Faculty Bibliography

PURPOSE: To evaluate the effect of different methods to convert magnetic resonance (MR) signal intensity (SI) to gadolinium concentration ([Gd]) on estimation and reproducibility of model-free and modeled hepatic perfusion parameters measured with dynamic contrast-enhanced (DCE)-MRI. MATERIALS AND METHODS: In this Institutional Review Board (IRB)-approved prospective study, 23 DCE-MRI examinations of the liver were performed on 17 patients. SI was converted to [Gd] using linearity vs. nonlinearity assumptions (using spoiled gradient recalled echo [SPGR] signal equations). The [Gd] vs. time curves were analyzed using model-free parameters and a dual-input single compartment model. Perfusion parameters obtained with the two conversion methods were compared using paired Wilcoxon test. Test-retest and interobserver reproducibility of perfusion parameters were assessed in six patients. RESULTS: There were significant differences between the two conversion methods for the following parameters: AUC60 (area under the curve at 60 s, P < 0.001), peak gadolinium concentration (Cpeak, P < 0.001), upslope (P < 0.001), Fp (portal flow, P = 0.04), total hepatic flow (Ft, P = 0.007), and MTT (mean transit time, P < 0.001). Our preliminary results showed acceptable to good reproducibility for all model-free parameters for both methods (mean coefficient of variation [CV] range, 11.87-23.7%), except for upslope (CV = 37%). Among modeled parameters, DV (distribution volume) had CV <22% with both methods, PV and MTT showed CV <21% and <29% using SPGR equations, respectively. Other modeled parameters had CV >30% with both methods. CONCLUSION: Linearity assumption is acceptable for quantification of model-free hepatic perfusion parameters while the use of SPGR equations and T1 mapping may be recommended for the quantification of modeled hepatic perfusion parameters. J. Magn. Reson. Imaging 2014;40:90-98 (c) 2013 Wiley Periodicals, Inc.

Established as a method to study anatomic changes, such as renal tumors or atherosclerotic vascular disease, magnetic resonance imaging (MRI) to interrogate renal function has only recently begun to come of age. In this review, we briefly introduce some of the most important MRI techniques for renal functional imaging, and then review current findings on their use for diagnosis and monitoring of major kidney diseases. Specific applications include renovascular disease, diabetic nephropathy, renal transplants, renal masses, acute kidney injury, and pediatric anomalies. With this review, we hope to encourage more collaboration between nephrologists and radiologists to accelerate the development and application of modern MRI tools in nephrology clinics.

OBJECTIVE. The purpose of this study was to evaluate split renal function, estimate single-kidney renal function, and identify cause of obstruction in patients with ureteropelvic junction (UPJ) obstruction by using contrast-enhanced dynamic MR renography (MRR). MATERIALS AND METHODS. Seventeen patients with UPJ obstruction underwent MRR and diuresis nuclear renography. Nuclear renography assessment of split renal function and mechanical versus functional obstruction served as the reference standard. The Baumann-Rudin model for determining glomerular filtration rate (GFR) was applied to generate single-kidney renal function (SK-GFRMRR) from MRR cortical and medullary enhancement curves. MRR split renal function of the right kidney (SK-GFRMRR of the right kidney normalized to the sum of SK-GFRMRR of both kidneys) was compared with nuclear renography. The MRR estimate of total GFR (eGFRMRR) was compared with that derived from Modification of Diet in Renal Disease (MDRD) formula (eGFRMDRD). Renal pelvic rate of signal intensity change (PUR) was compared between functionally and mechanically obstructed kidneys. RESULTS. There was excellent correlation between MRR and nuclear renography measure of split renal function ratio (r = 0.87, p < 0.01), with mean difference of less than 10%. There was moderate correlation (r = 0.60, p = 0.01) between eGFRMRR and eGFRMDRD. eGFRMRR underestimated eGFRMDRD, with mean difference of 13.3 mL/min/1.73 m(2). PUR in mechanically obstructed units was significantly lower (0.39 +/- 0.26 vs 2.0 +/- 1.38 min(-1); p < 0.01) compared with functionally obstructed units. PUR discriminated mechanical from functional obstruction with accuracy of 89%. CONCLUSION. In patients with UPJ obstruction, MRR can measure split renal function, estimate eGFRMDRD with moderate correlation, and accurately discriminate mechanical from functional obstruction, thus potentially providing a "one-stop shop" examination.

Blood oxygen level-dependent (BOLD) MRI data of kidney, while indicative of tissue oxygenation level (Po2), is in fact influenced by multiple confounding factors, such as R2, perfusion, oxygen permeability, and hematocrit. We aim to explore the feasibility of extracting tissue Po2 from renal BOLD data. A method of two steps was proposed: first, a Monte Carlo simulation to estimate blood oxygen saturation (SHb) from BOLD signals, and second, an oxygen transit model to convert SHb to tissue Po2. The proposed method was calibrated and validated with 20 pigs (12 before and after furosemide injection) in which BOLD-derived tissue Po2 was compared with microprobe-measured values. The method was then applied to nine healthy human subjects (age: 25.7 +/- 3.0 yr) in whom BOLD was performed before and after furosemide. For the 12 pigs before furosemide injection, the proposed model estimated renal tissue Po2 with errors of 2.3 +/- 5.2 mmHg (5.8 +/- 13.4%) in cortex and -0.1 +/- 4.5 mmHg (1.7 +/- 18.1%) in medulla, compared with microprobe measurements. After injection of furosemide, the estimation errors were 6.9 +/- 3.9 mmHg (14.2 +/- 8.4%) for cortex and 2.6 +/- 4.0 mmHg (7.7 +/- 11.5%) for medulla. In the human subjects, BOLD-derived medullary Po2 increased from 16.0 +/- 4.9 mmHg (SHb: 31 +/- 11%) at baseline to 26.2 +/- 3.1 mmHg (SHb: 53 +/- 6%) at 5 min after furosemide injection, while cortical Po2 did not change significantly at approximately 58 mmHg (SHb: 92 +/- 1%). Our proposed method, validated with a porcine model, appears promising for estimating tissue Po2 from renal BOLD MRI data in human subjects.

The precision, accuracy, and efficiency of a novel semi-automated renal segmentation technique for volumetric interpolated breath-hold sequence (VIBE) MRI sequences was analyzed using 7 clinical datasets (14 kidneys). Two observers performed whole-kidney segmentation using EdgeWave segmentation software based on constrained morphological growth. Ground truths were prepared by manual tracing of kidney on each of approximately 40 slices. Using the software, the average inter-observer disagreement was 2.7%+/- 2.1% for whole kidney volume, 2.1%+/- 1.8% for cortex, and 4.1%+/- 3.2% for medulla. In comparison to the ground truth kidney volume, the error was 2.8%+/- 2.5% for whole kidney volume, 3.1%+/- 1.7% for cortex, and 3.6%+/-.3.1% for medulla. It took an average of 4:14 +/- 1:42 minutes of operator time, plus 2:56 +/- 1:23 minutes of computer time to perform segmentation of one whole kidney, cortex, and medulla. Segmentation speed, inter-observer agreement and accuracy were several times better than those of our existing graph-cuts segmentation technique requiring approximately 20 minutes per case and with 7-10% error. With the expedited image processing, high inter-observer agreement, and volumes closely matching true values, kidney volumetry becomes a reality in many clinical applications.

In hypertension (HTN), cerebral blood flow regulation limits are changed, and the threshold for blood pressure (BP) at which perfusion is safely maintained is higher. This shift may increase the brain's vulnerability to lower BP in subjects with vascular disease. We investigated whether longitudinal reduction in mean arterial pressure (MAP) was related to changes in cerebrospinal fluid (CSF) biomarkers of Alzheimer's disease in a group of cognitively healthy elderly with and without HTN. The relationships among MAP, memory decline, and hippocampal atrophy were also examined. Seventy-seven subjects (age 63.4 +/- 9.4, range 44-86 years; education 16.9 +/- 2.1, range 10-22 years; 60% women) were assessed twice, 2 +/- 0.5 years apart. At both time points, all subjects underwent full medical and neuropsychological evaluations, lumbar punctures, and magnetic resonance imaging examinations. Twenty-five subjects had HTN. Hyper- and normotensive subjects did not differ in their CSF biomarkers, hippocampal volumes (HipVs), or memory scores at baseline. In the entire study group, the increase in tau phosphorylated at threonine 181 (p-tau181) was associated with a decline in verbal episodic memory (beta = -0.30, p = 0.01) and HipV reduction (beta = -0.27, p = 0.02). However, longitudinal decrease in MAP was related to memory decline (beta = 0.50, p = 0.01) and an increase in p-tau181 (beta = -0.50, p = 0.01) only in subjects with HTN. Our findings suggest that the hypertensive group may be sensitive to BP reductions.

PURPOSE: To assess the quality of the arterial input function (AIF) reconstructed using a dedicated pre-bolus low-dose contrast material injection imaged with a high temporal resolution and the resulting estimated liver perfusion parameters. MATERIALS AND METHODS: In this IRB-approved prospective study, 24 DCE-MRI examinations were performed in 21 patients with liver disease (M/F 17/4, mean age 56 y). The examination consisted of 1.3 mL and 0.05 mmol/kg of gadobenate dimeglumine for pre-bolus and main bolus acquisitions, respectively. The concentration-curve of the abdominal aorta in the pre-bolus acquisition was used to reconstruct the AIF. AIF quality and shape parameters obtained with pre-bolus and main bolus acquisitions and the resulting estimated hepatic perfusion parameters obtained with a dual-input single compartment model were compared between the 2 methods. Test-retest reproducibility of perfusion parameters were assessed in three patients. RESULTS: The quality of the pre-bolus AIF curve was significantly better than that of main bolus AIF. Shape parameters peak concentration, area under the time activity curve of gadolinium contrast at 60 s and upslope of pre-bolus AIF were all significantly higher, while full width at half maximum was significantly lower than shape parameters of main bolus AIF. Improved liver perfusion parameter reproducibility was observed using pre-bolus acquisition [coefficient of variation (CV) of 4.2%-38.7% for pre-bolus vs. 12.1-71.4% for main bolus] with the exception of distribution volume (CV of 23.6% for pre-bolus vs. 15.8% for main bolus). The CVs between pre-bolus and main bolus for the perfusion parameters were lower than 14%. CONCLUSION: The AIF reconstructed with pre-bolus low dose contrast injection displays better quality and shape parameters and enables improved liver perfusion parameter reproducibility, although the resulting liver perfusion parameters demonstrated no clinically significant differences between pre-bolus and main bolus acquisitions.

AIM: To assess a novel method of three-dimensional (3D) co-registration of prostate cancer digital histology and in-vivo multiparametric magnetic resonance imaging (mpMRI) image sets for clinical usefulness. MATERIAL AND METHODS: A software platform was developed to achieve 3D co-registration. This software was prospectively applied to three patients who underwent radical prostatectomy. Data comprised in-vivo mpMRI [T2-weighted, dynamic contrast-enhanced weighted images (DCE); apparent diffusion coefficient (ADC)], ex-vivo T2-weighted imaging, 3D-rebuilt pathological specimen, and digital histology. Internal landmarks from zonal anatomy served as reference points for assessing co-registration accuracy and precision. RESULTS: Applying a method of deformable transformation based on 22 internal landmarks, a 1.6 mm accuracy was reached to align T2-weighted images and the 3D-rebuilt pathological specimen, an improvement over rigid transformation of 32% (p = 0.003). The 22 zonal anatomy landmarks were more accurately mapped using deformable transformation than rigid transformation (p = 0.0008). An automatic method based on mutual information, enabled automation of the process and to include perfusion and diffusion MRI images. Evaluation of co-registration accuracy using the volume overlap index (Dice index) met clinically relevant requirements, ranging from 0.81-0.96 for sequences tested. Ex-vivo images of the specimen did not significantly improve co-registration accuracy. CONCLUSION: This preliminary analysis suggests that deformable transformation based on zonal anatomy landmarks is accurate in the co-registration of mpMRI and histology. Including diffusion and perfusion sequences in the same 3D space as histology is essential further clinical information. The ability to localize cancer in 3D space may improve targeting for image-guided biopsy, focal therapy, and disease quantification in surveillance protocols.

OBJECTIVE. The objective of our study was to report our initial experience with dynamic contrast-enhanced MRI (DCE-MRI) for perfusion quantification of hepatocellular carcinoma (HCC) and surrounding liver. SUBJECTS AND METHODS. DCE-MRI of the liver was prospectively performed on 31 patients with HCC (male-female ratio, 26:5; mean age, 61 years; age range, 41-83 years). A dynamic coronal 3D FLASH sequence was performed at 1.5 T before and after injection of gadolinium-based contrast agent with an average temporal resolution of 3.8 seconds. Regions of interest were drawn on the abdominal aorta, portal vein, liver parenchyma, and HCC lesions by two observers in consensus. Time-activity curves were analyzed using a dual-input single-compartment model. The following perfusion parameters were obtained: arterial flow, portal venous flow, arterial fraction, distribution volume, and mean transit time (MTT). RESULTS. Thirty-three HCCs (mean size, 3.9 cm; range, 1.1-12.6 cm) were evaluated in 26 patients. When compared with liver parenchyma, HCC showed significantly higher arterial hepatic blood flow and arterial fraction (p < 0.0001) and significantly lower distribution volume and portal venous hepatic blood flow (p < 0.0001-0.023), with no difference in MTT. Untreated HCCs (n = 16) had a higher arterial fraction and lower portal venous hepatic blood flow value than chemoembolized HCCs (n = 17, p < 0.04). CONCLUSION. DCE-MRI can be used to quantify perfusion metrics of HCC and liver parenchyma and to assess perfusion changes after HCC chemoembolization.