Faculty Bibliography

Echo-planar imaging (EPI) -based diffusion tensor imaging (DTI) is particularly prone to spike noise. However, existing spike noise correction methods are impractical for corrupted DTI data because the methods correct the complex MRI signal, which is not usually stored on clinical MRI systems. The present work describes a novel Outlier Detection De-spiking technique (ODD) that consists of three steps: detection, localization, and correction. Using automated outlier detection schemes, ODD exploits the data redundancy available in DTI data sets that are acquired with a minimum of six different diffusion-weighted images (DWIs) with similar signal and noise properties. A mathematical formulation, describing the effects of spike noise on magnitude images, yields appropriate measures for an outlier detection scheme used for spike detection while a normalization-dependent outlier detection scheme is used for spike localization. ODD performs accurately on diverse DTI data sets corrupted by spike noise and can be used for automated control of DTI data quality. ODD can also be extended to other MRI applications with data redundancy, such as dynamic imaging and functional MRI.

This study aims to assess the feasibility of a protocol to diagnose renovascular disease using dual MR renography (MRR) acquisitions: before and after administration of angiotensin converting enzyme inhibitor (ACEi). Results of our simulation study aimed at testing the reproducibility of glomerular filtration rate (GFR) and renal plasma flow (RPF) demonstrate that for a fixed overall dose of 12 ml gadolinium-based contrast material (500 mmol/L), the second dose should be approximately twice as large as the first dose. A three-compartment model for analyzing the second-injection data was shown to appropriately handle the tracer residue from the first injection. The optimized protocol was applied to 18 hypertensive patients without renovascular disease, showing minimal systematic difference in GFR measurements before and after ACEi of 0.8+/-4.4 ml/min or 2.7%+/-14.9%. For 10 kidneys with significant renal artery stenosis, GFR decreased significantly after ACEi (P < 0.001, T-value = 3.79), and the difference in GFR measurements before and after ACEi averaged 8.3+/-6.9 ml/min or 26.2%+/-43.9%. Dual-injection MRI with optimized dose distribution appears promising for ACEi renography by offering measures of GFR changes with clinically acceptable precision and accuracy. Key words: angiotensin converting enzyme inhibitor, glomerular filtration rate, renovascular disease, compartmental modeling

After institutional review board approval and informed consent were obtained for this HIPAA-compliant investigation, a three-dimensional electrocardiographically gated variable flip angle (VFA) fast spin-echo magnetic resonance (MR) angiography technique was evaluated as an unenhanced method for imaging hand arteries in 13 subjects (including four patients) at 3.0 T; this included evaluation of vessel visualization with warming and cooling in seven subjects. Examinations were evaluated for image quality and vessel conspicuity. Clear separation of arteries from veins was achieved in all subjects, with excellent vessel conspicuity and depiction of stenoses. Warming improved vessel visualization in healthy volunteers. VFA MR angiography is a high-spatial-resolution technique that enables the assessment of vascular reactivity in response to temperature challenge

PURPOSE: To validate a new method for converting MR arterial signal intensity versus time curves to arterial input functions (AIFs). MATERIALS AND METHODS: The method constrains AIF with patient's cardiac output (Q). Monte Carlo simulations of MR renography and tumor perfusion protocols were carried out for comparison with two alternative methods: direct measurement and population-averaged input function. MR renography was performed to assess the method's inter- and intraday reproducibility for renal parameters. RESULTS: In simulations of tumor perfusion, the precision of the parameters (K(trans) and v(e)) computed using the proposed method was improved by at least a factor of three compared to direct measurement. Similar improvements were obtained in simulations of MR renography. Volunteer study for testing interday reproducibility confirmed the improvement of precision in renal parameters when using the proposed method compared to conventional methods. In another patient study (two injections within one session), the proposed method significantly increased the correlation coefficient (R) between GFR of the two exams (0.92 vs. 0.83) compared to direct measurement. CONCLUSION: A new method significantly improves the precision of dynamic contrast-enhanced (DCE) parameters. The method may be especially useful for analyzing repeated DCE examinations, such as monitoring tumor therapy or angiotensin converting enzyme-inhibitor renography. J. Magn. Reson. Imaging 2009;30:656-665. (c) 2009 Wiley-Liss, Inc

The objective of the study was to assess predictive value of serial diffusion tensor MRI (DTI) for the white matter injury and neurodevelopmental outcome in a cohort of premature infants. Twenty-four infants less than 32 weeks' gestation were stratified to a control group (n = 11), mild brain injury with grades 1-2 of intraventricular hemorrhage (n = 6) and severe brain injury with grades 3-4 intraventricular hemorrhage (n = 4). Serial DTI studies were performed at around 30 and 36 weeks' gestation. Fractional anisotropy (FA) and apparent diffusion coefficient were calculated. Twelve infants were followed up for developmental outcome. Developmental testing was performed with the Bayley Scales of Infant Development to obtain psychomotor index (Performance Developmental Index). Apparent diffusion coefficient was higher in the severe injury group at the second MRI in the central and occipital white matter, and corona radiata; FA was lower in optic radiation compared to controls. Performance Developmental Index score correlated with FA on the scan taken at the 30th week and inversely with the change of FA between scans in internal capsule and occipital white matter. A low value of FA at 30 weeks and a higher change of FA predicted less favorable motor outcome at 2 years and suggests that early subtle white matter injury can be detected in premature infants even without obvious signs of injury.

PURPOSE: To determine normative R2* values in the liver and heart at 3T, and establish the relationship between R2* at 3T and 1.5T over a range of tissue iron concentrations. MATERIALS AND METHODS: A total of 20 healthy control subjects and 14 transfusion-dependent patients were scanned at 1.5T and 3T. At each field strength R2* imaging was performed in the liver and heart. RESULTS: Normative R2* values in the liver were estimated from the control group to be 39.2 +/- 9.0 second(-1) at 1.5T and 69.1 +/- 21.9 second(-1) at 3T. Normative cardiac values were estimated as 23.4 +/- 2.2 second(-1) at 1.5T and 30.0 +/- 3.7 second(-1) at 3T. The combined R2* data from patients and control subjects exhibited a linear relationship between 3T and 1.5T. In the liver, the line of best fit to the 3T vs. 1.5T data had a slope of 2.00 +/- 0.06 and an intercept of -11 +/- 4 second(-1). In the heart, it had a slope of 1.88 +/- 0.14 and an intercept of -15 +/- 4 second(-1). CONCLUSION: These preliminary data suggest that the iron-dependent component of R2* scales linearly with field strength over a wide range of tissue iron concentrations. The incidence of susceptibility artifacts may, however, also increase with field strength.

Partial k-space sampling is frequently used in single-shot diffusion-weighted echo-planar imaging (DW-EPI) to reduce the TE and thereby improve the SNR. However, it increases the sensitivity of the technique to bulk rotational motion, which introduces a phase gradient across the tissue that shifts the echo in k-space. If the echo is displaced into the high spatial frequencies, conventional homodyne reconstruction fails, causing intensity oscillations across the image. Zero-padding, on the other hand, compromises the image resolution and may cause truncation artifacts. We present an adaptive version of the homodyne algorithm that detects the location of the echo in k-space and adjusts the center and width of the homodyne filters accordingly. The adaptive algorithm produces artifact-free images when the echo is shifted into the high positive k-space range, and reduces to the standard homodyne algorithm in the absence of bulk motion.

We describe a novel physical basis and methodology for gadolinium (Gd)-enhanced MRA, which we call 'off-resonance contrast angiography' (ORCA). Unlike standard contrast-enhanced (CE) MR angiography (MRA), ORCA contrast depends not on T(1) but on Gd-induced shifts in intravascular resonance frequency due to the bulk magnetic susceptibility (BMS) effects of Gd. The method was tested at 3 Tesla in phantoms with a range of dilutions of Gd-DTPA and ultrasmall iron oxide contrast agent (CA). With the use of ORCA, complete background suppression was obtained without image subtraction. As a result, catheters filled with various Gd dilutions proved to be highly conspicuous in ORCA projection images. This feature may make ORCA particularly attractive for passive catheter tracking during MR-guided endovascular procedures. Gd-induced intravascular frequency shifts were measured in human subjects and found to be in the expected range. ORCA was used to create angiograms of forearm veins that were comparable in quality to standard CE-MRA. In addition, ORCA images of the extracranial carotid bifurcation were successfully acquired during intravenous contrast administration. However, significant technical restrictions also exist, including a dependence on vessel orientation with respect to B(0), and sensitivity to static field inhomogeneities. Further study is needed to determine the practicality and potential clinical utility of this method

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.