Faculty Bibliography

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

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

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

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.

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.

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.

A method for estimating T1 using a single breath-hold, segmented, inversion recovery prepared, true fast imaging with steady-state precession (sIR-TrueFISP) acquisition at low flip angle (FA) was implemented in this study. T1 values measured by sIR-TrueFISP technique in a Gd-DTPA-doped water phantom and the human brain and abdomen of healthy volunteers were compared with the results of the standard IR fast spin echo (FSE) technique. A good correlation between the two methods was observed (R2=0.999 in the phantom, and R2=0.943 in the brain and abdominal tissues). The T1 values of the tissues agreed well with published results. sIR-TrueFISP enables fast measurements of T1 to be obtained within a single breath-hold with good accuracy, which is particularly important for chest and abdominal imaging

PURPOSE: To investigate the tendency of proton MR signal intensity (SI) gradients to be steeper in the supine than in the prone body position, and to quantify the relation between gravity-related and isogravitational changes of SI on proton MR images of the lung. MATERIALS AND METHODS: In eight healthy volunteers, MR images were obtained in the supine and prone positions using a multiple inversion recovery turbo spin-echo (TSE) sequence. The variation in SI along the gravity-dependent direction and within isogravitational planes was measured on a pixel-by-pixel basis. Ratios of slopes were calculated for comparisons among volunteers. Comparisons of ratios were made using Fisher's exact test. Isogravitational variability was compared with the mean SI, the signal-to-noise ratio (SNR), and the image noise. RESULTS: The average ratios of slopes showed that the overall SI gradient was steeper in the supine than the prone position, with a substantial difference in the supine/prone ratios between inspiration (1.21) and expiration (1.72). In both the supine and prone positions, gravity-dependent gradients were steeper in expiration than in inspiration (P = 0.001). The SI variability along the gravitational direction was larger than the isogravitational variability. The isogravitational variability in turn was larger than the image noise but smaller than the mean SI of the MR images. CONCLUSION: Gravity-dependent gradients in proton MR SI are steeper in the supine than in the prone position. The magnitudes of these gradients were larger than the isogravitational signal variability, showing that MRI is sensitive to gravitationally induced effects

PURPOSE: To investigate the MRI characteristics of an intracellular manganese-based contrast agent, EVP 1001-1 (Eagle Vision Pharmaceutical Corp.), in a canine model of myocardial infarction. MATERIALS AND METHODS: Three dogs were imaged 14-37 days following permanent ligation of the left anterior descending coronary artery (LAD). Measurements of the longitudinal relaxation rate R(1) were made prior to EVP 1001-1 administration (20 micromol/kg i.v.) and for one hour thereafter. Triphenyl tetrazolium chloride (TTC) staining was used to document infarction. RESULTS: In normal myocardium, EVP 1001-1 produced a substantial increase in the longitudinal relaxation rate, which remained fairly constant over the postcontrast imaging period (DeltaR1= 1.47 +/- 0.58 sec(-1) (mean +/- SD) at 35 minutes, P < 0.05). In the infarct, the response to EVP 1001-1 was small or negligible (DeltaR1= 0.27 +/- 0.28 sec(-1)). This resulted in a significant postcontrast difference in relaxation rate between normal and infarcted tissue (R1(normal) - R1(infarct) = 1.08 +/- 0.26 sec(-1), P < 0.05). The infarct remained clearly delineated in all animals throughout the steady-state imaging period, and qualitatively matched TTC results. CONCLUSION: The persistent enhancement pattern revealed by MRI following EVP 1001-1 administration may be beneficial for identifying and characterizing myocardial infarction