MRI: a charmed past and an exciting future.

The invention and development of MRI took place under very desirable circumstances. Dr. Lauterbur, a distinguished NMR chemist, conceived of the basic idea. Once this concept was presented to the medical imaging community, a wonderful synergy developed between the two mature disciplines of NMR physical chemistry and medical imaging. This resulted in amazingly rapid progress and acceptance by the clinical community. This adventure is in sharp contrast to the history of x-ray imaging. This began with the accidental discovery of x-rays by Roentgen at the end of the 19th century. Unlike NMR, the basic x-ray mechanism was not understood and was mistakenly thought to not be an electromagnetic wave. MRI is now at an advanced stage where investigators study advanced hardware and software improvements. These studies include improved signal-to-noise ratio, resolution, and speed, which generally involve higher B(0). The high readout field results in numerous problems. These can be overcome by prepolarizing techniques using a high polarization field and a relatively low readout field.

Rapid polarizing field cycling in magnetic resonance imaging.

We describe the electronics for controlling the independently pulsed polarizing coil in a prepolarized magnetic resonance imaging (PMRI) system and demonstrate performance with free induction decay measurements and in vivo imaging experiments. A PMRI scanner retains all the benefits of acquiring MRI data at low field, but with the higher signal of the polarizing field. Rapidly and efficiently ramping the polarizing coil without disturbing the data acquisition is one of the major challenges of PMRI. With our modular hardware design, we successfully ramp the 0.4-T polarizing coil of a wrist-sized PMRI scanner at up to 100 T/s without causing image artifacts or otherwise degrading data acquisition.

Introduction to: A k-space analysis of small-tip-angle excitation.

The article "A k-space analysis of small-tip-angle excitation" introduced a spatial frequency interpretation of the effect of RF excitation pulses. This introduction describes where the initial ideas for this paper came from, and traces out some of the applications that have been developed using this perspective.

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.

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.

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.

Short echo time MR spectroscopic imaging of the lung parenchyma.

PURPOSE: To perform short echo time MR spectroscopic imaging of the lung parenchyma on normal volunteers. MATERIALS AND METHODS: A short echo time projection-reconstruction spectroscopic imaging sequence was implemented on a commercial 1.5T whole body MRI scanner. Images and spectra of the lung parenchyma were obtained from five normal volunteers. Breath-held spectroscopic imaging was also performed. RESULTS: Spectroscopic imaging of short-T2* species allows visualization of different anatomic structures based upon their frequency shifts. A characteristic peak from the parenchyma was seen at three ppm from water frequency. CONCLUSION: Short echo time MR spectroscopic imaging of the lung parenchyma was demonstrated in normal volunteers. This method may improve proton imaging of the lungs and add specificity to the diagnosis of pulmonary disease.