Four-dimensional 1H and 23Na imaging using continuously oscillating gradients.

A class of fast magnetic spectroscopic imaging methods using continuously oscillating gradients for four-dimensional (three spatial and one spectral) localization is introduced. Sampling may start immediately following the application of an RF excitation pulse, thus enabling measurement of spin density, chemical shift, and relaxation rates of short-T2 species. For spatial localization, steady-state sinusoidal gradient waveforms are used to sample a ball in k space. The two types of trajectories presented include: (1) continuously oscillating gradients with continuously rotating direction used for steady-state free-precession imaging and (2) continuously oscillating gradients followed by a spoiler directed along discrete projections. Design criteria are given and spatial-spectral and spatial-temporal reconstruction methods are developed. Theoretical point-spread functions and signal-to-noise ratios are derived while considering T2*, off-resonance effects, and RF excitation options. Experimental phantom, in vivo, and in vitro 1H and 23Na images collected at 2.35 T are presented. The 1H images were acquired with isotropic spatial resolution ranging from 0.03 to 0.27 cm3 and gradient-oscillation frequencies ranging from 600 to 700 Hz, thus allowing for the separation of water and lipid signals within a voxel. The 23Na images, acquired with 500 and 800 Hz gradient waveforms and 0.70 cm3 isotropic resolution, were resolved in the time domain, yielding spatially localized FIDs.

MR image-guided control of cryosurgery.

Cryoablation has recently become a useful procedure for the treatment of prostatic and hepatic tumors, primarily because of advances in the ability to monitor visually the freezing process with ultrasound. Success of the procedure depends in large part on how well the ice front can be positioned to destroy pathologic tissue, while sparing healthy tissue. This study describes a cryogen delivery system that can be used in conjunction with magnetic resonance (MR) image-guided cryoablation, and an automatic control system that uses MR image guidance in a feedback loop to control the ice front trajectory. Edge-detected MR images are used to determine the current ice front location at each time interval, providing feed-back to an automatic control system that adjusts the flow of cryogen to the cryoprobe. Numerical simulations and experimental results demonstrate that an ice front with cylindrical symmetry can be accurately controlled using this MR image-guided feedback control scheme.

Shielded gradient coils on hyperbolic surfaces of revolution.

A formalism for the calculation of fields generated by current distributions on hyperbolic surfaces of revolution is presented and used to generate designs for shielded gradient coils. One application is the development of small, insertable coils suitable for head imaging while allowing clearance for shoulders. This technique demonstrates that efficient designs that offer substantial advantages over conventional cylindrical designs are possible.

PET studies of cerebral glucose metabolism in conscious rhesus macaques.

A growing body of evidence suggests that rhesus macaques may be a good model of human brain aging. We used positron emission tomography (PET) and 18F-fluorodeoxyglucose (FDG) to measure regional cerebral metabolic rates for glucose (rCMRglc) in young and aged rhesus macaques to determine if age-related decreases, such as those reported in humans, also occur in macaques. Whereas the aged animals had lower metabolic rates in every brain region studied, the largest differences were in left temporal cortex. The largest differences were also observed in left temporal cortex when relative rCMRglc values were used. Both rCMRglc and relative rCMRglc were marked by substantial individual variation within the aged group. This variation may parallel the variation observed in behavioral studies. Future studies that include both PET and behavioral measures should help determine if there is a relationship between age-related changes in rCMRglc and behavior.

An integrated probe for magnetic resonance imaging monitored skin cryosurgery.

Cryosurgery of the skin is a common treatment for both benign and malignant skin cancers. Monitoring the depth of the frozen lesion during cryosurgery, either by estimation based on the lateral spread of freeze at the skin surface or via thermocouples, may be inaccurate because of the heterogeneous nature of tissue. We describe an integrated cryosurgical probe and magnitude resonance imaging probe which we use to obtain high resolution MR images of skin, subcutaneous muscle and the frozen lesion during cryosurgery.

A strategy for sampling on a sphere applied to 3D selective RF pulse design.

Conventional constant angular velocity sampling of the surface of a sphere results in a higher sampling density near the two poles relative to the equatorial region. More samples, and hence longer sampling time, are required to achieve a given sampling density in the equatorial region when compared with uniform sampling. This paper presents a simple expression for a continuous sample path through a nearly uniform distribution of points on the surface of a sphere. Sampling of concentric spherical shells in k-space with the new strategy is used to design 3D selective inversion and spin-echo pulses. These new 3D selective pulses have been implemented and verified experimentally.

Three-dimensional triple-quantum-filtered imaging of 0.012 and 0.024 M sodium-23 using short repetition times.

The gradient-selected triple-quantum-filtered (GS3Q) experiment, developed to improve the contrast in NMR imaging of sodium-23 (23Na) in the human brain, is limited by low signal-to-noise ratio (SNR). We have analyzed the GS3Q experiment and show here that the improvement in GS3Q-filtered 23Na SNR as the repetition time (TR) decreases is accompanied by the appearance of spurious one-quantum (1Q) 23Na signals. An improved filter with better suppression of spurious 1Q 23Na signals is obtained by adding a preparatory crusher gradient and two-step phase cycling to a conventional GS3Q filter. The relative contributions of 3Q coherence and spurious 1Q coherences to the conventional and modified-GS3Q-filtered signals are calculated, providing a measure of the effectiveness of each GS3Q filter. The filters were implemented on a 2.35 T medium-bore spectrometer and their predicted properties verified. SNR measurements from GS3Q-filtered three-dimensional images of an agarose gel phantom indicate that 0.012 M 23Na images in the human brain can be acquired with 8 cm3 voxels and SNR of 10 in 30 minutes at 2.35 T, assuming similar relaxation times.

Human corpus callosum in aging and Alzheimer's disease: a magnetic resonance imaging study.

The involvement of the corpus callosum in aging and Alzheimer's disease (AD) is not clear. We measured cross sectional areas of the entire corpus callosum (CC), as well as the front 20% (genu), middle 60% (body), and posterior 20% (splenium) of the structure from a midsagittal MRI slice in AD patients (N = 20), and young (N = 16) and old (N = 13) control subjects. We found that mean CC area in young controls was 570 +/- 107 mm2. Aging did not significantly affect the mean area of the CC (562 +/- 98 mm2). A small, significant reduction was seen in AD in comparison to the young control group (480 +/- 133 mm2). However, AD is accompanied by a large and statistically significant reduction in the genu area in comparison to both young and old control subjects. A trend toward an age-dependent reduction in the body area is also accentuated in AD patients who showed significantly smaller callosal bodies than young controls. We conclude that selective changes within corpus callosum accompany aging and AD pathology.

Monitoring cryosurgery in the brain and in the prostate with proton NMR.

This brief communication reports the results of preliminary studies performed to evaluate the feasibility of using NMR imaging to monitor the freezing of tissue during cryosurgery. Two tissues were studied, rabbit brain and dog prostate. NMR imaging of these tissues provided a clear distinction between frozen and unfrozen tissue and an accurate assessment of the extent of freezing in real time.

A study of radiation necrosis and edema in the canine brain using positron emission tomography and magnetic resonance imaging.

Radiation injury, a major hazard of central nervous system (CNS) radiotherapy, was investigated using sequential studies with positron emission tomography (PET) and magnetic resonance imaging (MRI) in beagle dogs with both helium and neon-ion hemibrain irradiation. All dogs receiving 7.5-11 Gy of neon showed no signs of radiation injury to 3 years after irradiation. Dogs receiving > or = 13 Gy neon or helium succumbed to radiation necrosis and died 21-32 weeks after irradiation. The findings of imaging studies for all dogs who succumbed to radiation necrosis were normal until 3-6 weeks before death. Sequential studies were performed using 0.5 T MRI spin-echo and inversion recovery imaging sequences, and high-resolution (2-3 mm) PET with 18F deoxyglucose and 82Rb. The same axial slices (within 1-2 mm) were imaged repeatedly (weekly) after irradiation until death. The earliest CNS changes were seen as decreased metabolic activity in the cortex of the irradiated hemisphere with PET or an increase in signal intensity in the periventricular white matter on T2-weighted spin-echo imaging on MRI. From the time this increase in signal intensity was first observed, T1 and T2 values increased steadily in both the gray and white matter until death. The changes in white matter were consistently greater than those in gray matter. The results of PET, MRI, and histopathological examinations support the theory that both cellular and vascular mechanisms are involved in radiation necrosis.

MRI-monitored cryosurgery in the rabbit brain.

The inability to observe the transient, irregular shape of the frozen region that develops during cryosurgery has inhibited the application of this surgical technique to the treatment of tumors in the brain and deep in visceral organs. We used proton NMR spin-echo and spoiled gradient-echo imaging to monitor the development of frozen lesions during cryosurgery in the rabbit brain and the resulting postcryosurgical changes up to 4 hr after freezing. Spoiled gradient-echo images (TE = 14 ms; TR = 50 ms) were acquired during freezing and thawing at a rate of 15 s/slice. Although the frozen region itself is invisible in MR images, its presence is distinguished easily from the surrounding unfrozen soft tissue because of the large contrast difference between frozen and unfrozen regions. T2-weighted spin-echo images (TE = 100 ms, TR = 2 s) obtained after thawing suggest that edema forms first at the margin of the region that was frozen (cryolesion) and then moves into the region's core. Histological examination showed complete necrosis in the cryolesion and a sharp transition to undamaged tissue at the margin of the lesion and its image. Blood-brain barrier (BBB) damage was investigated using gadolinium-DTPA. The region of edema in the T2-weighted spin-echo images was coincident with the area of BBB damage in the Gd-DTPA-enhanced T1-weighted spin-echo images (TE = 33 ms, TR = 400 ms) and both were distinguishable as areas of high signal relative to the surrounding normal tissue. The results of these experiments indicate that MR can both effectively monitor the cryosurgical freezing and thawing cycle and characterize the postcryosurgical changes in tissue during follow-up.

Electromagnetic fields of surface coil in vivo NMR at high frequencies.

A high frequency solution of the electromagnetic field produced by a circular surface coil adjacent to a homogeneous conducting, dielectric sphere is used to predict the attainable signal to noise ratio (S/N) and specific absorption rate (SAR) for in vivo 1H NMR spectroscopy experiments from 200 to 430 MHz (4.7-10 T). Above 200 MHz the S/N increases more rapidly with frequency and the SAR increases less rapidly compared with the respective S/N and SAR frequency dependence below 200 MHz. The difference in frequency dependence is due to dielectric resonances of the magnetic field inside the sphere at frequencies above 200 MHz. It is predicted that surface coil 1H NMR experiments may be performed on a head-sized sphere, having conductivity and relative dielectric constant of brain, at frequencies up to 430 MHz without exceeding 8 W/kg local SAR and 3.2 W/kg SAR. The calculations of the S/N and SAR are used to determine optimum surface coil geometries for NMR experiments. The power radiated by the surface coil in the absence of shielding and asymmetries in the received signal with respect to the plane defined by the surface coil axis and the direction of the static magnetic field are significant at high frequency. Experimental measurements of the magnetic field inside a head-sized sphere verify the presence of dielectric resonances at frequencies above 200 MHz.

Magnetohydrodynamics of blood flow.

The changes in hydrostatic pressure and electrical potentials across vessels in the human vasculature in the presence of a large static magnetic field are estimated to determine the feasibility of in vivo NMR spectroscopy at fields as high as 10 T.A 10-T magnetic field changes the vascular pressure in a model of the human vasculature by less than 0.2%. An exact solution to the magnetohydrodynamic equations describing a conducting fluid flowing transverse to a static magnetic field in a nonconducting, straight, circular tube is used. This solution is compared to an approximate solution that assumes that no magnetic fields are induced in the fluid and that has led previous investigators to predict significant biological effects from static magnetic fields. Experimental results show that the exact solution accurately predicts the magnetohydrodynamic slowing of 15% NaCl flowing transverse to 2.3- and 4.7-T magnetic fields for fluxes below 0.5 liter/min while the approximate solution predicts a much more retarded flow.

Missing pulse steady-state free precession.

A fast imaging technique, missing pulse steady-state free precession (MP-SSFP), is described. MP-SSFP is one of a class of steady-state free precession techniques in which every nth RF pulse is missing. MP-SSFP has been implemented for the case where every third RF pulse is omitted: [-theta 1-r-theta 2-r-echo-r-]. A RF-refocused echo forms at the time of the missing pulse. This echo is less sensitive to field inhomogeneities than the gradient-recalled echoes used in most fast imaging methods. An analytical expression is obtained for the signal strength as a function of the flip angles theta 1 and theta 2, the interpulse interval tau, and the amount of interpulse dephase. The expression shows that theta 1 and theta 2 provide two degrees of freedom to optimize the signal-to-noise ratio and improve tissue contrast. Two different cases, theta 1 = theta 2 and theta 1 = -theta 2, are described to demonstrate the difference in contrast. The first case gives a strong signal from spins with short T1 and short T2 while the second case has contrast very similar to a conventional SSFP technique. The theoretical expression predicts that the signal consists of multiple components which may be observed experimentally by adjusting the gradient pulses.

Quantitative NMR measurements of hippocampal atrophy in Alzheimer's disease.

Nuclear magnetic resonance (NMR) imaging was employed to study 10 patients with Alzheimer's disease (AD) and seven healthy elderly control subjects. Coronal sections were used to make volumetric measurements of the hippocampus, ventricles, subarachnoid space, and brain parenchyma. The hippocampal volume (normalized relative to the size of the lenticular nucleus) was reduced by 40% in the AD group compared to the controls, with no overlap between the two groups. Overall measures of brain atrophy and ventricular and sulcal enlargement also showed significantly different group means, although with overlap between the two groups. Hippocampal atrophy did not correlate with either overall brain atrophy or dementia severity, although the degree of brain atrophy was correlated with dementia severity. These results show that NMR is capable of providing in vivo quantification of diminished hippocampal size in AD which is not correlated with overall brain atrophy and which may differentiate AD from normal aging.

Application of 30-MHz acoustic scattering to the study of human red blood cells.

A technique for simultaneously measuring the scattering amplitude of individual particles at two angles is applied to human red blood cells. Using a Rayleigh scattering model, the density and compressibility of the cells may be determined given a priori knowledge of their volume. A calibration method relying on measurements of the bulk properties of particle suspensions is described. Red cell properties in hypotonic and hypertonic hosts are compared with a homogeneous mixture model, and a linear relation between hemoglobin content and scattering amplitude at a 90 deg scattering angle is established.

A technique for the study of acoustic scattering from microparticles.

A technique for observing acoustic scattering from individual particles with volumes on the order of 100 micron3 is described. The apparatus consists of an array of focused acoustic transducers, a fluid jet to position the particles, and electronics incorporating quadrature detection. A maximum likelihood estimator is developed for the scattering amplitude that combines the data received from a series of pulses as a particle is carried through the focal zone of the transducers. A calibration method and performance of the apparatus are discussed.

Effects of slice selection and diffusion on T2 measurement.

This study examines slice selection and diffusion as sources of systematic error in T2 estimates obtained from imaging experiments. The selective refocusing pulses in a multislice, two-echo, spin-warp experiment cause slice profiles of sequential echoes to change in shape. This results in bias in T2 images calculated from the two-echo data set. A method to correct the bias is presented. The effect of diffusion on T2 images obtained with typical two-echo imaging sequences was found to be negligible.

MRI of blood flow: correlation of image appearance with spin-echo phase shift and signal intensity.

Phase-sensitive imaging was used to correlate signal distribution with phase shift and velocity distribution in spin-echo magnetic resonance imaging (MRI). Flow-dependent, changing intensity patterns that were seen in a constant-flow phantom study were explained by the simultaneous effects of inflow signal enhancement, first-echo dephasing, and outflow signal loss occurring during laminar flow. In clinical studies, first-echo dephasing was shown during laminar flow in the inferior vena cava. Turbulent flow was demonstrated in the descending thoracic aorta during late systolic flow, and turbulent dephasing-rephasing was shown in the abdominal aorta.