Arterial spin labeling blood flow magnetic resonance imaging for the characterization of metastatic renal cell carcinoma(1)

A perfusion phantom for ASL MRI based on impinging jets

PURPOSE

We present a novel perfusion phantom for validation of arterial spin labeled (ASL) perfusion MRI methods and protocols.

METHODS

Impinging jets, driven by a peristaltic pump, were used to achieve perfusion-like mixing of magnetically labeled inflowing fluid within a perfusion compartment. The phantom was validated by varying pump rates and obtaining ASL-MRI data at multiple postlabeling delays using a pseudo-continuous ASL sequence with a 3D stack-of-spirals readout. An additional data set was acquired using a pseudo-continuous ASL sequence with a 2D EPI readout. Phantom sensitivity to pseudo-continuous ASL labeling efficiency was also tested.

RESULTS

Fluid dynamics simulations predicted that maximum mixing would occur near the central axis of the perfusion compartment. Experimentally observed signal changes within this region were reproducible and well fit by the standard Buxton general kinetic model. Simulations and experimental data showed no label outflow from the perfusion chamber and calculated perfusion rates, averaged over the entire phantom volume, agreed with the expected volumetric flow rates provided by the flow pump. Phantom sensitivity to pseudo-continuous ASL labeling parameters was also demonstrated.

CONCLUSION

Perfusion-like signal can be simulated using impinging jets to create a well-mixed compartment. Observed perfusion and transit time values were reproducible and within the physiological range for brain perfusion. This phantom design has a broad range of potential applications in both basic and clinical research involving ASL MRI.

Optimizing MRF-ASL scan design for precise quantification of brain hemodynamics using neural network regression

PURPOSE

Arterial Spin Labeling (ASL) is a quantitative, non-invasive alternative for perfusion imaging that does not use contrast agents. The magnetic resonance fingerprinting (MRF) framework can be adapted to ASL to estimate multiple physiological parameters simultaneously. In this work, we introduce an optimization scheme to increase the sensitivity of the ASL fingerprint. We also propose a regression based estimation framework for MRF-ASL.

METHODS

To improve the sensitivity of MRF-ASL signals to underlying parameters, we optimized ASL labeling durations using the Cramer-Rao Lower Bound (CRLB). This paper also proposes a neural network regression based estimation framework trained using noisy synthetic signals generated from our ASL signal model. We tested our methods in silico and in vivo, and compared with multiple post labeling delay (multi-PLD) ASL and unoptimized MRF-ASL. We present comparisons of estimated maps for the six parameters of our signal model.

RESULTS

The scan design process facilitated precise estimates of multiple hemodynamic parameters and tissue properties from a single scan, in regions of normal gray and white matter, as well as regions with anomalous perfusion activity in the brain. In particular, there was a 86.7% correlation of perfusion estimates with the ground truth in silico, using our proposed techniques. In vivo, there was roughly a 7 fold improvement in the Coefficient of Variation (CoV) for white matter perfusion, and 2 fold improvement in gray matter perfusion CoV in comparison to a reference Multi PLD method. The regression based estimation approach provided perfusion estimates rapidly, with estimation times of around 1s per map.

CONCLUSIONS

Scan design optimization, coupled with regression-based estimation is a powerful tool for improving precision in MRF-ASL.

Consensus-based technical recommendations for clinical translation of renal ASL MRI

OBJECTIVES

This study aimed at developing technical recommendations for the acquisition, processing and analysis of renal ASL data in the human kidney at 1.5 T and 3 T field strengths that can promote standardization of renal perfusion measurements and facilitate the comparability of results across scanners and in multi-centre clinical studies.

METHODS

An international panel of 23 renal ASL experts followed a modified Delphi process, including on-line surveys and two in-person meetings, to formulate a series of consensus statements regarding patient preparation, hardware, acquisition protocol, analysis steps and data reporting.

RESULTS

Fifty-nine statements achieved consensus, while agreement could not be reached on two statements related to patient preparation. As a default protocol, the panel recommends pseudo-continuous (PCASL) or flow-sensitive alternating inversion recovery (FAIR) labelling with a single-slice spin-echo EPI readout with background suppression and a simple but robust quantification model.

DISCUSSION

This approach is considered robust and reproducible and can provide renal perfusion images of adequate quality and SNR for most applications. If extended kidney coverage is desirable, a 2D multislice readout is recommended. These recommendations are based on current available evidence and expert opinion. Nonetheless they are expected to be updated as more data become available, since the renal ASL literature is rapidly expanding.

Consensus-based technical recommendations for clinical translation of renal ASL MRI

Objectives

This study aimed at developing technical recommendations for the acquisition, processing and analysis of renal ASL data in the human kidney at 1.5 T and 3 T field strengths that can promote standardization of renal perfusion measurements and facilitate the comparability of results across scanners and in multi-centre clinical studies.

Methods

An international panel of 23 renal ASL experts followed a modified Delphi process, including on-line surveys and two in-person meetings, to formulate a series of consensus statements regarding patient preparation, hardware, acquisition protocol, analysis steps and data reporting.

Results

Fifty-nine statements achieved consensus, while agreement could not be reached on two statements related to patient preparation. As a default protocol, the panel recommends pseudo-continuous (PCASL) or flow-sensitive alternating inversion recovery (FAIR) labelling with a single-slice spin-echo EPI readout with background suppression and a simple but robust quantification model.

Discussion

This approach is considered robust and reproducible and can provide renal perfusion images of adequate quality and SNR for most applications. If extended kidney coverage is desirable, a 2D multislice readout is recommended. These recommendations are based on current available evidence and expert opinion. Nonetheless they are expected to be updated as more data become available, since the renal ASL literature is rapidly expanding.

Electronic supplementary material

The online version of this article (10.1007/s10334-019-00800-z) contains supplementary material, which is available to authorized users.

Volumetric abdominal perfusion measurement using a pseudo-randomly sampled 3D fast-spin-echo (FSE) arterial spin labeling (ASL) sequence and compressed sensing reconstruction

PURPOSE

To improve image quality and spatial coverage for abdominal perfusion imaging by implementing an arterial spin labeling (ASL) sequence that combines variable-density 3D fast-spin-echo (FSE) with Cartesian trajectory and compressed-sensing (CS) reconstruction.

METHODS

A volumetric FSE sequence was modified to include background-suppressed pseudo-continuous ASL labeling and to support variable-density (VD) Poisson-disk sampling for acceleration. We additionally explored the benefits of center oversampling and variable outer k-space sampling. Fourteen healthy volunteers were scanned on a 3T scanner to test acceleration factors as well as the various sampling schemes described under synchronized-breathing to limit motion issues. A CS reconstruction was implemented using the BART toolbox to reconstruct perfusion-weighted ASL volumes, assessing the impact of acceleration, different reconstruction, and sampling strategies on image quality.

RESULTS

CS acceleration is feasible with ASL, and a strong renal perfusion signal could be observed even at very high acceleration rates (≈15). We have shown that ASL k-space complex subtraction was desirable before CS reconstruction. Although averaging of multiple highly accelerated images helped to reduce artifacts from physiologic fluctuations, superior image quality was achieved by interleaving of different highly undersampled pseudo-random spatial sampling patterns and using 4D-CS reconstruction. Combination of these enhancements produces high-quality ASL volumes in under 5 min.

CONCLUSIONS

High-quality isotropic ASL abdominal perfusion volumes can be obtained in healthy volunteers with a VD-FSE and CS reconstruction. This lays the groundwork for future developments toward whole abdomen free-breathing non-contrast perfusion imaging.

Enabling free-breathing background suppressed renal pCASL using fat imaging and retrospective motion correction

Purpose

For free‐breathing renal perfusion imaging using arterial spin labeling (ASL), retrospective image realignment has been found essential to reduce subtraction artifacts and, independently, background suppression has been demonstrated to reduce physiologic noise. However, negative results on ASL precision and accuracy have been reported for the combination of both. In this study, the effect of background suppression ‐level in combination with image registration on free‐breathing renal ASL signal quality, with registration either on ASL‐images themselves or guided by additionally acquired fat‐images, was investigated. The results from free‐breathing acquisitions were compared with the reference paced‐breathing motion compensation strategy.

Methods

Pseudocontinuous ASL (pCASL) data with additional fat‐images were acquired from 10 subjects at 1.5T with varying background suppression levels during free‐breathing and paced‐breathing. Images were registered using the ASL‐images themselves (ASLReg) or using their corresponding fat‐images (FatReg). Temporal signal‐to‐noise ratio (tSNR) served to evaluate precision and perfusion weighted signal (PWS) to assess accuracy.

Results

In combination with image registration, background suppression significantly improved tSNR by 50% (P < .05). For heavy suppression, ASLReg and FatReg showed similar performance in terms of tSNR and PWS. Background suppression with two inversion pulses induced a small, nonsignificant (P > .05) PWS reduction, but increased PWS accuracy. When applying heavy background suppression, free‐breathing acquisitions resulted in similar ASL‐quality to paced‐breathing acquisitions.

Conclusion

Background suppression was found beneficial for free‐breathing renal pCASL precision without compromising accuracy, despite motion challenges. In combination with ASLReg or FatReg, background suppression enabled clinically viable free‐breathing renal pCASL.

Influence of background suppression and retrospective realignment on free-breathing renal perfusion measurement using pseudo-continuous ASL

PURPOSE

To assess the influence of background suppression and retrospective realignment on physiological noise and image quality in free-breathing renal pseudo-continuous arterial spin labeling (pCASL).

METHODS

Ten subjects were scanned at 3T with a pCASL prepared single-slice coronal acquisition through the kidneys under free breathing. Multiple acquisitions were performed with various levels of residual background signal based on optimization of pulse timings to achieve specific background suppression levels (<2%, <5%, <10%, <20%). A retrospective non-rigid motion-correction strategy was also implemented.

RESULTS

Decreasing level of residual background signal was associated with higher temporal SNR. The retrospective motion-correction provided an additional but not statistically significant improvement in tSNR. The highest image quality was obtained with the lowest level of residual background signal accompanied by the retrospective motion-correction, although no significant difference in quantitative renal blood-flow could be observed.

CONCLUSIONS

Renal perfusion measurement with ASL under free breathing is feasible and robust against physiological noise when using strong background suppression strategies. Finally, retrospective motion-correction further improves image quality but cannot replace background suppression.

Estimation of perfusion properties with MR Fingerprinting Arterial Spin Labeling

In this study, the acquisition of ASL data and quantification of multiple hemodynamic parameters was explored using a Magnetic Resonance Fingerprinting (MRF) approach. A pseudo-continuous ASL labeling scheme was used with pseudo-randomized timings to acquire the MRF ASL data in a 2.5 min acquisition. A large dictionary of MRF ASL signals was generated by combining a wide range of physical and hemodynamic properties with the pseudo-random MRF ASL sequence and a two-compartment model. The acquired signals were matched to the dictionary to provide simultaneous quantification of cerebral blood flow, tissue time-to-peak, cerebral blood volume, arterial time-to-peak, B₁, and T_1. A study in seven healthy volunteers resulted in the following values across the population in grey matter (mean ± standard deviation): cerebral blood flow of 69.1 ± 6.1 ml/min/100 g, arterial time-to-peak of 1.5 ± 0.1 s, tissue time-to-peak of 1.5 ± 0.1 s, T₁ of 1634 ms, cerebral blood volume of 0.0048 ± 0.0005. The CBF measurements were compared to standard pCASL CBF estimates using a one-compartment model, and a Bland-Altman analysis showed good agreement with a minor bias. Repeatability was tested in five volunteers in the same exam session, and no statistical difference was seen. In addition to this validation, the MRF ASL acquisition's sensitivity to the physical and physiological parameters of interest was studied numerically.

Non-Invasive Renal Perfusion Imaging Using Arterial Spin Labeling MRI: Challenges and Opportunities

Tissue perfusion allows for delivery of oxygen and nutrients to tissues, and in the kidneys is also a key determinant of glomerular filtration. Quantification of regional renal perfusion provides a potential window into renal (patho) physiology. However, non-invasive, practical, and robust methods to measure renal perfusion remain elusive, particularly in the clinic. Arterial spin labeling (ASL), a magnetic resonance imaging (MRI) technique, is arguably the only available method with potential to meet all these needs. Recent developments suggest its viability for clinical application. This review addresses several of these developments and discusses remaining challenges with the emphasis on renal imaging in human subjects.

Value of intravoxel incoherent motion in assessment of pathological grade of clear cell renal cell carcinoma

Background Intravoxel incoherent motion (IVIM) can provide a unique view of tissue perfusion without the use of exogenous contrast agents. Purpose To investigate the value of IVIM in assessing grades of clear cell renal cell carcinoma (CRCC). Material and Methods A total of 107 patients with pathologically proven CRCC were included, 26 with grade I, 27 with grade II, 25 with grade III, and 29 with grade IV. These tumors were divided into low (I + II) and high grades (III + IV). Nine b values (0, 30, 50, 80, 150, 300, 500, 800, and 1500 s/mm²) were used in diffusion-weighted imaging (DWI). The tissue diffusivity (D), pseudodiffusivity (D*), and perfusion fraction (f) were calculated using bi-exponential fitting of the diffusion data. Results The D values of the four groups were 1.83 ± 0.38, 1.23 ± 0.19, 1.07 ± 0.26, and 0.37 ± 0.11 × 10^-3 mm²/s ( P < 0.05). The D* values of the four groups were 0.079 ± 0.021, 0.053 ± 0.019, 0.047 ± 0.022, and 0.033 ± 0.017 ( P < 0.05). The f values of the four groups were 0.208 ± 0.09, 0.341 ± 0.12, 0.373 ± 0.15, and 0.461 ± 0.17 ( P < 0.05). Both the D and D* values correlated negatively with CRCC grading ( r = -0.677 and -0.693, P < 0.05). The f values correlated positively with CRCC grading (r = 0.699, P < 0.05). The areas of the D, D*, and f values under the ROC curves to diagnose low and high CRCC grades were 0.934, 0.837, and 0.793, respectively. The cutoff values of D, D*, and f were 1.13, 0.056, and 0.376, respectively; the diagnostic performance for low and high CRCC grading had a sensitivity of 82.0%, 80.7%, and 83.2% and a specificity of 90.8%, 86.3%, and 82.6%. Conclusion IVIM may provide information for differentiating CRCC grades.

Arterial spin labelling MRI for detecting pseudocapsule defects and predicting renal capsule invasion in renal cell carcinoma

AIM

To evaluate prospectively the performance of combining morphological and arterial spin labelling (ASL) magnetic resonance imaging (MRI) for detecting pseudocapsule defects in renal cell carcinoma (RCC), and to predict renal capsule invasion confirmed histopathologically.

MATERIALS AND METHODS

Twenty consecutive patients with suspicious renal tumours underwent MRI. Renal ASL imaging was performed and renal blood flow was measured quantitatively. The diagnostic performance of T2-weighted images alone, and a combination of T2-weighted and ASL images for predicting renal capsule invasion were assessed.

RESULTS

Twenty renal lesions were evaluated in 20 patients. All lesions were clear cell RCCs (ccRCCs) confirmed at post-surgical histopathology. Fifteen ccRCCs showed pseudocapsule defects on T2-weighted images, of which 12 cases showed existing blood flow in defect areas on perfusion images. To predict renal capsule invasion, the sensitivity, specificity, positive predictive value, and negative predictive value were 100%, 71.4%, 86.7%, 100%, respectively, for T2-weighted images alone, and 92.3%, 100%, 100%, 87.5%, respectively, for the combination of T2-weighted and ASL images.

CONCLUSION

ASL images can reflect the perfusion of pseudocapsule defects and as such, the combination of T2-weighted and ASL images produces promising diagnostic accuracy for predicting renal capsule invasion.

Quantification of Mouse Renal Perfusion Using Arterial Spin Labeled MRI at 1 T

RATIONALE AND OBJECTIVES

Quantitative measurement of renal perfusion in murine models provides important information on the organ physiology and disease states. The 1-T desktop magnetic resonance imaging has a small footprint and a self-contained fringe field. This resultant flexibility in siting makes the system ideal for preclinical imaging research. Our objective was to evaluate the capability of the 1-T desktop magnetic resonance imaging to measure mouse renal perfusion without the administration of exogenous contrast agents.

MATERIALS AND METHODS

We implemented a flow-sensitive alternating inversion recovery (FAIR)-based arterial spin labeling sequence with a mouse volume coil on a 1-T desktop magnetic resonance scanner. The validity of the implementation was tested by comparing obtained renal perfusion results with literature values for normal mice and challenging the technique with mice treated with furosemide, a blood vessel vasoconstrictor drug.

RESULTS

The measured cortical and medullary perfusions were quantified to be 402 ± 95 and 184 ± 52 mL/100 g/min, respectively, in agreement with literature values. The ratio of cortical to medullary renal blood flow was between 2 and 3 and was independent of the mouse weight. As expected, upon furosemide injection, a decrease (~50%) in cortical perfusion was observed in the mice population, at 1 hour post injection compared to baseline (P < 0.0001), which returned to baseline after 24 hours (P = 0.68).

CONCLUSIONS

We reported the successful application of FAIR-based arterial spin labeling for noncontrast perfusion measurement of mouse kidneys using a 1-T desktop scanner. The easy implementation of FAIR sequence on a 1-T desktop scanner offers the potential for longitudinal perfusion studies in limited access areas such as behind the barrier in mouse facilities and in multimodality preclinical imaging laboratories without the administration of exogenous contrast agents.

Arterial Spin Labeling Perfusion of the Brain: Emerging Clinical Applications

Arterial spin labeling (ASL) is a magnetic resonance (MR) imaging technique used to assess cerebral blood flow noninvasively by magnetically labeling inflowing blood. In this article, the main labeling techniques, notably pulsed and pseudocontinuous ASL, as well as emerging clinical applications will be reviewed. In dementia, the pattern of hypoperfusion on ASL images closely matches the established patterns of hypometabolism on fluorine 18 fluorodeoxyglucose (FDG) positron emission tomography (PET) images due to the close coupling of perfusion and metabolism in the brain. This suggests that ASL might be considered as an alternative for FDG, reserving PET to be used for the molecular disease-specific amyloid and tau tracers. In stroke, ASL can be used to assess perfusion alterations both in the acute and the chronic phase. In arteriovenous malformations and dural arteriovenous fistulas, ASL is very sensitive to detect even small degrees of shunting. In epilepsy, ASL can be used to assess the epileptogenic focus, both in peri- and interictal period. In neoplasms, ASL is of particular interest in cases in which gadolinium-based perfusion is contraindicated (eg, allergy, renal impairment) and holds promise in differentiating tumor progression from benign causes of enhancement. Finally, various neurologic and psychiatric diseases including mild traumatic brain injury or posttraumatic stress disorder display alterations on ASL images in the absence of visualized structural changes. In the final part, current limitations and future developments of ASL techniques to improve clinical applicability, such as multiple inversion time ASL sequences to assess alterations of transit time, reproducibility and quantification of cerebral blood flow, and to measure cerebrovascular reserve, will be reviewed. ^© RSNA, 2016 Online supplemental material is available for this article.

Respiratory motion prediction and prospective correction for free-breathing arterial spin-labeled perfusion MRI of the kidneys

PURPOSE

Respiratory motion prediction using an artificial neural network (ANN) was integrated with pseudocontinuous arterial spin labeling (pCASL) MRI to allow free-breathing perfusion measurements in the kidney. In this study, we evaluated the performance of the ANN to accurately predict the location of the kidneys during image acquisition.

METHODS

A pencil-beam navigator was integrated with a pCASL sequence to measure lung/diaphragm motion during ANN training and the pCASL transit delay. The ANN algorithm ran concurrently in the background to predict organ location during the 0.7-s 15-slice acquisition based on the navigator data. The predictions were supplied to the pulse sequence to prospectively adjust the axial slice acquisition to match the predicted organ location. Additional navigators were acquired immediately after the multislice acquisition to assess the performance and accuracy of the ANN. The technique was tested in eight healthy volunteers.

RESULTS

The root-mean-square error (RMSE) and mean absolute error (MAE) for the eight volunteers were 1.91 ± 0.17 mm and 1.43 ± 0.17 mm, respectively, for the ANN. The RMSE increased with transit delay. The MAE typically increased from the first to last prediction in the image acquisition. The overshoot was 23.58% ± 3.05% using the target prediction accuracy of ± 1 mm.

CONCLUSION

Respiratory motion prediction with prospective motion correction was successfully demonstrated for free-breathing perfusion MRI of the kidney. The method serves as an alternative to multiple breathholds and requires minimal effort from the patient.

Diagnostic value of renal perfusion in patients with chronic kidney disease using 3D arterial spin labeling

PURPOSE

To evaluate the diagnostic value of 3D arterial spin labeling (ASL) for noninvasive quantification of renal blood flow (RBF) in patients with chronic kidney disease (CKD).

MATERIALS AND METHODS

CKD patients (n = 27) and healthy volunteers (n = 36) underwent renal 3T ASL magnetic resonance imaging, with inversion times from 1200 to 2000 msec for volunteers in the preliminary test, and 1800 to 2000 msec for volunteers and CKD patients in the formal experiments. The cortical RBFs were compared, and a correlation between RBF and estimated glomerular filtration rate (eGFR) was evaluated.

RESULTS

For healthy volunteers, RBF values increased with TIs from 1200 to 1600 msec, but were almost constant at TIs from 1600 to 2000 msec. The cortical RBF values of CKD patients were lower than that of healthy volunteers at TIs from 1800 to 2000 msec. In addition, the CKD patients had lower cortical RBF values than the healthy volunteers (P < 0.01 for both), and their RBF values positively correlated with eGFR.

CONCLUSION

3D ASL is a potential noninvasive method for measuring renal perfusion that can provide valuable information for clinical CKD diagnosis.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 3 J. MAGN. RESON. IMAGING 2017;46:589-594.

Volumetric Arterial Spin-labeled Perfusion Imaging of the Kidneys with a Three-dimensional Fast Spin Echo Acquisition

RATIONALE AND OBJECTIVES

Renal perfusion measurements using noninvasive arterial spin-labeled (ASL) magnetic resonance imaging techniques are gaining interest. Currently, focus has been on perfusion in the context of renal transplant. Our objectives were to explore the use of ASL in patients with renal cancer, and to evaluate three-dimensional (3D) fast spin echo (FSE) acquisition, a robust volumetric imaging method for abdominal applications. We evaluate 3D ASL perfusion magnetic resonance imaging in the kidneys compared to two-dimensional (2D) ASL in patients and healthy subjects.

MATERIALS AND METHODS

Isotropic resolution (2.6 × 2.6 × 2.8 mm(3)) 3D ASL using segmented FSE was compared to 2D single-shot FSE. ASL used pseudo-continuous labeling, suppression of background signal, and synchronized breathing. Quantitative perfusion values and signal-to-noise ratio (SNR) were compared between 3D and 2D ASL in four healthy volunteers and semiquantitative assessments were made by four radiologists in four patients with known renal masses (primary renal cell carcinoma).

RESULTS

Renal cortex perfusion in healthy subjects was 284 ± 21 mL/100 g/min, with test-retest repeatability of 8.8%. No significant differences were found between the quantitative perfusion value and SNR in volunteers between 3D ASL and 2D ASL, or in 3D ASL with synchronized or free breathing. In patients, semiquantitative assessment by radiologists showed no significant difference in image quality between 2D ASL and 3D ASL. In one case, 2D ASL missed a high perfusion focus in a mass that was seen by 3D ASL.

CONCLUSIONS

3D ASL renal perfusion imaging provides isotropic-resolution images, with comparable quantitative perfusion values and image SNR in similar imaging time to single-slice 2D ASL.

Multiparametric MR Imaging in Abdominal Malignancies

Modern MR imaging protocols can yield both anatomic and functional information for the assessment of hepatobiliary and pancreatic malignancies. Diffusion-weighted imaging is fully integrated into state-of-the-art protocols for tumor detection, characterization, and therapy monitoring. Hepatobiliary contrast agents have gained ground in the evaluation of focal liver lesions during the last years. Perfusion MR imaging is expected to have a central role for monitoring therapy in body tumors treated with antivascular drugs. Approaches such as Magnetic resonance (MR) elastography and (1)H-MR spectroscopy are still confined to research centers, but with the potential to grow in a short time frame.

Functional imaging of the human placenta with magnetic resonance

Abnormal placentation is responsible for most failures in pregnancy; however, an understanding of placental functions remains largely concealed from noninvasive, in vivo investigations. Magnetic resonance imaging (MRI) is safe in pregnancy for magnetic fields of up to 3 Tesla and is being used increasingly to improve the accuracy of prenatal imaging. Functional MRI (fMRI) of the placenta has not yet been validated in a clinical setting, and most data are derived from animal studies. FMRI could be used to further explore placental functions that are related to vascularization, oxygenation, and metabolism in human pregnancies by the use of various enhancement processes. Dynamic contrast-enhanced MRI is best able to quantify placental perfusion, permeability, and blood volume fractions. However, the transplacental passage of Gadolinium-based contrast agents represents a significant safety concern for this procedure in humans. There are alternative contrast agents that may be safer in pregnancy or that do not cross the placenta. Arterial spin labeling MRI relies on magnetically labeled water to quantify the blood flows within the placenta. A disadvantage of this technique is a poorer signal-to-noise ratio. Based on arterial spin labeling, placental perfusion in normal pregnancy is 176 ± 91 mL × min(-1) × 100 g(-1) and decreases in cases with intrauterine growth restriction. Blood oxygen level-dependent and oxygen-enhanced MRIs do not assess perfusion but measure the response of the placenta to changes in oxygen levels with the use of hemoglobin as an endogenous contrast agent. Diffusion-weighted imaging and intravoxel incoherent motion MRI do not require exogenous contrast agents, instead they use the movement of water molecules within tissues. The apparent diffusion coefficient and perfusion fraction are significantly lower in placentas of growth-restricted fetuses when compared with normal pregnancies. Magnetic resonance spectroscopy has the ability to extract information regarding metabolites from the placenta noninvasively and in vivo. There are marked differences in all 3 metabolites N-acetyl aspartate/choline levels, inositol/choline ratio between small, and adequately grown fetuses. Current research is focused on the ability of each fMRI technique to make a timely diagnosis of abnormal placentation that would allow for appropriate planning of follow-up examinations and optimal scheduling of delivery. These research programs will benefit from the use of well-defined sequences, standardized imaging protocols, and robust computational methods.

Childhood extracranial neoplasms: the role of imaging in drug development and clinical trials

Cancer is the leading cause of death in children older than 1 year of age and new drugs are necessary to improve outcomes. Imaging is crucial to the drug development process and assessment of therapeutic response. In adults, tumours are often assessed with CT using size criteria. Unfortunately, techniques established in adults are not necessarily applicable in children due to differing pathophysiology, ability to cooperate and increased susceptibility to ionising radiation. MRI, in particular quantitative MRI, has to date not been fully utilised in children with extracranial neoplasms. The specific challenges of imaging in children, the potential for functional imaging techniques to inform upon and their inclusion in clinical trials are discussed.

Arterial spin labeling to determine tumor viability in head and neck cancer before and after treatment

PURPOSE

To evaluate the feasibility of arterial spin-labeling (ASL) in head and neck cancer for noninvasive measurement of tumor blood flow (TBF), by comparing 1) the TBF change before and after the treatment, and 2) posttreatment TBF and its reduction rate between residual and nonresidual tumors after treatment.

MATERIALS AND METHODS

Twenty-two patients with head and neck cancer were evaluated using ASL on 3.0-T magnetic resonance imaging (MRI) before and after nonsurgical treatment. A pulsed ASL sequence with Look–Locker readout was used to calculate quantitative TBF. TBF reduction rates between pre- and posttreatment values were also calculated. Residual tumors were confirmed when present with either histopathologically or clinical follow-up.

RESULTS

Pre- and posttreatment mean TBF values were 121.4 ± 27.8 (standard deviation) and 24.9 ± 14.9 mL/100g/min, respectively. Pre- and posttreatment TBF differed significantly. Posttreatment TBF was significantly higher in patients with residual tumors (five patients, 46.9 ± 7.1 mL/100g/min) than in those without (17 patients, 18.4 ± 9.2 mL/100g/min). The TBF reduction rate was significantly lower in patients with residual tumors (0.540.55 ± 0.120.12) than in those without (0.85 ± 0.06).

CONCLUSION

ASL allows quantitative assessment of TBF in head and neck cancer. ASL may be useful for noninvasive assessment of tumor viability in head and neck cancer.

Imaging of Tumor Angiogenesis for Radiologists--Part 2: Clinical Utility

Angiogenesis is a key cancer hallmark involved in tumor growth and metastasis development. Angiogenesis and tumor microenvironment significantly influence the response of tumors to therapies. Imaging techniques have changed our understanding of the process of angiogenesis, the resulting vascular performance, and the tumor microenvironment. This article reviews the status and potential clinical value of the imaging modalities used to assess the status of tumor vasculature in vivo, before, during, and after treatment.

Quantifying vascular heterogeneity using microbubble disruption-replenishment kinetics in patients with renal cell cancer

PURPOSE

The purposes of this study were to establish the physiological interpretation of the shape parameter of the dynamic contrast-enhanced ultrasound (DCE-US) lognormal perfusion model and to evaluate the clinical significance of the parameter in a sample of patients undergoing antiangiogenic therapy for metastatic renal cell carcinoma (mRCC).

MATERIALS AND METHODS

The physiological interpretation of the lognormal shape parameter was explored using computer simulations of disruption-replenishment in fractal models of the microcirculation generated by a piecewise iterative algorithm in MATLAB. Architectural variety was accomplished by introducing random perturbations to the diameter, length, and branching angles to the growing vascular tree. The shape parameter was extracted from the time-intensity curves and compared with the transit time distributions calculated directly from the simulations. Dynamic contrast-enhanced ultrasound data were obtained from 31 consenting patients with mRCC being treated with antiangiogenic therapy. Lognormal parameters related to the blood volume, mean flow speed, and vascular morphology/heterogeneity extracted before, during, and after therapy were correlated with progression-free survival (PFS). Cox proportional hazard ratios were calculated alongside receiver operator characteristics for different combinations of the vascular parameters to determine their ability to distinguish patients who would progress early (less than the median PFS) versus late (greater than the median PFS).

RESULTS

The lognormal shape parameter correlated strongly to the width of the transit time distribution calculated directly from the simulations, and by extension, to the morphology/heterogeneity of the microvascular network (Spearman r = 0.80, P < 0.001, n = 28). Shorter time to progression was predicted by higher baseline heterogeneity (P = 0.003) and a reduction in tumor blood volume less than 43% (P = 0.002) after 2 weeks of treatment. Combining baseline parameters with changes that occur shortly after starting treatment increased the sensitivity and specificity of DCE-US to identify which patients would progress/resist therapy early versus late compared with when the vascular parameters were considered in isolation.

CONCLUSIONS

The DCE-US shape parameter from the lognormal perfusion model is representative of microvascular morphology/heterogeneity and may be used to noninvasively characterize the vascular architecture of cancer lesions. A more abnormal flow distribution at baseline predicts for poorer outcome for patients treated with antiangiogenic therapy for metastatic renal cell cancer. Combining pretreatment and on-treatment measurements of vascularity can improve the performance of DCE-US to predict which patients will progress earlier versus later when on antiangiogenic therapy for mRCC.

Arterial spin labeling-fast imaging with steady-state free precession (ASL-FISP): a rapid and quantitative perfusion technique for high-field MRI

Arterial spin labeling (ASL) is a valuable non-contrast perfusion MRI technique with numerous clinical applications. Many previous ASL MRI studies have utilized either echo-planar imaging (EPI) or true fast imaging with steady-state free precession (true FISP) readouts, which are prone to off-resonance artifacts on high-field MRI scanners. We have developed a rapid ASL-FISP MRI acquisition for high-field preclinical MRI scanners providing perfusion-weighted images with little or no artifacts in less than 2 s. In this initial implementation, a flow-sensitive alternating inversion recovery (FAIR) ASL preparation was combined with a rapid, centrically encoded FISP readout. Validation studies on healthy C57/BL6 mice provided consistent estimation of in vivo mouse brain perfusion at 7 and 9.4 T (249 ± 38 and 241 ± 17 mL/min/100 g, respectively). The utility of this method was further demonstrated in the detection of significant perfusion deficits in a C57/BL6 mouse model of ischemic stroke. Reasonable kidney perfusion estimates were also obtained for a healthy C57/BL6 mouse exhibiting differential perfusion in the renal cortex and medulla. Overall, the ASL-FISP technique provides a rapid and quantitative in vivo assessment of tissue perfusion for high-field MRI scanners with minimal image artifacts.

[Quantitative imaging in uro-oncology]

INTRODUCTION

Imaging currently performed in uro-oncology could provide useful information. The use of all this information could help to better understand tumor growth and response to treatment. Therefore, it seems interesting to review the knowledge, to describe the main techniques currently available in many centers or in process and to clarify their results.

MATERIALS AND METHODS

A systematic literature review was conducted in the PubMed database to identify all imaging techniques performed for therapeutic evaluation in uro-oncology. The keywords used were: cancer, kidney, bladder, prostate, urology biomarkers, imaging, ultrasound, CT-scan, MRI, PET-CT, RECIST, BOLD, ASL, gold DWI Diffusion, contrast, F-miso. The first publications identified were analyzed to search unidentified studies by the selected keywords.

RESULTS

From simple to more complex morphology data from functional imaging (PET, MRI), data obtained from imaging helps to better understand tumor growth and response to treatment. Although optimizations are coming, all the techniques reported are available in many centers or going to be.

CONCLUSION

The imaging evaluation in onco-urology can bring a large amount of information. Integrating to research protocols is now essential to sustain this activity.

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.

Balanced steady state free precession for arterial spin labeling MRI: Initial experience for blood flow mapping in human brain, retina, and kidney

We implemented pseudo-continuous ASL (pCASL) with 2D and 3D balanced steady state free precession (bSSFP) readout for mapping blood flow in the human brain, retina, and kidney, free of distortion and signal dropout, which are typically observed in the most commonly used echo-planar imaging acquisition. High resolution functional brain imaging in the human visual cortex was feasible with 3D bSSFP pCASL. Blood flow of the human retina could be imaged with pCASL and bSSFP in conjunction with a phase cycling approach to suppress the banding artifacts associated with bSSFP. Furthermore, bSSFP based pCASL enabled us to map renal blood flow within a single breath hold. Control and test-retest experiments suggested that the measured blood flow values in retina and kidney were reliable. Because there is no specific imaging tool for mapping human retina blood flow and the standard contrast agent technique for mapping renal blood flow can cause problems for patients with kidney dysfunction, bSSFP based pCASL may provide a useful tool for the diagnosis of retinal and renal diseases and can complement existing imaging techniques.

Diffusion-weighted intravoxel incoherent motion imaging of renal tumors with histopathologic correlation

PURPOSE

The aim of this study was to use intravoxel incoherent motion diffusion-weighted imaging to discriminate subtypes of renal neoplasms and to assess agreement between intravoxel incoherent motion (perfusion fraction, fp) and dynamic contrast-enhanced magnetic resonance imaging (MRI) metrics of tumor vascularity.

SUBJECTS AND METHODS

In this Health Insurance Portability and Accountability Act-compliant, institutional review board-approved prospective study, 26 patients were imaged at 1.5-T MRI using dynamic contrast-enhanced MRI with high temporal resolution and diffusion-weighted imaging using 8 b values (range, 0-800 s/mm). Perfusion fraction (fp), tissue diffusivity (Dt), and pseudodiffusivity (Dp) were calculated using biexponential fitting of the diffusion data. Apparent diffusion coefficient (ADC) was calculated with monoexponential fit using 3 b values of 0, 400, and 800 s/mm. Dynamic contrast-enhanced data were processed with a semiquantitative method to generate model-free parameter cumulative initial area under the curve of gadolinium concentration at 60 seconds (CIAUC60). Perfusion fraction, Dt, Dp, ADC, and CIAUC60 were compared between different subtypes of renal lesions. Perfusion fraction was correlated with CIAUC60.

RESULTS

We examined 14 clear cell, 4 papillary, 5 chromophobe, and 3 cystic renal cell carcinomas (RCCs). Although fp had higher accuracy (area under the curve, 0.74) for a diagnosis of clear cell RCC compared with Dt or ADC, the combination of fp and Dt had the highest accuracy (area under the curve, 0.78). The combination of fp and Dt diagnosed papillary RCC and cystic RCC with 100% accuracy, and clear cell RCC and chromophobe RCC, with 86.5% accuracy. There was significant strong correlation between fp and CIAUC60 (r = 0.82; P < 0.001).

CONCLUSION

Intravoxel incoherent motion parameters fp and Dt can discriminate renal tumor subtypes. Perfusion fraction demonstrates good correlation with CIAUC60 and can assess degree of tumor vascularity without the use of exogenous contrast agent.

The role of functional imaging in the era of targeted therapy of renal cell carcinoma

Antiangiogenic therapies interacting with tumor-specific pathways have been established for targeted therapy of renal cell carcinoma (RCC). However, evaluation of tumor response based on morphologic tumor diameter measurements has limitations, as tumor shrinkage may lag behind pathophysiological response. Functional imaging techniques such as dynamic contrast-enhanced (DCE) ultrasound (US), computed tomography (CT) and magnetic resonance imaging (MRI), unenhanced diffusion-weighted MRI (DW-MRI), and also metabolic imaging with positron emission tomography (PET) have the ability to assess physiological parameters and to predict and monitor therapy response. Assessment of changes in vascularity, cellularity, oxygenation, and glucose uptake with functional imaging during targeted therapy may correlate with progression-free survival and can predict tumor response or progression. In this review, we explore the potential of functional imaging techniques for assessing the effects of targeted therapy of RCC and as well review the reproducibility and limitations.

Imaging techniques as predictive and prognostic biomarkers in renal cell carcinoma

A number of imaging modalities are showing promise as predictive and prognostic biomarkers in advanced renal cell carcinoma. This review discusses progress to date in this exciting area and identifies areas of future promise.

Feasibility of measuring prostate perfusion with arterial spin labeling

Prostate perfusion has the potential to become an important pathophysiological marker for the monitoring of disease progression or the assessment of the therapeutic response of prostate cancer. The feasibility of arterial spin labeling, an MRI approach for the measurement of perfusion without an exogenous contrast agent, is demonstrated in the prostate for the first time. Although various arterial spin labeling methods have been demonstrated previously in highly perfused organs, such as the brain and kidneys, the prospect of obtaining such measurements in the prostate is challenging because of the relatively low blood flow, long transit times, susceptibility-induced image distortion and local motion. However, despite these challenges, this study demonstrates that, with a whole-body transmit coil and external receiver array, global prostate perfusion can be measured with arterial spin labeling at 3 T. In five healthy subjects with a mean age of 44 years, the mean total prostate blood flow was measured to be 25.8 ± 7.1 mL/100 cm(3) /min, with an estimated bolus duration and arterial transit time of 884 ± 209 ms and 721 ± 131 ms, respectively.

A comparative study between arterial spin labeling and CT perfusion methods on hepatic portal venous flow

PURPOSE

The purpose of this study was to evaluate the feasibility and potential usefulness of unenhanced magnetic resonance (MR) hepatic portal perfusion using arterial spin labeling (ASL) among healthy volunteers and hepatocellular carcinoma patients.

MATERIALS AND METHODS

The five healthy volunteers underwent unenhanced MR perfusion with inversion time 2 (TI2) values at 500-ms intervals between 2,000 and 4,000 ms, and the 12 patients underwent unenhanced MR perfusion using ASL and computed tomography (CT) perfusion during superior mesenteric artery (SMA) portography. The regions of interest were placed in both the right and left lobes of the liver or both the right anterior and posterior segments of the liver and were placed over the tumor if a lesion was located within a particular perfusion study slice.

RESULTS

In the healthy volunteer study, perfusion rate in hepatic parenchyma showed a peak at the TI2 value of 3,000 ms (254.3 ml/min/100 g ± 58.3). In patients, a fair correlation was observed between CT and MR perfusion (r = 0.795, P < 0.01).

CONCLUSION

Our results demonstrate a significant fair correlation between unenhanced MR hepatic portal perfusion imaging using ASL and CT perfusion during SMA portography.

Comparing kidney perfusion using noncontrast arterial spin labeling MRI and microsphere methods in an interventional swine model

OBJECTIVE

The purpose of this study was to assess the ability of a flow-sensitive alternating inversion recovery-arterial spin labeling (FAIR-ASL) technique to track renal perfusion changes during pharmacologic and physiologic alterations in renal blood flow using microspheres as a gold standard.

MATERIALS AND METHODS

Fluorescent microsphere and FAIR-ASL perfusion were compared in the cortex of the kidney for 11 swine across 4 interventional time points: (1) under baseline conditions, (2) during an acetylcholine and fluid bolus challenge to increase perfusion, (3) initially after switching to isoflurane anesthesia, and (4) after 2 hours of isoflurane anesthesia. In 10 of the 11 swine, a bag of ice was placed on the hilum of 1 kidney at the beginning of isoflurane administration to further reduce perfusion in 1 kidney.

RESULTS

Both ASL and microspheres tracked the expected cortical perfusion changes (P < 0.02) across the interventions, including an increase in perfusion during the acetylcholine challenge and decrease during the administration of isoflurane. Both techniques also measured lower cortical perfusion in the iced compared with the non-iced kidneys (P ≤ 0.01). The ASL values were systematically lower compared with microsphere perfusion. Very good correlation (r = 0.81, P < 0.0001) was observed between the techniques, and the relationship appeared linear for perfusion values in the expected physiologic range (microsphere perfusion <550 mL/min/100 g) although ASL values saturated for perfusion >550 mL/min/100 g.

CONCLUSION

Cortical perfusion measured with ASL correlated with microspheres and reliably detected changes in renal perfusion in response to physiologic challenge.

Arterial spin labeling blood flow magnetic resonance imaging for evaluation of renal injury

A multitude of evidence suggests that iodinated contrast material causes nephrotoxicity; however, there have been no previous studies that use arterial spin labeling (ASL) blood flow functional magnetic resonance imaging (fMRI) to investigate the alterations in effective renal plasma flow between normointensive and hypertensive rats following injection of contrast media. We hypothesized that FAIR-SSFSE arterial spin labeling MRI may enable noninvasive and quantitative assessment of regional renal blood flow abnormalities and correlate with disease severity as assessed by histological methods. Renal blood flow (RBF) values of the cortex and medulla of rat kidneys were obtained from ASL images postprocessed at ADW4.3 workstation 0.3, 24, 48, and 72 h before and after injection of iodinated contrast media (6 ml/kg). The H&E method for morphometric measurements was used to confirm the MRI findings. The RBF values of the outer medulla were lower than those of the cortex and the inner medulla as reported previously. Iodinated contrast media treatment resulted in decreases in RBF in the outer medulla and cortex in spontaneously hypertensive rats (SHR), but only in the outer medulla in normotensive rats. The iodinated contrast agent significantly decreased the RBF value in the outer medulla and the cortex in SHR compared with normotensive rats after injection of the iodinated contrast media. Histological observations of kidney morphology were also consistent with ASL perfusion changes. These results demonstrate that the RBF value can reflect changes of renal perfusion in the cortex and medulla. ASL-MRI is a feasible and accurate method for evaluating nephrotoxic drugs-induced kidney damage.

Evaluation of treatment response in patients with metastatic renal cell carcinoma: role of state-of-the-art cross-sectional imaging

Recent advances in genetics and oncology have led to development of a wide array of molecular therapeutics in the management of patients with metastatic renal cell carcinoma. These drugs have revolutionized the treatment of advanced disease by significantly improving patient outcomes. State-of-the-art cross-sectional imaging techniques play a seminal role in the evaluation of treatment response by providing reproducible, objective data, thereby permitting accurate quantification of tumor burden. Evolving functional imaging techniques such as perfusion and diffusion studies continue to advance the technology beyond assessing changes in tumor size and morphology.

Pulmonary magnetic resonance imaging for airway diseases

Pulmonary magnetic resonance (MR) imaging has been put forward as a new research and diagnostic tool mainly to overcome the limitations of computed tomography and nuclear medicine studies. However, pulmonary MR imaging has been difficult to use because of inherently low proton density, a multitude of air-tissue interfaces, which create significant magnetic field distortions and are commonly referred to as susceptibility artifacts; diminishing signal in the lung; and respiratory and/or cardiac motion artifacts. To overcome these drawbacks of pulmonary MR imaging, technical advances made during the last decade in sequencing, scanner and coil, adaptation of parallel imaging techniques, and utilization of contrast media have been reported as being useful for functional and morphologic assessment of various pulmonary diseases including airway diseases. This review article covers (1) pulmonary MR techniques for morphologic and functional assessment of airway diseases, and (2) pulmonary MR imaging for cystic fibrosis, asthma, and chronic obstructive pulmonary disease. Pulmonary MR imaging provides not only morphology-related but also pulmonary function-related information. It has the potential to replace nuclear medicine studies for the identification of regional pulmonary function and may perform a complementary role in airway disease assessment instead of nuclear medicine study. We believe that the findings of further basic studies as well as clinical applications of this new technique will validate the real significance of pulmonary MR imaging for the future of airway disease assessment and its usefulness for diagnostic radiology and pulmonary medicine.

Renal perfusion 3-T MR imaging: a comparative study of arterial spin labeling and dynamic contrast-enhanced techniques

PURPOSE

To investigate the feasibility of and correlation between arterial spin-labeling (ASL) and dynamic contrast material-enhanced (DCE) 3-T magnetic resonance (MR) imaging in the measurement of renal blood flow (RBF).

MATERIALS AND METHODS

The review board approved this study. Nineteen healthy volunteers (seven women, 12 men; age range, 25-68 years) were recruited, and each provided written informed consent. MR imaging was performed with a 3-T whole-body system. Each subject underwent back-to-back ASL and DCE MR imaging. Ten runs of ASL imaging were performed by using the pseudocontinuous tagging scheme, and each run required an 18-second breath hold. For DCE imaging, a gadopentetate dimeglumine bolus (0.0125 mmol per kilogram of body weight) was administrated intravenously in all subjects except two; in the latter subjects, a 0.025 mmol/kg gadopentetate dimeglumine bolus was administered to evaluate the T1 saturation effect. RBF was quantified with both techniques and in both the cortex and the medulla. Agreement was evaluated for RBF measurements obtained with ASL imaging and those obtained with DCE imaging by using correlation analysis.

RESULTS

RBF was apparently overestimated with 0.025 mmol/kg gadopentetate dimeglumine, which is a concentration that is commonly adopted for 1.5-T DCE. RBF was 227 mL/100 mL/min ± 30 (standard deviation) in the cortex and 101 mL/100 mL/min ± 21 in the medulla, as measured with ASL imaging, and 272 mL/100 mL/min ± 60 in the cortex and 122 mL/100 mL/min ± 30 in the medulla, as measured with DCE imaging. In the cortex, measurements obtained with ASL and DCE imaging exhibited a linear correlation (r = 0.66; statistical power, 0.8 at the 5% significance level) and fair agreement (intraclass correlation coefficient, 0.41).

CONCLUSION

ASL and DCE 3-T MR imaging are feasible in the quantification of cortical renal perfusion, yielding measurements that are correlated but not entirely comparable. Intermodality differences have yet to be solved.

Repeatability of renal arterial spin labelling MRI in healthy subjects

OBJECT

Arterial spin labelling (ASL) can be used to measure renal perfusion non-invasively. The aim of this study was to determine the repeatability of this technique in healthy kidneys to vindicate its use in clinic.

MATERIALS AND METHODS

Two groups of healthy volunteers were imaged two different days to assess intra- and inter-session repeatability. Oblique-coronal data volumes were acquired on a 1.5 T scanner with a dedicated abdominal 32-channel body phased array coil. ASL was performed using a multi-TI FAIR labelling scheme and 3D GRASE imaging module. Background suppression and respiratory triggering were used. T(1) maps of the kidney were acquired using the same sequence with background suppression disabled.

RESULTS

For the group with multiple intra-session ASL measurements, the average cortical perfusion was 197 mL min(-1)100 g(-1) and average cortical T(1) was 1265 ms. For both perfusion and T(1) the variation shown by the within-subject standard deviation (SDws) (14.6 mL min(-1)100 g(-1) and 33.4 ms) and coefficient of variation (CVws) (7.52 and 2.69%, respectively) was small for all the analyses carried out. Bland-Altman plots were also used to visualise the variation between the same parameters collected from the different scanning sessions in both groups, and demonstrated good reproducibility.

CONCLUSION

We have shown that in healthy volunteers, ASL parameters are repeatable over a short and long period. This supports the overall aim of using ASL in the clinic to assess longitudinal renal perfusion changes in patients.

Imaging in renal cell carcinoma

Imaging plays a central role in the detection, diagnosis, staging, and follow-up of renal cell carcinoma (RCC). Most renal masses are incidentally detected with modern, high-resolution imaging techniques and a variety of management options exist for the renal masses encountered today. This article discusses the role of multiple imaging modalities in the diagnosis of RCC and the imaging features of specific pathologic subtypes and staging techniques. Future directions in RCC imaging are presented, including dynamic contrast-enhanced and unenhanced techniques, as well as the development of novel tracers for positron emission tomography.

Unique in utero identification of fetuses in multifetal mouse pregnancies by placental bidirectional arterial spin labeling MRI

Noninvasive imaging is a critical part of the study of developing embryos/fetuses, particularly in the context of alterations of gene expression in genetically modified animals. However, in litter-bearing animals, such as mice, the inability to accurately identify individual embryo/fetus in utero is a major obstacle to longitudinal, noninvasive in vivo studies. Arterial spin labeling MRI was adopted here to determine the fetal order along the uterine horns in vivo, based on the specific pattern of dual arterial blood supply within the mouse uterine horns. Blood enters the mouse uterus cranially through the ovarian artery and caudally through the uterine artery. Saturation slices were alternately placed on the maternal heart or on the bifurcation point of the common iliac artery, thereby saturating either downward inflow via the ovarian arteries or upward inflow via the uterine arteries, respectively. Saturation maps provided a unique signature with highly significant correlation between the direction-dependent magnetization transfer and the position of the fetuses/placentas along the uterine horns. The bidirectional arterial spin labeling-MRI method reported here opens possibilities to determine and pursue phenotypic alterations in fetuses and placentas in longitudinal studies of transgenic and knockout mice models, and for studying defects in placental vascular architecture.

In vivo, label-free, three-dimensional quantitative imaging of kidney microcirculation using Doppler optical coherence tomography

Doppler optical coherence tomography (DOCT) is a functional extension of optical coherence tomography (OCT) and is currently being employed in several clinical arenas to quantify blood flow in vivo. In this study, the objective was to investigate the feasibility of DOCT to image kidney microcirculation, specifically, glomerular blood flow. DOCT is able to capture three-dimensional (3D) data sets consisting of a series of cross-sectional images in real time, which enables label-free and non-destructive quantification of glomerular blood flow. The kidneys of adult, male Munich-Wistar rats were exposed through laparotomy procedure after being anesthetized. Following exposure of the kidney beneath the DOCT microscope, glomerular blood flow was observed. The effects of acute mannitol and angiotensin II infusion were also observed. Glomerular blood flow was quantified for the induced physiological states and compared with baseline measurements. Glomerular volume, cumulative Doppler volume, and Doppler flow range parameters were computed from 3D OCT/DOCT data sets. Glomerular size was determined from OCT, and DOCT readily revealed glomerular blood flow. After infusion of mannitol, a significant increase in blood flow was observed and quantified, and following infusion of angiontensin II, a significant decrease in blood flow was observed and quantified. Also, blood flow histograms were produced to illustrate differences in blood flow rate and blood volume among the induced physiological states. We demonstrated 3D DOCT imaging of rat kidney microcirculation in the glomerulus in vivo. Dynamic changes in blood flow were detected under altered physiological conditions demonstrating the real-time imaging capability of DOCT. This method holds promise to allow non-invasive imaging of kidney blood flow for transplant graft evaluation or monitoring of altered-renal hemodynamics related to disease progression.

Arterial spin labeling MR imaging for characterisation of renal masses in patients with impaired renal function: initial experience

OBJECTIVES

To retrospectively evaluate the feasibility of arterial spin labeling (ASL) magnetic resonance imaging (MRI) for the assessment of vascularity of renal masses in patients with impaired renal function.

METHODS

Between May 2007 and November 2008, 11/67 consecutive patients referred for MRI evaluation of a renal mass underwent unenhanced ASL-MRI due to moderate-to-severe chronic or acute renal failure. Mean blood flow in vascularised and non-vascularised lesions and the relation between blood flow and final diagnosis of malignancy were correlated with a 2-sided homogeneous variance t-test and the Fisher Exact Test, respectively. A p value <0.05 was considered statistically significant.

RESULTS

Seventeen renal lesions were evaluated in 11 patients (8 male; mean age = 70 years) (range 57-86). The median eGFR was 24 mL/min/1.73 m(2) (range 7-39). The average blood flow of 11 renal masses interpreted as ASL-positive (134 +/- 85.7 mL/100 g/min) was higher than that of 6 renal masses interpreted as ASL-negative (20.5 +/- 8.1 mL/100 g/min)(p = 0.015). ASL-positivity correlated with malignancy (n = 3) or epithelial atypia (n = 1) at histopathology or progression at follow up (n = 7).

CONCLUSIONS

ASL detection of vascularity in renal masses in patients with impaired renal function is feasible and seems to indicate neoplasia although the technique requires further evaluation.

KEY POINTS

Arterial spin labeling may help to characterise renal masses in patients with renal failure Detection of blood flow on ASL in a renal mass supports the presence of a neoplasm Renal masses with high blood-flow levels on ASL seem to progress rapidly.