Faculty Bibliography

OBJECTIVE: The objective of this study was to implement a three-dimensional (3-D) T1 mapping sequence (3DLL) at 3.0 T for dGEMRIC based on the Look Locker scheme. MATERIALS AND METHODS: Because all current reports on dGEMRIC are at 1.5 T and mostly using 2-D IR fast spin echo (FSE), data were acquired at 1.5 T and 3.0 T with both 3DLL and 2-D IR-FSE sequence. Phantoms with different concentrations of Gd(DTPA) were used and seven subjects (three asymptomatic, four symptomatic) were scanned using the dGEMRIC technique. RESULTS: The T1 measurements obtained on the phantom with 3DLL show very good agreement with those acquired with 2-D IR-FSE. Using a two-tailed paired t test, the T1 (Gd) measurements in two sections obtained in all subjects with both sequences were found to be statistically indistinguishable at either field strength (P = 0.07 at 1.5 T and P = 0.07 at 3.0 T). CONCLUSIONS: The preliminary data presented here suggest that the 3DLL sequence provides accurate T1 values with sufficient in-plane resolution and allows full joint coverage in less than 10 minutes.

This chapter provides a brief introduction to the principles and practice of magnetic resonance imaging and spectroscopy. Its goal is to equip researchers in the life sciences with a basic understanding of the capabilities and limitations of magnetic resonance techniques, and a command of the terminology used in more technical publications, including the methods sections of this book. Magnetic resonance is extremely versatile, and this introductory chapter attempts to provide an indication of its current range of applications, as well as emerging possibilities. Many of the applications mentioned here are described in greater detail in the later chapters. It is hoped that this introduction may provide some guidance to the reader in navigating the rest of the book, and in identifying ways to exploit magnetic resonance imaging and spectroscopy in his or her own research.

Diffusion tensor imaging (DTI) was used to derive in vivo tissue status measurements of subcortical brain regions that are vulnerable to injury in human immunodeficiency virus (HIV)-infected patients. Quantitative measurements, including the mean diffusivity (MD) and fractional anisotropy (FA), were determined in lateralized basal ganglia (caudate and putamen) and centrum semiovale in 11 well-characterized HIV patients and in 11 control subjects. DTI measurements were examined for patterns of relationship with markers of clinical and cognitive progression. DTI measures acquired in subcortical regions were significantly correlated with loss of function in specific cognitive domains. Significant relationships were identified between measures for putamen and verbal memory (MD), visual memory (FA), working memory (FA), and overall cognitive impairment (MD). Measures for caudate (FA) were significantly correlated with visual memory. Measures for centrum semiovale were significantly correlated with visual memory deficits (MD) and visuoconstruction (FA). Relationships between anisotropy measures and anemia (basal ganglia) and CD4 counts (centrum semiovale) were also observed. Findings from this investigation indicate that DTI is a sensitive tool for correlating neuroanatomic pathologic features with specific cognitive deficits in patients with HIV infection.

PURPOSE: To demonstrate a differential response following administration of a free radical scavenger, tempol, in kidneys of hypertensive compared to normotensive rats. MATERIAL AND METHODS: Data were obtained in spontaneously hypertensive rats (SHR, N = 5). Wistar-Kyoto rats (WKY, N = 6) were used as normotensive controls. RESULTS: Consistent with prior reports, SHRs show a significant response to tempol (R(2)*decreased from 40.56 +/- 0.66 second(-1) to 28.58 +/- 0.6 second(-1) in medulla, P < 0.05), while WKY rats exhibit a minimal change (R(2)* measuring 22.36 +/- 4.38 second(-1) pre-tempol and 21.57 +/- 4.78 second(-1) post-tempol, in medulla). The post-tempol R(2)* in SHR was found to be comparable to pre-tempol values in WKY rats, suggesting an improved medullary oxygenation in SHRs. CONCLUSION: Based on both baseline R(2)* values and the differential effect of the free radical scavenger on renal medullary oxygenation, BOLD MRI can distinguish hypertensive from normal kidney in rats

PURPOSE: To test the hypothesis that, in magnetic resonance (MR) imaging of healthy individuals, equal relative changes in lung volume cause equal relative changes in MR signal intensity of the lung parenchyma. MATERIALS AND METHODS: In two experimental runs, 10 volunteers underwent spirometrically monitored MR imaging of the lungs, with MR images acquired at 10 incremental lung volumes ranging from total lung capacity to 10% above residual volume. Average signal intensity, signal variability, and signal intensity integrals were calculated for each volunteer and for each lung volume. The effect of lung volume on signal intensity was quantified using linear regression analysis complemented by the runs test. Slopes and intercepts of regression lines were compared with an analysis of covariance. Slopes of the lines of best fit for lung volumes and signal intensities from the two runs were compared to the slope of the line of identity. Comparisons between the two runs were visualized using Bland and Altman plots. RESULTS: The slopes of the 10 individual regression lines yielded no significant differences (F = 1.703, P = 0.101; F = 1.321, P = 0.239). The common slopes were -0.556 +/- 0.027 (P = 0.0001) for the first and -0.597 +/- 0.0031 (P = 0.0001) for the second experimental run. Both slopes displayed no significant nonlinearity (P = 0.419 and P = 0.067). There was a strong association between changes in lung volumes (rs = 0.991, P = 0.0001) and changes in signal intensity (rs = 0.889, P = 0.0001) in the two experimental runs. Lines of best fit for lung volume and signal intensities were not significantly different from the slope of the line of identity (P = 0.321 and P = 0.212, respectively). CONCLUSION: Equal changes in lung volume cause equal changes in MR signal intensity of the lung parenchyma. This linear and reproducible phenomenon could be helpful in comparing pulmonary MR signal intensity between individuals

Use of a parallel imaging technique to improve temporal and spatial resolution at three-dimensional contrast-enhanced magnetic resonance (MR) angiography was investigated. Thirty experiments were performed in five groups of healthy subjects. In groups 1-3, the technique was used to improve imaging speed by a factor of two or four while maintaining spatial resolution. Contrast agent concentration was two to four times higher than at standard MR angiography, to take advantage of the faster imaging speed. In groups 4 and 5, the technique was used to double spatial resolution in the phase-encoding direction while maintaining imaging speed and contrast agent concentration. At a two to four times faster imaging speed, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) almost equaled those at standard MR angiography, likely a result of increased contrast agent concentration. The use of parallel imaging to achieve higher spatial resolution was also proved feasible, but with substantial reduction in SNR and CNR

PURPOSE: To estimate the reproducibility of BOLD MRI measurements in the evaluation of intrarenal oxygenation levels. MATERIALS AND METHODS: In this study, the reproducibility of semiquantitative BOLD MRI measurements performed on a 1.5 T scanner with a multiple gradient-echo sequence in the renal medulla and cortex, and their response to furosemide and waterload, were assessed in eight healthy young subjects (25.6 +/- 4.1 years). Each subject underwent an identical experimental procedure on two separate days. RESULTS: Renal R2* measurements were shown to be reproducible within approximately 12% from day to day based on a coefficient of variance (CV) analysis. The changes in R2* (DeltaR2*) following administration of furosemide were statistically significant, as shown by ANOVA and a paired Student's t-test, and were deemed reliable based on the reliable change index (RCI). However, DeltaR2* values following waterload were not statistically significant, and were not deemed reliable. CONCLUSION: R2* measurements were reproducible over 270 days within 12%. Furosemide produced a significant and reliable change (approximately 30%), and the magnitude of change (5.7 s(-1)) was reproducible and consistent with our previous data. The response to waterload, however, did not reach statistical significance, and the magnitude did not reach the level that we had previously reported

BACKGROUND AND PURPOSE: Quantitative MR imaging strategies may have considerable potential for in vivo assessment of neuropathologic changes associated with HIV. This investigation evaluated the prognostic significance of whole brain histogram-derived diffusion tensor imaging indices with respect to severity of cognitive impairment and measures of clinical status in cases of HIV. METHODS: Quantitative indices derived with diffusion tensor imaging, including whole brain fractional anisotropy and the apparent diffusion coefficient, were compared for six patients with HIV and eight control volunteers. Relationships between whole brain indices and specific measures of dementia severity and clinical status were examined. RESULTS: Whole brain fractional anisotropy was reduced in patients with HIV and was significantly associated with severity of dementia, as indicated by several widely used clinical and functional status measures. Summary fractional anisotropy measures were more prognostic of dementia status than were apparent diffusion coefficient measures. CONCLUSION: Findings from this investigation support the use of diffusion tensor imaging for noninvasive MR imaging measurement of neuropathologic changes in studies of HIV-associated cognitive impairment.

Steady-state free precession cine images from cardiac magnetic resonance imaging studies of 24 patients were reviewed retrospectively to identify dark flow artifacts. The cause and features of the artifacts were studied in flow phantom experiments. Dark flow artifacts were recognized in eight of the 24 cases and were characterized by low or inhomogeneous signal intensity in blood pools with little change in adjacent tissues. The artifacts could be mimicked in flow phantom experiments by deliberately deshimming the gradients and appeared periodically during imaging with off-centered frequencies. These artifacts appeared to be caused by spins moving within an inhomogeneous magnetic field

Steady-state free precession (SSFP) cardiac cine images are frequently corrupted by dark flow artifacts, which can usually be eliminated by reshimming and retuning the scanner. A theoretical explanation for these artifacts is provided in terms of spins moving through an off-resonant point in the magnetic field, and the theory is validated using phantom experiments. The artifacts can be reproduced in vivo by detuning the center frequency by an amount in the range of half the inverse repetition time (TR). Since this offset is similar in magnitude to the frequency difference between the water and lipid peaks, a likely cause of the artifacts in vivo is that the center frequency is tuned incorrectly to the lipid peak rather than the water peak