Christopher Judson Hardy

A review of 1H nuclear magnetic resonance relaxation in pathology: are T1 and T2 diagnostic?

The longitudinal (T1) and transverse (T2) proton (1H) nuclear magnetic resonance (NMR) relaxation times of pathological human and animal tissues in the frequency range 1-100 MHz are archived, reviewed, and analyzed as a function of tissue of origin, NMR frequency, temperature, species, and in vivo versus in vitro status. T1 data from specific disease states of the bone, brain, breast, kidney, liver, muscle, pancreas, and spleen can be characterized by simple dispersions of the form T1 = AvB in the range 1-100 MHz with A and B empirically determined pathology-dependent constants. Pathological tissue T2 values are essentially independent of NMR frequency. Raw relaxation data, best-fit T1 parameters A and B, and the mean T2 values, are tabulated along with standard deviations and sample size to establish the normal range of pathological tissue relaxation times applicable to NMR imaging or in vitro NMR examination. Statistical analysis of relaxation data, assumed independent, reveals that most tumor and edematous tissue T1 values and some breast, liver, and muscle tumor T2 values are significantly elevated (p greater than or equal to 0.95) relative to normal, but do not differ significantly from other tumors and pathologies. Statistically significant abnormalities in the T1 values of some brain, breast, and lung tumors, and most pathological tissue T2 values could not, however, be demonstrated in the presence of large statistical errors. Both T1 and T2 in uninvolved tissue from tumor-bearing animals or organs do not demonstrate statistically significant differences from normal when considered as a group, suggesting no appreciable systemic effects associated with the presence of tumors compared to the statistical uncertainty. Statistical prediction analysis for both T1 and T2 indicates that of all the tissues studied, only liver hepatoma can be reliably distinguished from normal liver based on a single T1 measurement (p greater than or equal to 0.95) given the scatter in the current published data. Indeed, data scatter, not easily attributable to temperature, species, in vivo versus in vitro status, the inclusion of implanted or chemical induced tumors, or the possible existence of multiple component relaxation, is recognized as the major factor inhibiting the diagnostic utility of quantitative NMR relaxation measurements. Malignancy indexes that combine T1 and T2 data as a diagnostic indicator suffer similar problems of uncertainty. The literature review reveals a dearth of information on the temperature and frequency dependence of pathological tissue relaxation and the possible existence of multiple relaxation components.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional Myocardial Metabolism of High-Energy Phosphates during Isometric Exercise in Patients with Coronary Artery Disease

BACKGROUND The maintenance of cellular levels of high-energy phosphates is required for myocardial function and preservation. In animals, severe myocardial ischemia is characterized by the rapid loss of phosphocreatine and a decrease in the ratio of phosphocreatine to ATP. METHODS To determine whether ischemic metabolic changes are detectable in humans, we recorded spatially localized phosphorus-31 nuclear-magnetic-resonance (31P NMR) spectra from the anterior myocardium before, during, and after isometric hand-grip exercise. RESULTS The mean (+/- SD) ratio of phosphocreatine to ATP in the left ventricular wall when subjects were at rest was 1.72 +/- 0.15 in normal subjects (n = 11) and 1.59 +/- 0.31 in patients with nonischemic heart disease (n = 9), and the ratio did not change during hand-grip exercise in either group. However, in patients with coronary heart disease and ischemia due to severe stenosis (greater than or equal to 70 percent) of the left anterior descending or left main coronary arteries (n = 16), the ratio decreased from 1.45 +/- 0.31 at rest to 0.91 +/- 0.24 during exercise (P less than 0.001) and recovered to 1.27 +/- 0.38 two minutes after exercise. Only three patients with coronary heart disease had clinical symptoms of ischemia during exercise. Repeat exercise testing in five patients after revascularization yielded values of 1.60 +/- 0.20 at rest and 1.62 +/- 0.18 during exercise (P not significant), as compared with 1.51 +/- 0.19 at rest and 1.02 +/- 0.26 during exercise before revascularization (P less than 0.02). CONCLUSIONS The decrease in the ratio of phosphocreatine to ATP during hand-grip exercise in patients with myocardial ischemia reflects a transient imbalance between oxygen supply and demand in myocardium with compromised blood flow. Exercise testing with 31P NMR is a useful method of assessing the effect of ischemia on myocardial metabolism of high-energy phosphates and of monitoring the response to treatment.

Altered myocardial high-energy phosphate metabolites in patients with dilated cardiomyopathy.

Myocardial high-energy phosphate metabolism in patients with dilated cardiomyopathy (DCM) of ischemic or idiopathic etiology was assessed at rest by one-dimensional phase-encoded 31P-nuclear magnetic resonance (NMR) spectroscopy studies performed in conjunction with 1H imaging in 20 patients with DCM and in 12 normal volunteers. The measured values of anterior myocardial phosphocreatine/beta-adenosine triphosphate (PCr/beta-ATP), corrected for partial saturation and contamination of the spectra by blood metabolites, averaged 1.80 +/- 0.06 (mean +/- SE) in normal volunteers and 1.46 +/- 0.07 in the patients overall, a highly significant (p less than 0.001) decrease. In patients with DCM accompanied by coronary artery disease (n = 9), the PCr/beta-ATP ratio averaged 1.53 +/- 0.07, while in those with DCM alone it was 1.41 +/- 0.12 (n = 11), a value that was not significantly different. There was no significant correlation (r = 0.34) between myocardial PCr/ATP ratio and left ventricular ejection fraction in patients. These studies demonstrate that myocardial PCr/ATP ratios are reduced at rest in human ischemic and idiopathic dilated cardiomyopathy.

Comparison of linear and circular polarization for magnetic resonance imaging

Abstract A comparison of experimental imaging results obtained with linearly polarized and circularly polarized radiofrequency excitation and reception is presented. Simulation images in good agreement with the experimental scans are described. The simulations are calculated with a model in which a homogeneous, isotropic cylinder of lossy dielectric material and infinite axial extent is immersed in a uniform rf magnetic field perpendicular to the axis. It is found that with the usual linear polarization, reconstructions of uniform objects have regions of decreased intensity. These artifacts are shown to arise from dielectric standing wave effects and eddy currents. The effects become more severe as the frequency or object size is increased, and depend upon the complex conductivity of the object. Results indicate that a significant reduction in the artifact intensity is achieved when circular polarization is employed for both transmission and reception. The expected benefits of circular polarization over linear polarization in reduction of excitation power (up to 50% reduction) and signal-to-noise advantage (√2) have been realized in practice with cylindrical objects and human subjects.

Broadband nuclear magnetic resonance pulses with two-dimensional spatial selectivity

Two‐dimensional spatial localization can be achieved using a single nuclear magnetic resonance (NMR) pulse if the pulse is applied in the presence of a magnetic field gradient which is reorienting through two dimensions. The application of these pulses to whole‐body NMR imaging systems is hampered, however, by amplitude and slew‐rate constraints on the magnetic field gradients, resulting in longer pulses with relatively limited bandwidth. We describe here a variable rate method for shortening these pulses by as much as 36% for pulses limited by slew rate and 55% for pulses limited by gradient amplitude, without changing their spatial excitation profiles on resonance.

Highly parallel volumetric imaging with a 32-element RF coil array.

The improvement of MRI speed with parallel acquisition is ultimately an SNR‐limited process. To offset acquisition‐ and reconstruction‐related SNR losses, practical parallel imaging at high accelerations should include the use of a many‐element array with a high intrinsic signal‐to‐noise ratio (SNR) and spatial‐encoding capability, and an advantageous imaging paradigm. We present a 32‐element receive‐coil array and a volumetric paradigm that address the SNR challenge at high accelerations by maximally exploiting multidimensional acceleration in conjunction with noise averaging. Geometric details beyond an initial design concept for the array were determined with the guidance of simulations. Imaging with the support of 32‐channel data acquisition systems produced in vivo results with up to 16‐fold acceleration, including images from rapid abdominal and MRA studies. Magn Reson Med 52:869–877, 2004.

Coupling and decoupling theory and its application to the MRI phased array

In classical MRI phased‐array design, optimal coil overlapping is used to minimize coupling between nearest‐neighbor coils, and low input impedance preamplifiers are used to isolate the relatively weak coupling between non‐nearest neighbors. However, to make the complex sensitivities of phased‐array coils sufficiently distinct in parallel spatially‐encoded MRI, it is desirable to have no overlapping between coils. Also, if phased arrays are used as transmit coils in MRI, one can no longer rely on the low input impedance of the preamplifiers for decoupling. Here a coupling and decoupling theory is introduced to provide a better understanding of the relations between coupled and uncoupled signals in the MRI phased array, and to offer a new method for decoupling phased‐array coils without overlapping the nearest coil pairs. The new decoupling method is based on the assumption that any n‐element phased array can be decoupled by a 2n‐port interface system between phased array and preamplifiers. The detailed analysis and the experimental results show that a four‐port interface can be used to decouple a two‐element phased array. Furthermore, the four‐port interfaces can serve as building blocks to construct a 2n‐port decoupling interface. This new method allows one to place the coil elements anywhere that could optimize parallel spatial encoding without concern for coupling between the coils. The method can also be used for phased‐array transmit coils. Magn Reson Med 48:203–213, 2002.

Toward single breath-hold whole-heart coverage coronary MRA using highly accelerated parallel imaging with a 32-channel MR system†

Coronary MR angiography (CMRA) is generally confined to the acquisition of multiple targeted slabs with coverage dictated by the competing constraints of signal‐to‐noise ratio (SNR), physiological motion, and scan time. This work addresses these obstacles by demonstrating the technical feasibility of using a 32‐channel coil array and receiver system for highly accelerated volumetric breath‐hold CMRA. The use of the 32‐element array in unaccelerated CMRA studies provided a baseline SNR increase of as much as 40% over conventional cardiac‐optimized phased array coils, which resulted in substantially enhanced image quality and improved delineation of the coronary arteries. Modest accelerations were used to reduce breath‐hold durations for tailored coverage of the coronary arteries using targeted multi‐oblique slabs to as little as 10 s. Finally, high net accelerations were combined with the SNR advantages of a 3D steady‐state free precession (SSFP) technique to achieve previously unattainable comprehensive volumetric coverage of the coronary arteries in a single breath‐hold. The merits and limitations of this simplified volumetric imaging approach are discussed and its implications for coronary MRA are considered. Magn Reson Med, 2006.

Accelerated diffusion spectrum imaging in the human brain using compressed sensing

We developed a novel method to accelerate diffusion spectrum imaging using compressed sensing. The method can be applied to either reduce acquisition time of diffusion spectrum imaging acquisition without losing critical information or to improve the resolution in diffusion space without increasing scan time. Unlike parallel imaging, compressed sensing can be applied to reconstruct a sub‐Nyquist sampled dataset in domains other than the spatial one. Simulations of fiber crossings in 2D and 3D were performed to systematically evaluate the effect of compressed sensing reconstruction with different types of undersampling patterns (random, gaussian, Poisson disk) and different acceleration factors on radial and axial diffusion information. Experiments in brains of healthy volunteers were performed, where diffusion space was undersampled with different sampling patterns and reconstructed using compressed sensing. Essential information on diffusion properties, such as orientation distribution function, diffusion coefficient, and kurtosis is preserved up to an acceleration factor of R = 4. Magn Reson Med, 2011.

Efficient adiabatic fast passage for NMR population inversion in the presence of radiofrequency field inhomogeneity and frequency offsets

Abstract Adiabatic fast passage is a powerful tool for uniformly inverting magnetization in the presence of rf magnetic field inhomogeneity and frequency offsets. Long pulses or large rf powers are necessary for this technique to be effective, however, when the usual linear frequency sweep is used. A sweep of the form ω ( t ) − ω 0 = ω 1 tan( αω 1 t ), on the other hand, is shown to be an order of magnitude more efficient than the linear sweep. Here ω ( t ) is the rf field frequency, ω 0 is the Larmor frequency, w 1 = - γB 1 (gyromagnetic ratio times rf field strength), and α is a constant determined by w 1 , the total sweep time, and the range of the frequency sweep. This tangential frequency sweep can produce substantially better inversion than composite pulses of the same amplitude.