Three-dimensional prepolarized magnetic resonance imaging using rapid acquisition with relaxation enhancement.

Prepolarized MRI (PMRI) with pulsed electromagnets has the potential to produce diagnostic quality 0.5- to 1.0-T images with significantly reduced cost, susceptibility artifacts, specific absorption rate, and gradient noise. In PMRI, the main magnetic field cycles between a high field (B(p)) to polarize the sample and a homogeneous, low field (B(0)) for data acquisition. This architecture combines the higher SNR of the polarizing field with the imaging benefits of the lower field. However, PMRI can only achieve high SNR efficiency for volumetric imaging with 3D rapid imaging techniques, such as rapid acquisition with relaxation enhancement (RARE) (FSE, TSE), because slice-interleaved acquisition and longitudinal magnetization storage are both inefficient in PMRI. This paper demonstrates the use of three techniques necessary to achieve efficient, artifact-free RARE in PMRI: quadratic nulling of concomitant gradient fields, electromotive force cancelation during field ramping, and phase compensation of CPMG echo trains. This paper also demonstrates the use of 3D RARE in PMRI to achieve standard T(1) and fat-suppressed T(2) contrast in phantoms and in vivo wrists. These images show strong potential for future clinical application of PMRI to extremity musculoskeletal imaging and peripheral angiography.

Magnetic resonance imaging with T1 dispersion contrast.

Prepolarized MRI uses pulsed magnetic fields to produce MR images by polarizing the sample at one field strength (approximately 0.5 T) before imaging at a much lower field (approximately 50 mT). Contrast reflecting the T(1) of the sample at an intermediate field strength is achieved by polarizing the sample and then allowing the magnetization to decay at a chosen "evolution" field before imaging. For tissues whose T(1) varies with field strength (T(1) dispersion), the difference between two images collected with different evolution fields yields an image with contrast reflecting the slope of the T(1) dispersion curve between those fields. Tissues with high protein content, such as muscle, exhibit rapid changes in their T(1) dispersion curves at 49 and 65 mT due to cross-relaxation with nitrogen nuclei in protein backbones. Tissues without protein, such as fat, have fairly constant T(1) over this range; subtracting images with two different evolution fields eliminates signal from flat T(1) dispersion species. T(1) dispersion protein-content images of the human wrist and foot are presented, showing clear differentiation between muscle and fat. This technique may prove useful for delineating regions of muscle tissue in the extremities of patients with diseases affecting muscle viability, such as diabetic neuropathy, and for visualizing the protein content of tissues in vivo.

Dual in vivo magnetic resonance evaluation of magnetically labeled mouse embryonic stem cells and cardiac function at 1.5 t.

Cell therapy has demonstrated the potential to restore injured myocardium. A reliable in vivo imaging method to localize transplanted cells and monitor their restorative effects will enable a systematic investigation of this therapeutic modality. The dual MRI capability of imaging both magnetically labeled mouse embryonic stem cells (mESC) and their restorative effects on cardiac function in a murine model of acute myocardial infarction is demonstrated. Serial in vivo MR detection of transplanted mESC and monitoring of the mESC-treated myocardium was conducted over a 4-week period using a 1.5 T clinical scanner. During the 4-week duration, the mESC-treated myocardium demonstrated sustained improvement of the left ventricular (LV) ejection fraction and conservation of LV mass. Furthermore, no significant difference of their restorative effects on the cardiac function was created by the magnetic labeling of mESC. Thus, in vivo MRI enables simultaneous detection of transplanted mESC and their therapeutic effect on the injured myocardium.

Prepolarized magnetic resonance imaging around metal orthopedic implants.

A prepolarized MRI (PMRI) scanner was used to image near metal implants in agar gel phantoms and in in vivo human wrists. Comparison images were made on 1.5- and 0.5-T conventional whole-body systems. The PMRI experiments were performed in a smaller bore system tailored to extremity imaging with a prepolarization magnetic field of 0.4 T and a readout magnetic field of 27-54 mT (1.1-2.2 MHz). Scan parameters were chosen with equal readout gradient strength over a given field of view and matrix size to allow unbiased evaluation of the benefits of lower readout frequency. Results exhibit substantial reduction in metal susceptibility artifacts under PMRI versus conventional scanners. A new artifact quantification technique is also presented, and phantom results confirm that susceptibility artifacts improve as expected with decreasing readout magnetic field using PMRI. This proof-of-concept study demonstrates that prepolarized techniques have the potential to provide diagnostic cross-sectional images for postoperative evaluation of patients with metal implants.

Automatic tuning of flexible interventional RF receiver coils.

Microcontroller-based circuitry was built and tested for automatically tuning flexible RF receiver coils at the touch of a button. This circuitry is robust to 10% changes in probe center frequency, is in line with the scanner, and requires less than 1 s to tune a simple probe. Images were acquired using this circuitry with a varactor-tunable 1-inch flexible probe in a phantom and in an in vitro porcine knee model. The phantom experiments support the use of automatic tuning by demonstrating 30% signal-to-noise ratio (SNR) losses for 5% changes in coil center frequency, in agreement with theoretical calculations. Comparisons between patellofemoral cartilage images obtained using a 3-inch surface coil and the surgically-implanted 1-inch flexible coil reveal a worst-case local SNR advantage of a factor of 4 for the smaller coil. This work confirms that surgically implanted coils can greatly improve resolution in small-field-of-view (FOV) applications, and demonstrates the importance and feasibility of automatically tuning such probes.