Pediatric magnetic resonance angiography: to contrast or not to contrast.

Magnetic resonance (MR) angiography and MR venography imaging with contrast and non-contrast techniques are widely used for pediatric vascular imaging. However, as with any MRI examination, imaging the pediatric population can be challenging because of patient motion, which sometimes requires sedation. There are multiple benefits of non-contrast MR angiographic techniques, including the ability to repeat sequences if motion is present, the decreased need for sedation, and avoidance of potential risks associated with gadolinium administration and radiation exposure. Thus, MR angiography is an attractive alternative to CT or conventional catheter-based angiography in pediatric populations. Contrast-enhanced MR angiographic techniques have the advantage of increased signal to noise. Blood pool imaging allows long imaging times that result in high-spatial-resolution imaging, and thus high-quality diagnostic images. This article outlines the technique details, indications, benefits and downsides of non-contrast-enhanced and contrast-enhanced MR angiographic techniques to assist in protocol decision-making.

Non-contrast magnetic resonance angiography/venography techniques: what are my options?

Non-contrast magnetic resonance (MR) angiography and MR venography techniques are gaining popularity for vascular imaging because they are faster, more forgiving and less costly compared with contrast-enhanced MR angiography. Non-contrast MR angiography also avoids gadolinium deposition, which is especially important in imaging children. Non-contrast MR angiography has an array of specific applications for numerous clinical indications. This review summarizes the non-contrast MR angiography methods and their relative advantages and disadvantages. The paper also guides the reader on which technique to consider when determining the optimal imaging modality for each individual patient.

Prospective comparison of longitudinal change in hepatic proton density fat fraction (PDFF) estimated by magnitude-based MRI (MRI-M) and complex-based MRI (MRI-C).

PURPOSE: To compare longitudinal hepatic proton density fat fraction (PDFF) changes estimated by magnitude- vs. complex-based chemical-shift-encoded MRI during a weight loss surgery (WLS) program in severely obese adults with biopsy-proven nonalcoholic fatty liver disease (NAFLD). METHODS: This was a secondary analysis of a prospective dual-center longitudinal study of 54 adults (44 women; mean age 52 years; range 27-70 years) with obesity, biopsy-proven NAFLD, and baseline PDFF ≥ 5%, enrolled in a WLS program. PDFF was estimated by confounder-corrected chemical-shift-encoded MRI using magnitude (MRI-M)- and complex (MRI-C)-based techniques at baseline (visit 1), after a 2- to 4-week very low-calorie diet (visit 2), and at 1, 3, and 6 months (visits 3 to 5) after surgery. At each visit, PDFF values estimated by MRI-M and MRI-C were compared by a paired t test. Rates of PDFF change estimated by MRI-M and MRI-C for visits 1 to 3, and for visits 3 to 5 were assessed by Bland-Altman analysis and intraclass correlation coefficients (ICCs). RESULTS: MRI-M PDFF estimates were lower by 0.5-0.7% compared with those of MRI-C at all visits (p < 0.001). There was high agreement and no difference between PDFF change rates estimated by MRI-M vs. MRI-C for visits 1 to 3 (ICC 0.983, 95% CI 0.971, 0.99; bias = - 0.13%, p = 0.22), or visits 3 to 5 (ICC 0.956, 95% CI 0.919-0.977%; bias = 0.03%, p = 0.36). CONCLUSION: Although MRI-M underestimates PDFF compared with MRI-C cross-sectionally, this bias is consistent and MRI-M and MRI-C agree in estimating the rate of hepatic PDFF change longitudinally. KEY POINTS: • MRI-M demonstrates a significant but small and consistent bias (0.5-0.7%; p < 0.001) towards underestimation of PDFF compared with MRI-C at 3 T. • Rates of PDFF change estimated by MRI-M and MRI-C agree closely (ICC 0.96-0.98) in adults with severe obesity and biopsy- proven NAFLD enrolled in a weight loss surgery program. • Our findings support the use of either MRI technique (MRI-M or MRI-C) for clinical care or by individual sites or for multi-center trials that include PDFF change as an endpoint. However, since there is a bias in their measurements, the same technique should be used in any given patient for longitudinal follow-up.

Monitoring Fatty Liver Disease with MRI Following Bariatric Surgery: A Prospective, Dual-Center Study.

Purpose To longitudinally monitor liver fat before and after bariatric surgery by using quantitative chemical shift-encoded (CSE) MRI and to compare with changes in body mass index (BMI), weight, and waist circumference (WC). Materials and Methods For this prospective study, which was approved by the internal review board, a total of 126 participants with obesity who were undergoing evaluation for bariatric surgery with preoperative very low calorie diet (VLCD) were recruited from June 27, 2010, through May 5, 2015. Written informed consent was obtained from all participants. Participants underwent CSE MRI measuring liver proton density fat fraction (PDFF) before VLCD (2-3 weeks before surgery), after VLCD (1-3 days before surgery), and 1, 3, and 6-10 months following surgery. Linear regression was used to estimate rates of change of PDFF (ΔPDFF) and body anthropometrics. Initial PDFF (PDFF0), initial anthropometrics, and anthropometric rates of change were evaluated as predictors of ΔPDFF. Mixed-effects regression was used to estimate time to normalization of PDFF. Results Fifty participants (mean age, 51.0 years; age range, 27-70 years), including 43 women (mean age, 50.8 years; age range, 27-70 years) and seven men (mean age, 51.7 years; age range, 36-62 years), with mean PDFF0 ± standard deviation of 18.1% ± 8.6 and mean BMI0 of 44.9 kg/m2 ± 6.5 completed the study. By 6-10 months following surgery, mean PDFF decreased to 4.9% ± 3.4 and mean BMI decreased to 34.5 kg/m2 ± 5.4. Mean estimated time to PDFF normalization was 22.5 weeks ± 11.5. PDFF0 was the only strong predictor for both ΔPDFF and time to PDFF normalization. No body anthropometric correlated with either outcome. Conclusion Average liver proton density fat fraction (PDFF) decreased to normal (< 5%) by 6-10 months following surgery, with mean time to normalization of approximately 5 months. Initial PDFF was a strong predictor of both rate of change of PDFF and time to normalization. Body anthropometrics did not predict either outcome. Online supplemental material is available for this article. © RSNA, 2018.

Development and validation of an open source Monte Carlo dosimetry model for wide-beam CT scanners using Fluka.

PURPOSE: Development and validation of an open source Fluka-based Monte Carlo source model for diagnostic patient dose calculations. METHODS: A framework to simulate a computed tomography (CT) scanner using Fluka Monte Carlo particle transport code was developed. The General Electric (GE) Revolution scanner with the large body filter and 120 kV tube potential was characterized using measurements. The model was validated on benchmark CT test problems and on dose measurements in computed tomography dose index (CTDI) and anthropomorphic phantoms. Axial and helical operation modes with provision for tube current modulation (TCM) were implemented. The particle simulations in Fluka were accelerated by executing them on a high-performance computing cluster. RESULTS: The simulation results agreed to better than an average of 4% of the reference simulation results from the AAPM Report 195 test scenarios, namely: better than 2% for both test problems in case 4 using the PMMA phantom, and better than 5% of the reference result for 14 of 17 organs in case 5, and within 10% for the three remaining organs. The Fluka simulation results agreed to better than 2% of the air kerma measured in-air at isocenter of the GE Revolution scanner. The simulated air kerma in the center of the CTDI phantom overestimated the measurement by 7.5% and a correction factor was introduced to account for this. The simulated mean absorbed doses for a chest scan of the pediatric anthropomorphic phantom was completed in ~9 min and agreed to within the 95% CI for bone, soft tissue, and lung measurements made using MOSFET detectors for fixed current axial and helical scans as well as helical scan with TCM. CONCLUSION: A Fluka-based Monte Carlo simulation model of axial and helical acquisition techniques using a wide-beam collimation CT scanner demonstrated good agreement between measured and simulated results for both fixed current and TCM in complex and simple geometries. Code and dataset will be made available at https://github.com/chezhia/FLUKA_CT.

Fully phase-encoded MRI near metallic implants using ultrashort echo times and broadband excitation.

PURPOSE: To develop a fully phase-encoded MRI method for distortion-free imaging near metallic implants, in clinically feasible acquisition times. THEORY AND METHODS: An accelerated 3D fully phase-encoded acquisition with broadband excitation and ultrashort echo times is presented, which uses a broadband radiofrequency pulse to excite the entire off-resonance induced by the metallic implant. Furthermore, fully phase-encoded imaging is used to prevent distortions caused by frequency encoding, and to obtain ultrashort echo times for rapidly decaying signal. RESULTS: Phantom and in vivo acquisitions were used to describe the relationship among excitation bandwidth, signal loss near metallic implants, and T1 weighting. Shorter radiofrequency pulses captured signal closer to the implant by improving spectral coverage and allowing shorter echo times, whereas longer pulses improved T1 weighting through larger maximum attainable flip angles. Comparisons of fully phase-encoded acquisition with broadband excitation and ultrashort echo times to T1 -weighted multi-acquisition with variable resonance image combination selective were performed in phantoms and subjects with metallic knee and hip prostheses. These acquisitions had similar contrast and acquisition efficiency. CONCLUSIONS: Accelerated fully phase-encoded acquisitions with ultrashort echo times and broadband excitation can generate distortion free images near metallic implants in clinically feasible acquisition times. Magn Reson Med 79:2156-2163, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

The role of routine imaging in pediatric cutaneous melanoma.

BACKGROUND: Optimal imaging for children with pediatric malignant melanoma (MM) is unknown. METHODS: We reviewed clinical and imaging findings of patients with American Joint Commission on Cancer (AJCC) stage IIC-IV MM treated on our institutional MEL06 trial. All patients had baseline brain magnetic resonance imaging/computed tomography (MRI/CT), positron emission tomography/computed tomography (PET/CT), CT chest, abdomen, and pelvis (CTCAP). Patients on stratum A (PEG-interferon, where PEG is pegylated; AJCC IIC, IIIA, IIIB; n = 16) had imaging every 6 months; stratum B1 (PEG-interferon and temozolomide; unresectable measurable disease, metastatic, or recurrent; n = 2) had PET/CT scans every 2 months and brain imaging studies every 4 months; stratum B2 patients (PEG-interferon and temozolomide; unresectable nonmeasurable, metastatic, or recurrent, n = 3) had imaging every 4 months. Off-therapy imaging was done every 6 months for 3 years. RESULTS: There were 21 patients (11 females, 11 spitzoid, median age 14 years, head/neck [6], trunk [7], extremities [8]). Patients with spitzoid melanoma underwent 236 imaging studies in total (86 PET/CT, 81 CTCAP, 11 CT chest, 10 CT brain, 48 MRI brain) at a median cost per patient of $32,718. Thirteen studies (5.8%) had findings that led to two biopsies (one positive). For conventional MM, 162 studies (61 PET/CT, 57 CTCAP, 8 CT chest, 7 CT brain, and 29 MRI brain) were performed with a median cost per patient of $23,420. Twenty (14%) had findings leading to six biopsies (four positive). At 6.3 years (range 0.4-9.2), 17 patients remain disease-free. CONCLUSION: Children with spitzoid melanoma require minimal imaging at diagnosis and follow-up. Patients with conventional MM should be imaged according to adult guidelines.

Externally calibrated parallel imaging for 3D multispectral imaging near metallic implants using broadband ultrashort echo time imaging.

PURPOSE: To develop an externally calibrated parallel imaging technique for three-dimensional multispectral imaging (3D-MSI) in the presence of metallic implants. THEORY AND METHODS: A fast, ultrashort echo time (UTE) calibration acquisition is proposed to enable externally calibrated parallel imaging techniques near metallic implants. The proposed calibration acquisition uses a broadband radiofrequency (RF) pulse to excite the off-resonance induced by the metallic implant, fully phase-encoded imaging to prevent in-plane distortions, and UTE to capture rapidly decaying signal. The performance of the externally calibrated parallel imaging reconstructions was assessed using phantoms and in vivo examples. RESULTS: Phantom and in vivo comparisons to self-calibrated parallel imaging acquisitions show that significant reductions in acquisition times can be achieved using externally calibrated parallel imaging with comparable image quality. Acquisition time reductions are particularly large for fully phase-encoded methods such as spectrally resolved fully phase-encoded three-dimensional (3D) fast spin-echo (SR-FPE), in which scan time reductions of up to 8 min were obtained. CONCLUSION: A fully phase-encoded acquisition with broadband excitation and UTE enabled externally calibrated parallel imaging for 3D-MSI, eliminating the need for repeated calibration regions at each frequency offset. Significant reductions in acquisition time can be achieved, particularly for fully phase-encoded methods like SR-FPE. Magn Reson Med 77:2303-2309, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Accelerating fully phase-encoded MRI near metal using multiband radiofrequency excitation.

PURPOSE: To develop a multiband radiofrequency (RF) excitation strategy for simultaneous excitation of multiple RF offsets to accelerate fully phase-encoded imaging near metallic prostheses. METHODS: Multiband RF excitation was designed and incorporated into a spectrally resolved fully phase-encoded (SR-FPE) imaging scheme. A triband (-6, 0, 6 kHz) acquisition was compared with three separate single-band acquisitions at the corresponding RF offsets with a phantom containing the head of a hip prosthesis. In vivo multiband data with continuous spectral coverage were acquired in the knee of a healthy volunteer with the head of a hip prosthesis placed posteriorly and in a volunteer with a total knee prosthetic implant. RESULTS: Phantom images acquired with triband excitation were essentially identical to the composite of three single-band excitations, but with an acceleration factor of three. In vivo multiband images of the healthy knee with adjacent metal demonstrated very good depiction of knee anatomy. In vivo images of the total knee replacement were successfully acquired, allowing visualization of native tissue with far less signal dropout than 2D-FSE. CONCLUSIONS: FPE imaging with multiband excitation is feasible in the presence of extreme off-resonance. This approach can reduce scan time and/or increase off-resonance coverage, enabling in vivo FPE imaging near metallic prostheses over a broad off-resonance spectrum. Magn Reson Med 77:1223-1230, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Characterizing the limits of MRI near metallic prostheses.

PURPOSE: To characterize the fundamental limits of MRI near metallic implants on RF excitation, frequency encoding, and chemical shift-encoding water-fat separation. METHODS: Multicomponent three-dimensional (3D) digital models of a total hip and a total knee replacement were used to construct material-specific susceptibility maps. The fundamental limits and spatial relationship of imaging near metallic prostheses were investigated as a function of distance from the prosthetic surface by calculating 3D field map perturbations using a well-validated k-space based dipole kernel. RESULTS: Regions limited by the bandwidth of RF excitation overlap substantially with those fundamentally limited by frequency encoding. Rapid breakdown of water-fat separation occurs once the intravoxel off-resonance exceeds ∼6 ppm over a full range of fat fractions (0%-100%) and SNR (5-100). CONCLUSION: Current 3D multispectral imaging methods would not benefit greatly from exciting spins beyond ±12 kHz despite the presence of signal that lies outside of this range from tissue directly adjacent to the metallic implants. Methods such as phase encoding in all three spatial dimensions are necessary to spatially resolve spins beyond an excitation bandwidth of ±12 kHz. The approach described in this study provides a benchmark for the capabilities of current imaging techniques to guide development of new MRI methods for imaging near metal.

Improving chemical shift encoded water-fat separation using object-based information of the magnetic field inhomogeneity.

PURPOSE: The purpose of this work was to improve the robustness of existing chemical shift encoded water-fat separation methods by incorporating object-based information of the B0 field inhomogeneity. THEORY: The primary challenge in water-fat separation is the estimation of phase shifts that arise from B0 field inhomogeneity, which is composed of the background field and susceptibility-induced field. The susceptibility-induced field can be estimated if the susceptibility distribution is known or can be approximated. In this work, the susceptibility distribution is approximated from the source images using the known susceptibility values of water, fat, and air. The field estimate is then demodulated from the source images before water-fat separation. METHODS: Chemical shift encoded source images were acquired in anatomical regions that are prone to water-fat swaps. The images were processed using algorithms from the ISMRM Fat-Water Toolbox, with and without the object-based field map information. The estimates were compared to examine the benefit of using the object-based field map information. RESULTS: Multiple cases are shown in which water-fat swaps were avoided by using the object-based information of the B0 field map. CONCLUSION: Object-based information of the B0 field may improve the robustness of existing chemical shift encoded water-fat separation methods.

Reproducibility of MR-based liver fat quantification across field strength: Same-day comparison between 1.5T and 3T in obese subjects.

PURPOSE: To examine the reproducibility of quantitative magnetic resonance (MR) methods to estimate hepatic proton density fat-fraction (PDFF) at different magnetic field strengths. MATERIALS AND METHODS: This Health Insurance Portability and Accountability Act (HIPAA)-compliant study was approved by the Institutional Review Board. Following informed consent, 25 severely obese subjects (mean body mass index [BMI]: 45 ± 4, range: 38-53 kg/m(2) ) were scanned at 1.5T and 3T on the same day. Two confounder-corrected multiecho chemical shift-encoded gradient-echo-based imaging methods were acquired to estimate PDFF over the entire liver: 3D complex-based (MRI-C) and 2D magnitude-based (MRI-M) MRI. Single-voxel MR spectroscopy (MRS) was performed in the right liver lobe. Using linear regression, pairwise comparisons of estimated PDFF were made between methods (MRI-C, MRI-M, MRS) at each field strength and for each method across field strengths. RESULTS: 1.5T vs. 3T regression analyses for MRI-C, MRI-M, and MRS PDFF measurements yielded R(2) values of 0.99, 0.97, and 0.90, respectively. The best-fit line was near unity (slope(m) = 1, intercept(b) = 0), indicating excellent agreement for each case: MRI-C (m = 0.92 [0.87, 0.99], b = 1.4 [0.7, 1.8]); MRI-M (m = 1.0 [0.90, 1.08], b = -1.4 [-2.4, -0.5]); MRS (m = 0.98 [0.82, 1.15], b = 1.2 [-0.2, 3.0]). Comparing MRI-C and MRI-M yielded an R(2) = 0.98 (m = 1.1 [1.02, 1.16], b = -1.8 [-2.8, -1.1]) at 1.5T, and R(2) = 0.99 (m = 0.98 [0.93, 1.03], b = 1.2 [0.7, 1.7]) at 3T. CONCLUSION: This study demonstrates that PDFF estimation is reproducible across field strengths and across two confounder-corrected MR-based methods.

New magnetic resonance imaging methods in nephrology.

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.

Spectrally resolved fully phase-encoded three-dimensional fast spin-echo imaging.

PURPOSE: To develop and test the feasibility of a spectrally resolved fully phase-encoded (SR-FPE) three-dimensional fast spin-echo technique and to demonstrate its application for distortion-free imaging near metal and chemical species separation. METHODS: In separate scans at 1.5 T, a hip prosthesis phantom and a sphere filled with gadolinium solution were imaged with SR-FPE and compared to conventional three-dimensional-fast spin-echo. Spectral modeling was performed on the SR-FPE data to generate the following parametric maps: species-specific signal (ρspecies), B0 field inhomogeneity, and R*2. The prosthesis phantom was also scanned using a 16-channel coil at 1.5 T. The fully sampled k-space data were retrospectively undersampled to demonstrate the feasibility of parallel imaging acceleration in all three phase-encoding directions, in combination with corner-cutting and half-Fourier sampling. Finally, SR-FPE was performed with an acetone/water/oil phantom to test chemical species separation. RESULTS: High quality distortion-free images and parametric maps were generated from SR-FPE. A 4 h SR-FPE scan was retrospectively accelerated to 12 min while preserving spectral information and 7.5 min without preserving spectral data. Chemical species separation was demonstrated in the acetone/water/oil phantom. CONCLUSION: This work demonstrates the feasibility of SR-FPE to perform chemical species separation and spectrally resolved imaging near metal without distortion, in scan times appropriate for the clinical setting.

Accelerating sequences in the presence of metal by exploiting the spatial distribution of off-resonance.

PURPOSE: To demonstrate feasibility of exploiting the spatial distribution of off-resonance surrounding metallic implants for accelerating multispectral imaging techniques. THEORY AND METHODS: Multispectral imaging (MSI) techniques perform time-consuming independent three-dimensional acquisitions with varying radio frequency offsets to address the extreme off-resonance from metallic implants. Each off-resonance bin provides a unique spatial sensitivity that is analogous to the sensitivity of a receiver coil and, therefore, provides a unique opportunity for acceleration. Fully sampled MSI was performed to demonstrate retrospective acceleration. A uniform sampling pattern across off-resonance bins was compared with several adaptive sampling strategies using a total hip replacement phantom. Monte Carlo simulations were performed to compare noise propagation of two of these strategies. With a total knee replacement phantom, positive and negative off-resonance bins were strategically sampled with respect to the B0 field to minimize aliasing. Reconstructions were performed with a parallel imaging framework to demonstrate retrospective acceleration. RESULTS: An adaptive sampling scheme dramatically improved reconstruction quality, which was supported by the noise propagation analysis. Independent acceleration of negative and positive off-resonance bins demonstrated reduced overlapping of aliased signal to improve the reconstruction. CONCLUSION: This work presents the feasibility of acceleration in the presence of metal by exploiting the spatial sensitivities of off-resonance bins.

Low-Field Portable MR Imaging to Evaluate Ventricular Volumes: A Single-Center Retrospective Study.

This study assesses the efficacy of low-field portable MR imaging in measuring ventricular volumes in the pediatric population in the hospital setting. We compared portable and standard of care MR images from the same patient. The estimated ventricular volumes had excellent agreement with a mean bias of 2.06% by Bland-Altman analysis and a correlation of 0.99. From this initial data set, our results suggest that low-field, portable MR imaging is a promising technique for imaging and quantifying ventricular volumes.

Feasibility of and experience using a portable MRI scanner in the neonatal intensive care unit.

OBJECTIVE: A portable, low-field MRI system is now Food and Drug Administration cleared and has been shown to be safe and useful in adult intensive care unit settings. No neonatal studies have been performed. The objective is to assess our preliminary experience and assess feasibility of using the portable MRI system at the bedside in a neonatal intensive care unit (NICU) at a quaternary children's hospital. STUDY DESIGN: This was a single-site prospective cohort study in neonates ≥2 kg conducted between October and December 2020. All parents provided informed consent. Neonates underwent portable MRI examination in the NICU with support equipment powered on and attached to the neonate during the examination. A paediatric radiologist interpreted each portable MRI examination. The study outcome variable was percentage of portable MRI examinations completed without artefacts that would hinder diagnosis. Findings were compared between portable MRI examinations and standard of care examinations. RESULTS: Eighteen portable, low-field MRI examinations were performed on 14 neonates with an average age of 29.7 days (range 1-122 days). 94% (17 of 18) of portable MRI examinations were acquired without significant artefact. Significant intracranial pathology was visible on portable MRI, but subtle abnormalities were missed. The examination reads were concordant in 59% (10 of 17) of cases and significant pathology was missed in 12% (2 of 17) of cases. CONCLUSION: This single-centre series demonstrated portable MRI examinations can be performed safely with standard patient support equipment present in the NICU. These findings demonstrate that portable MRI could be used in the future to guide care in the NICU setting. TRIAL REGISTRATION NUMBER: NCT04629469.

MR measures of renal perfusion, oxygen bioavailability and total renal blood flow in a porcine model: noninvasive regional assessment of renal function.

BACKGROUND: Magnetic resonance imaging (MRI) may be a useful adjunct to current methods of evaluating renal function. MRI is a noninvasive imaging modality that has the ability to evaluate the kidneys regionally, which is lacking in current clinical methods. Other investigators have evaluated renal function with MRI-based measurements, such as with techniques to measure cortical and medullary perfusion, oxygen bioavailability and total renal blood flow (TRBF). However, use of all three techniques simultaneously, and therefore the relationships between these MRI-derived functional parameters, have not been reported previously. METHODS: To evaluate the ability of these MRI techniques to track changes in renal function, we scanned 11 swine during a state of hyperperfusion with acetylcholine and a saline bolus and subsequently scanned during a state of hypoperfusion with the prolonged use of isoflurane anesthesia. For each time point, measurements of perfusion, oxygen bioavailability and TRBF were acquired. Measurements of perfusion and oxygen bioavailability were compared with measurements of TRBF for all swine across all time points. RESULTS: Cortical perfusion, cortical oxygen bioavailability, medullary oxygen bioavailability and TRBF significantly increased with the acetylcholine challenge. Cortical perfusion, medullary perfusion, cortical oxygen bioavailability and TRBF significantly decreased during isoflurane anesthesia. Cortical perfusion (Spearman's correlation coefficient = 0.68; P < 1 × 10(-6)) and oxygen bioavailability (Spearman's correlation coefficient = -0.60; P < 0.0001) correlated significantly with TRBF, whereas medullary perfusion and oxygen bioavailability did not correlate with TRBF. CONCLUSIONS: Our results demonstrate expected changes given the pharmacologically induced changes in renal function. Maintenance of the medullary oxygen bioavailability in low blood flow states may reflect the autoregulation particular to this region of the kidney. The ability to non-invasively measure all three parameters of kidney function in a single MRI examination and to evaluate the relationships between these functional parameters is potentially useful for evaluating the state of the human kidneys in situ in future studies.

Measurement and comparison of T1 relaxation times in native and transplanted kidney cortex and medulla.

PURPOSE: To measure and compare cortical and medullary T1 values in native and transplanted kidneys with a wide range of function as measured by estimated glomerular filtration rate (eGFR). MATERIALS AND METHODS: A total of 27 subjects (12 native and 15 transplants) were studied. Two magnetic resonance imaging (MRI) exams of T1 measurement were performed on separate days for reproducibility study. Group-wise comparisons of renal T1 on day 1 were performed between subjects with native and transplanted kidneys and also between subjects based on an eGFR threshold of 60 mL/min/1.73 m(2). RESULTS: Transplanted kidneys had higher cortical renal T1 (1183 ± 136 msec) than native kidneys (1057 ± 94 msec) with similar results in the medulla. Subjects with an eGFR < 60 mL/min/1.73 m(2) had higher renal T1 than subjects with an eGFR > 60 mL/min/1.73 m(2) (cortical T1: P < 0.0001; medullary T1: P = 0.008). Renal T1 was highly reproducible for both native and transplant groups (with percent differences less than 10%). CONCLUSION: There are differences in cortical and medullary T1 between native and transplanted kidneys at different levels of function.

Reproducibility of renal perfusion MR imaging in native and transplanted kidneys using non-contrast arterial spin labeling.

PURPOSE: To examine both inter-visit and intra-visit reproducibility of a MR arterial spin labeling (ASL) perfusion technique in native and transplanted kidneys over a broad range of renal function. MATERIALS AND METHODS: Renal perfusion exams were performed at 1.5 T in a total of 24 subjects: 10 with native and 14 with transplanted kidneys. Using a flow-sensitive alternating inversion recovery (FAIR) ASL scheme, 32 control/tag pairs were acquired and processed using a single-compartment model. Two FAIR-ASL MR exams were performed at least 24 h apart on all the subjects to assess inter-visit reproducibility. ASL perfusion measurements were also repeated back-to-back within one scanning session in 8 native subjects and in 12 transplant subjects to assess intra-visit reproducibility. Intra-class correlations (ICCs) and coefficients of variation (CVs) were calculated as metrics of reproducibility. RESULTS: Intra-visit ICCs ranged from 0.96 to 0.98 while CVs ranged from 4.8 to 6.0%. Inter-visit measurements demonstrated slightly more variation with ICCs from 0.89 to 0.94 and CVs from 7.6 to 13.1%. Medullary perfusion demonstrated greater variability compared with cortical blood flow: intra-visit ICCs from 0.72 to 0.78 and CVs from 16.7 to 26.7%, inter-visit ICCs from 0.13 to 0.63 and CVs from 19.8 to 37%. CONCLUSION: This study indicates that a FAIR-ASL perfusion technique is reproducible in the cortex of native and transplanted kidneys over a broad range in renal function. In contrast, perfusion measurements in the medulla demonstrated moderate to poor reproducibility for intra-visit and inter-visit measures respectively.