Henrik J. Michaely

MRI-measurement of perfusion and glomerular filtration in the human kidney with a separable compartment model.

Objectives:Recent animal studies with dynamic contrast enhanced magnetic resonance imaging have demonstrated that a separable compartment model provides more accurate assessments of glomerular filtration than the Patlak model. In this study, the feasibility of the separable compartment model for the measurement of perfusion and filtration in healthy humans is investigated. Methods:Dynamic contrast enhanced magnetic resonance imaging was performed in 15 healthy volunteers. Contrast enhancement curves were analyzed with the separable compartment model on whole kidney regions, cortex regions, and the pixel level. The region of interest values for the kinetic parameters were compared with those obtained from the Patlak model and from a model-free deconvolution analysis. Results:The separable compartment model provides a good fit to the data over the entire dynamic range. All values of filtration (30 ± 7.2 and 20 ± 11 mL/100 mL/Min for kidney and cortex, respectively) are significantly higher than those of the Patlak model (24 ± 6.4 and 15 ± 11 mL/100 mL/Min). Values produced by the Patlak model have a higher variability. Whole kidney values of perfusion (229 ± 57 mL/100 mL/Min) are significantly higher than those of a deconvolution analysis (210 ± 50 mL/100 mL/Min). Conclusions:The separable compartment model is feasible for application in humans and sufficiently robust for a pixel analysis. Increased filtration values compared with the Patlak model suggest that the difference in accuracy observed in animal studies is relevant in humans. Increased perfusion values suggest that the separable compartment model corrects for known underestimations in the deconvolution analysis.

Diffusion tensor imaging of the kidney with parallel imaging: initial clinical experience.

Objective:To evaluate the clinical feasibility of diffusion tensor imaging (DTI) of the kidney in volunteers and patients with renal diseases. Material and Methods:Ten volunteers and 22 patients (mean age, 56 ± 14.3) with renal masses and renal artery stenosis underwent breath-hold coronal fat-saturated echo-planar DTI (as provided by the manufacturer, 6 diffusion directions, diffusion weightings b = 0 and 300 s/mm2, repetition time 730 ms/echo time 72 ms; 5 slices; slice thickness, 6 mm; inplane resolution, 2.1 × 2.1 mm2; acquisition time, 26 seconds) of the kidneys at 1.5 T (MAGNETOM Avanto, Siemens Medical Solutions, Erlangen, Germany). The parallel imaging technique, generalized autocalibrating partially parallel acquisitions with an acceleration factor 2, was applied. Using the commercially available Syngo DTI task card software, regions of interests were placed in the cortex, medulla, and in renal masses if present. Fractional anisotropy (FA) and apparent diffusion coefficients (ADC) were determined, and tractography was used to visualize the renal diffusion properties. Statistical analysis was performed using the Wilcoxon signed-rank sum test and paired t tests. Results:In all volunteers, FA was significantly (P < 0.01) higher in the medulla (0.36 ± 0.03) than in the cortex (0.21 ± 0.02), whereas the ADC was significantly (P < 0.01) higher in the cortex (2.43 ± 0.19) than in the medulla (2.16 ± 0.22). Tractography typically revealed a radial preferred direction of medullary diffusion basically reflecting medullary flow. FA/ADC of simple renal cysts (n = 8) was 0.14 ± 0.05/2.86 ± 0.15. Renal cell carcinoma (n = 10) showed a wide FA range from 0.11 to 0.56. Using tractography, the structural organization of renal cell carcinoma such as pseudocapsules could be visualized. In 1 patient with unilateral high-grade renal artery stenosis, the cortical ADC of the affected kidney was lower than on the contralateral side (1.77/2.27) and the FA was increased (0.33/0.18). The FA of the medulla was increased (0.70/0.41) and the ADC decreased (1.43/1.90). Conclusions:Using parallel imaging, DTI measurements of the kidneys are feasible within a single breath-hold with good discrimination between cortex and medulla. Parallel imaging allows more slices and a superior resolution. DTI measurements of the kidney allows visualization of medullary flow, in pathology ADC and FA were altered. Further investigations will be required to evaluate the role of DTI for studying and monitoring renal ultrastructure.

Dual-energy CT angiography of the lung in patients with suspected pulmonary embolism: initial results.

To evaluate the feasibility of dual-energy CT angiography (CTA) of the lung in patients with suspected pulmonary embolism (PE). 24 patients with suspected PE were examined with a single-acquisition, dual-energy CTA protocol (A-system: 140 kV/65 mAsref, B-system: 80kV/190 mAsref) on a dual-source CT system. Lung perfusion was visualized by color-coding voxels containing iodine and air using dedicated dual-energy postprocessing software. Perfusion defects were classified by two blinded radiologists as being consistent or non-consistent with PE. Subjective image quality of perfusion maps and CTA was rated using a 5-point scale (1: excellent, 5: poor). The reading of a third independent radiologist served as the standard of reference for the diagnosis of PE. In all patients with PE (n=4), perfusion defects classified as being consistent with PE were identified in lung areas affected by PE. Both readers did not record perfusion defects classified as being consistent with PE in any of the patients without PE. Thus, on a per patient basis the sensitivity and specificity for the assessment of PE was 100% for both readers. On a per segment basis the sensitivity and specificity ranged between 60-66.7% and 99.5-99.8%. The interobserver agreement was good (k= 0.81). Perfusion defects rated as non-consistent with PE were most frequently caused by streak artifacts from dense contrast material in the great thoracic vessels. The median score of the image quality of both the perfusion maps and CTA was 2. In conclusion, dual-energy CTA of pulmonary embolism is feasible and allows the assessment of perfusion defects caused by pulmonary embolism. Further optimization of the injection protocol is required to reduce artifacts from dense contrast material.

Renal BOLD-MRI does not reflect renal function in chronic kidney disease

Renal blood oxygen level-dependent magnetic resonance imaging (BOLD-MRI) is a noninvasive fast technique to characterize renal function. Here we evaluated the impact of renal function on the relaxation rate (R2(*)) in the cortex and medulla to provide baseline data for further use of renal BOLD-MRI. This parameter was evaluated in 400 patients scheduled for abdominal imaging who underwent transversal blood oxygen level-dependent measurements with a multi-echo gradient-echo sequence with 12 echo times. The loss of phase coherence (T2(*)) maps were generated in which kidney regions of interest were selected to differentiate the medulla and cortex, and R2(*) was equated to 1/T2(*). Individual R2(*) values were, in turn, correlated to the eGFR (MDRD formula of 280 patients with available serum creatinine measurements), age, and gender each for 1.5 and 3.0 T field-strength scans of 342 patients. At both the field strengths, no significant differences in R2(*) of the cortex and medulla were found between patient gender, age, eGFR, or between different stages of chronic kidney disease determined using the KDOQI system. Thus, BOLD-MRI of a non-specific patient population failed to discriminate between the patients with various stages of chronic kidney disease.

Renal disease: value of functional magnetic resonance imaging with flow and perfusion measurements.

Purpose:To differentiate healthy kidneys from diseased kidneys, we propose a combined magnetic resonance (MR) examination that includes measurements of renal arterial blood flow and parenchymal perfusion. Materials and Methods:A total of 130 kidneys (patients/healthy volunteers: 83/47) were examined using renal artery MR flow measurements and renal parenchymal perfusion measurements, as well as contrast-enhanced MR angiography. Cine phase-contrast-flow measurements were performed using an ECG-gated fast low angle shot pulse sequence; perfusion was measured with an arterial spin labeling flow-sensitive alternating inversion recovery technique. Contrast-enhanced MR angiography was performed with a fast 3D gradient echo sequence in a single breath hold. For evaluation, kidneys were divided into groups based on nephrologic diagnosis of the patient. Recursive partitioning and Wilcoxon rank-sum tests were used to separate the different groups. Results:Significant differences in mean renal artery flow and parenchymal perfusion were found in kidneys with renal artery stenosis as well as parenchymal disease as compared with healthy kidneys. Using a classification tree derived from the recursive partitioning, a specificity of 99% and sensitivity of 69% with a positive/negative predictive value of 97%/84% was achieved for the separation of healthy kidneys from kidneys with vascular, parenchymal or combined disease. The overall accuracy was 88%. Conclusion:The combination of cine PC flow measurements and MR perfusion measurements offers a comprehensive assessment of both renovascular and renoparenchymal disease and provide a noninvasive approach to differentiate between these kidneys and normal kidneys.

Measuring perfusion and permeability in renal cell carcinoma with dynamic contrast-enhanced MRI: A pilot study

To retrospectively assess an improved quantitative methodology with separate assessment of perfusion and permeability for characterization of primary renal cell carcinoma (RCC) and monitoring antiangiogenic treatment.

Whole-body MR imaging in psoriatic arthritis: Additional value for therapeutic decision making

PURPOSE In psoriatic arthritis (PsA) multiple locations may show inflammatory changes not always readily accessible to clinical exam. Often, clinical exam is inconclusive and the decision to initiate or adapt therapy is difficult. Whole body (WB)-MRI may help in this situation by providing a comprehensive overview of affected areas/joints. The purpose of this study was to make a proof of concept whether WB-MRI in psoriatic arthritis is feasible and can provide additional information compared to clinical examination alone with regard to therapeutic decision making in patients with PsA and inconclusive clinical situation. MATERIALS AND METHODS 30 patients with PsA and diffuse musculoskeletal pain were examined. A WB-MRI protocol was implemented on a 1.5 Tesla scanner using coronal and sagittal STIR- (TR: 5800, TE: 54, matrix 384(2) pixels, FOV 400 mm) and pre- and steady-state-post-Gadolinium-VIBE sequences (TR: 9.82, TE: 4.53, matrix 384×307 pixels, FOV: 400 mm). MRI was evaluated for image quality and inflammatory findings by two readers in consensus and compared to clinical exam. RESULTS The WB-MR-exam was well tolerated by all patients. Image quality was rated good to excellent in 26 of 30 patients (86.6%). WB-MRI detected significantly (p<0.001) more areas of synovitis/enthesitis than clinical exam except for the hands and feet. MRI was able to detect unknown destructive bony changes in 10 patients (53%). In 22 patients (73.3%) the therapy regimen was modified, in 18 patients (62%) TNF-alpha-inhibitors were started. CONCLUSION Whole-body MRI (WB-MRI) may be integrated in the diagnostic work-up of patients with psoriatic arthropathy facilitating individual adaptation of therapeutic strategy.

CAIPIRINHA-Dixon-TWIST (CDT)-volume-interpolated breath-hold examination (VIBE): a new technique for fast time-resolved dynamic 3-dimensional imaging of the abdomen with high spatial resolution.

PurposeThe purpose of this study was to assess the feasibility and image quality of a novel, highly accelerated T1-weighted sequence for time-resolved imaging of the abdomen during the first pass of contrast media transit using controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) under sampling, view-sharing techniques, and Dixon water-fat separation (CAIPRINHA–Dixon–time-resolved imaging with interleaved stochastic trajectories–volumetric interpolated breath-hold examination [CDT-VIBE]). Materials and MethodsIn this retrospective, institutional review board–approved study, 47 patients (median age, 62 years; 25 men, 22 women) scanned on a 3.0-T magnetic resonance system (Skyra; Siemens) were included. The CDT-VIBE (repetition time/echo time1/echo time2, 4.1/1.33/2.56 milliseconds; acquisition time, 29 seconds) was used in place of the standard arterial phase acquisition and started 15 seconds after the injection of 0.1 mmol/kg Gd-DOTA (Dotarem, Guerbet). Within 29 seconds, 14 high spatial resolution (1.2 × 1.2 × 3 mm3) 3-dimensional data sets were acquired and reconstructed using view sharing (temporal resolution, 2.1 seconds). The CDT-VIBE images were evaluated independently by 2 blinded, experienced radiologists with regard to image quality and the number of hepatic arterial–dominant phases present on an ordinal 5-point scale (5, excellent; 1, nondiagnostic). Added diagnostic information with CDT-VIBE relative to portal venous phase VIBE was assessed. ResultsIn all patients, CDT-VIBE measurements were successfully acquired. The image quality was diagnostic in 46 of the 47 patients. Both readers assessed the highest image quality present in the data sets with a median score of 4 (range, 3–5 for both readers; &kgr;, 0.789) and the worst image quality with a median score of 3 (range, 1–4 for both readers; &kgr;, 0.689). With a range between 1 and 8 (median, 5), hepatic arterial–dominant data sets (of the 14 acquired) were obtained in each case. There was an added diagnostic value with CDT-VIBE in 10 of the 47 patients (21%). ConclusionsThe CDT-VIBE is a robust approach allowing, for the first time, dynamic imaging of the upper abdomen with high temporal resolution and preservation of high spatial resolution.

Peripheral magnetic resonance angiography with continuous table movement in combination with high spatial and temporal resolution time-resolved MRA With a total single dose (0.1 mmol/kg) of gadobutrol at 3.0 T.

Purpose:To prove the concept of peripheral continuous table movement (CTM) MR-angiography (MRA) in combination with high spatial and temporal resolution time-resolved TWIST-MRA in a single MR-examination at 3.0 T with a single dose (0.1 mmol/kg) of gadobutrol in total. Materials and Methods:We included 22 consecutive patients (15 m/7 f, mean age: 64 years) referred for peripheral MRA with clinical symptoms of peripheral arterial occlusive disease Fontaine stages II–IV. All of them underwent both CTM-MRA (TR: 2.4 ms/TE: 1.0 ms/flip angle: 21 degree) of the entire run-off vessels and TWIST-MRA (TR: 2.8 ms/TE: 1.1 ms/flip angle: 20 degree) of the calf station during a single MR-examination. All examinations were performed on a 3.0 T MR system (Tim Trio). Spatial resolution of the CTM-MRA datasets was technically limited to an acquired resolution of 1.2 × 1.2 × 1.96 mm3 reconstructed to 1.2 mm isotropic. The TWIST-MRA was acquired with 1.1 × 1.1 × 1.35 mm3 and reconstructed to 1.1 mm isotropic with a temporal resolution of 5.5 seconds in the calf station. A total of 0.1 mmol/kg BW gadobutrol diluted 1:1 with saline was injected at a flow rate of 1.5 mL/s of which 0.07 mmol/kg was administered for the CTM-MRA and 0.03 mmol/kg for the TWIST-MRA.CTM-MRA run off datasets were qualitatively assessed using a 4 point scale (4 = excellent, 1 = nondiagnostic) followed by TWIST-MRA datasets for the calf using the same scale. Additional relevant findings only visible in the TWIST-MRA were documented. Results:All datasets could be evaluated with a total of 397 assessable segments. CTM-MRA was diagnostic in 99% (393/397 segments) with image quality judged as excellent in 54% (213/397 segments), good in 42% (14/397), and moderate in 4% (14/397) of analyzed segments respectively. Nondiagnostic image quality was seen in 1% (4/397 segments). Venous overlay in the calf station was found in 27% (6/22 patients). TWIST-MRA was diagnostic in 100% (115/115 segments), throughout with good or excellent image quality. In 14 of 22 patients additional relevant findings were detected by TWIST-MRA. Conclusion:Single-dose gadobutrol CTM-MRA in combination with a high spatial and temporal resolution TWIST-MRA at 3.0 T is a reliable technique with good image quality. Despite the use of single dose contrast agent large field of view coverage and dynamic images can be acquired. Because of its robustness, this imaging approach of the vasculature has great potential for a broad clinical use.

Temporal Constraints in Renal Perfusion Imaging With a 2-Compartment Model

Objective:To assess the required temporal resolution and total acquisition time for renal perfusion and filtration measurements with a 2-compartment model. Material and Methods:Saturation-recovery TurboFLASH perfusion measurements of 15 healthy volunteers were acquired at 1.5 T, with a temporal resolution of 1 second during the first pass and a total acquisition time of 270 seconds. The time courses were then regridded and truncated to yield new data sets with temporal resolutions from 1 to 30 seconds in 1-second increments and with total acquisition times from 30 to 270 seconds in 5-second increments, respectively. Each new dataset was postprocessed by fitting the time courses to a 2-compartment model producing measures of perfusion and filtration: plasma volume (PV), plasma flow (PF), tubular volume (TV), and tubular flow (TF). The effect of reducing the temporal resolution or the total acquisition time was investigated by comparing the model parameters with those obtained at full temporal resolution and acquisition time and quantified by defining a discretization error (DE) and a truncation error (TE), respectively. For each parameter, the required temporal resolution and total acquisition times were defined by demanding a DE and TE of less than 10%. Results:It can be concluded from the analysis of the DE and TE that the acquisition of the parameters PF and TF requires a temporal resolution of at least 4 and 5 seconds, respectively. For the other 2 parameters, a temporal resolution of at least 9 seconds is sufficient. The required total acquisition times for PF and PV were 35 and 85 seconds, whereas for the parameters TF and TV, 230 and 255 seconds, respectively, are required. Conclusion:Renal perfusion measurements should be acquired with a temporal resolution of at least 4 seconds. To evaluate the renal excretory function adequately, the total acquisition time should be at least 255 seconds.