improved sensitivity as compared with spin-echo EPI approach in the brain regions affected by the susceptibility artifact. ASL images, either with or without subtraction of the control, provide a robust alternative to blood oxygenation level dependant (BOLD) methods for activation imaging in regions of high static field inhomogeneity.     

Are hepatic and muscle T2 values different at 0.5 and 1.5 Tesla?

In an effort to determine whether T2 values of liver and muscle change with increasing field strength, 144 abdominal MR examinations were retrospectively evaluated. These patients were evaluated with a dual echo T2-weighted spin-echo sequence. Eighty-two of the examinations were performed at 0.5 Tesla and 72 at 1.5 Tesla (T). Eleven of the patients were evaluated with both MR systems with the same sequences. T2 values were also obtained from a Fe NH4(SO4)2 12H2O phantom. The T2 values of liver decreased from 57.8 +/- 11.3 at 0.5 T to 43.7 +/- 8.3 at 1.5 T. The T2 values of muscle decreased from 44.2 +/- 9 at 0.5 T to 35.4 +/- 7.2 at 1.5 T. Patients who were examined on both systems also demonstrated a decrease in both liver and muscle T2 values. For concentrations in the range of hepatic T2's, the phantom demonstrated a decrease in T2 values from 0.5 to 1.5 T ranging from 20.3 to 23.4%. All the T2 changes were statistically significant (p less than .05). The findings suggest that T2 values may depend on field strength, or may vary due to other hardware-related differences.     

MR imaging of experimental and clinical thrombi at 1.5 T

We have previously reported that the T1 and T2 of experimental clots at 0.47 T varies considerably depending upon the method used in their preparation. However, these studies, while relevant to midfield imaging, may not reflect accurately the behavior of such thrombi at higher field strengths. Accordingly, we studied the T1 and T2 at 1.5 T of experimental thrombi prepared by several methods and compared these results with the relaxation times of clinical deep venous thrombi measured in situ in patients. The relationship between the T2 values for the different clot preparation methods was different at 1.5 T than at 0.47 T. The combined use of thrombin and epsilon-amino caproic acid produced thrombi with T1 and T2 indistinguishable from clinical deep venous thrombi.     

Feasibility of fast MR imaging of the liver at 1.5 T

A phantom with T1 and T2 relaxation times encompassing normal liver and liver lesions was constructed to evaluate fast magnetic resonance pulse sequences using TR from 21-100 milliseconds, TE 12-60 milliseconds and flip angles from 5 degrees-90 degrees. Ten of these fast MR sequences were then selected and compared with conventional spin-echo sequences in normal volunteers (n = 3) and in patients with liver lesions (n = 6). Subjectively, the fast MR sequences eliminated motion artefacts. Objectively, 8 of 10 fast sequences had signal-to-noise ratios comparable to spin-echo imaging whereas only 2 of 10 had contrast-to-noise ratios that were similar to spin-echo imaging. This preliminary study, performed at 1.5 Tesla, does not show any clear-cut advantage of fast imaging over spin-echo imaging in the detection of liver lesions. The use of a liver tissue equivalent phantom provides a rapid, practical approach in evaluation of fast scans.     

Bone marrow imaging using STIR at 0.5 and 1.5 T.

We retrospectively examined MR images in 82 patients to evaluate the usefulness of short inversion time inversion recovery (STIR) in bone marrow imaging at 0.5 and 1.5 T. The study included 56 patients at 1.5 T and 26 patients at 0.5 T with a variety of pathologic bone marrow lesions (principally oncological), and compared the contrast and image quality of STIR imaging with spin-echo short repetition time/echo time (TR/TE), long TR/TE, and gradient-echo sequences. The pulse sequences were adjusted for optimal image quality, contrast, and fat nulling. STIR appears especially useful for the evaluation of red marrow (e.g., spine), where contrast between normal and infiltrated marrow is greater than with either gradient-echo or T1-weighted images. STIR is also extremely sensitive for evaluation of osteomyelitis, including soft tissue extent. In more peripheral (yellow) marrow, T1-weighted images are usually as sensitive as STIR. Limitations of STIR include artifacts, in particular motion artifact that at high field strength necessitates motion compensation. At 0.5 T, however, motion compensation is usually not necessary. Also, because of extreme sensitivity to water content, STIR may overstate the margins of a marrow lesion. With these limitations in mind, STIR is a very effective pulse sequence at both 0.5 and 1.5 T for evaluation of marrow abnormalities.     

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.     

Surface-based functional magnetic resonance imaging analysis of partial brain echo planar imaging data at 1.5 T

Surface-based functional magnetic resonance imaging (fMRI) analysis is more sensitive and accurate than volume-based analysis for detecting neural activation. However, these advantages are less important in practical fMRI experiments with commonly used 1.5-T magnetic resonance devices because of the resolution gap between the echo planar imaging data and the cortical surface models. We expected high-resolution segmented partial brain echo planar imaging (EPI) data to overcome this problem, and the activation patterns of the high-resolution data could be different from the low-resolution data. For the practical applications of surface-based fMRI analysis using segmented EPI techniques, the effects of some important factors (e.g., activation patterns, registration and local distortions) should be intensively evaluated because the results of surface-based fMRI analyses could be influenced by them. In this study, we demonstrated the difference between activations detected from low-resolution EPI data, which were covering whole brain, and high-resolution segmented EPI data covering partial brain by volume- and surface-based analysis methods. First, we compared the activation maps of low- and high-resolution EPI datasets detected by volume- and surface-based analyses, with the spatial patterns of activation clusters, and analyzed the distributions of activations in occipital lobes. We also analyzed the high-resolution EPI data covering motor areas and fusiform gyri of human brain, and presented the differences of activations detected by volume- and surface-based methods.     

Whole body slotted tube resonator (STR) for proton NMR imaging at 2.0 Tesla

Design principles of a whole body Slotted Tube Resonator (STR) RF coil are given and fabrication details of the RF coil for a 2.0 Tesla NMR imaging system are presented. Experimental study proved efficient operation of the coil at 2.0 Tesla field and indicated potential for the extension of the technique to the even higher fields.     

Venous cavernoma at 8 Tesla MRI

Cavernous angiomas or cavernomas are vascular malformations, which may be associated with risk of bleeding episodes. We present a case report comparing high resolution 8 Tesla gradient echo (GE) imaging with routine fast spin echo (FSE) at 1.5 Tesla in a patient with venous cavernoma. A 55-year-old male with a history of hemorrhagic stroke was studied using high-resolution 8 Tesla magnetic resonance imaging (MRI) system, which revealed venous cavernoma (9 x 8.6 mm) in the left parietal region and visualized adjacent microvascular supply. Signal loss was prominent in the cavernoma region compared to surrounding brain tissue, and signal intensity declined by factor 7.3 +/- 2.4 (679 +/- 62%) on GE images at 8 Tesla. Cavernoma was not apparent on routine T(2)-weighted FSE images at 1.5 Tesla MRI. This case report indicates that GE images at 8 Tesla can be useful for evaluation of vascular pathologies and microvasculature.     

Single-shot fast spin-echo diffusion tensor imaging of the lumbar spine at 1.5 and 3 T

Diffusion tensor imaging (DTI) of the lumbar spine could improve diagnostic specificity. The purpose of this work was to determine the feasibility of and to validate DTI with single-shot fast spin-echo (SSFSE) for lumbar intervertebral discs at 1.5 and 3 T. Six normal volunteers were scanned with DTI-SSFSE using an eight- and a three-b-value protocol at 1.5 and 3 T, respectively. Apparent diffusion coefficient (ADC) values were computed and validated based on those obtained at 1.5 T from corresponding diffusion tensor scans using line scan diffusion imaging (LSDI), a technique that has been previously validated for use in the spine. Pearson correlation coefficients for LSDI and DTI-SSFSE ADC values were .88 and .89 for 1.5 and 3 T, respectively, with good quantitative agreement according to the Bland-Altman method. Results indicate that DTI-SSFSE is a candidate as a clinical sequence for obtaining diffusion tensor images of the lumbar intervertebral discs with scan times shorter than 4 min.     

In vivo imaging of the human brain at 1.5 T with 0.6-mm isotropic resolution

We present high-resolution in vivo anatomical scans with 3D whole-brain coverage and an isotropic resolution of 0.6 mm, obtained at a clinical field of 1.5 T. The data are acquired in 10 independent scans over two sessions using a 3D magnetization-prepared, gradient echo sequence, modified to output phase images in addition to magnitude images. The independent scans are coregistered to correct for head motion, prior to performing complex averaging. The resolution of the final, averaged image, is found to be equal to the nominal one.