Reduction of motion effects in myocardial arterial spin labeling

Purpose To evaluate the accuracy and reproducibility of myocardial blood flow measurements obtained under different breathing strategies and motion correction techniques with arterial spin labeling. Methods A prospective cardiac arterial spin labeling study was performed in 12 volunteers at 3 Tesla. Perfusion images were acquired twice under breath-hold, synchronized-breathing, and free-breathing. Motion detection based on the temporal intensity variation of a myocardial voxel, as well as image registration based on pairwise and groupwise approaches, were applied and evaluated in synthetic and in vivo data. A region of interest was drawn over the mean perfusion-weighted image for quantification. Original breath-hold datasets, analyzed with individual regions of interest for each perfusion-weighted image, were considered as reference values. Results Perfusion measurements in the reference breath-hold datasets were in line with those reported in literature. In original datasets, prior to motion correction, myocardial blood flow quantification was significantly overestimated due to contamination of the myocardial perfusion with the high intensity signal of blood pool. These effects were minimized with motion detection or registration. Synthetic data showed that accuracy of the perfusion measurements was higher with the use of registration, in particular after the pairwise approach, which probed to be more robust to motion. Conclusion Satisfactory results were obtained for the free-breathing strategy after pairwise registration, with higher accuracy and robustness (in synthetic datasets) and higher intrasession reproducibility together with lower myocardial blood flow variability across subjects (in in vivo datasets). Breath-hold and synchronized-breathing after motion correction provided similar results, but these breathing strategies can be difficult to perform by patients

Multiparametric renal magnetic resonance imaging: A reproducibility study in renal allografts with stable function

Monitoring renal allograft function after transplantation is key for the early detection of allograft impairment, which in turn can contribute to preventing the loss of the allograft. Multiparametric renal MRI (mpMRI) is a promising noninvasive technique to assess and characterize renal physiopathology; however, few studies have employed mpMRI in renal allografts with stable function (maintained function over a long time period). The purposes of the current study were to evaluate the reproducibility of mpMRI in transplant patients and to characterize normal values of the measured parameters, and to estimate the labeling efficiency of Pseudo-Continuous Arterial Spin Labeling (PCASL) in the infrarenal aorta using numerical simulations considering experimental measurements of aortic blood flow profiles. The subjects were 20 transplant patients with stable kidney function, maintained over 1 year. The MRI protocol consisted of PCASL, intravoxel incoherent motion, and T1 inversion recovery. Phase contrast was used to measure aortic blood flow. Renal blood flow (RBF), diffusion coefficient (D), pseudo-diffusion coefficient (D*), flowing fraction ( f ), and T1 maps were calculated and mean values were measured in the cortex and medulla. The labeling efficiency of PCASL was estimated from simulation of Bloch equations. Reproducibility was assessed with the within-subject coefficient of variation, intraclass correlation coefficient, and Bland-Altman analysis. Correlations were evaluated using the Pearson correlation coefficient. The significance level was p less than 0.05. Cortical reproducibility was very good for T1, D, and RBF, moderate for f , and low for D*, while medullary reproducibility was good for T1 and D. Significant correlations in the cortex between RBF and f (r = 0.66), RBF and eGFR (r = 0.64), and D* and eGFR (r = -0.57) were found. Normal values of the measured parameters employing the mpMRI protocol in kidney transplant patients with stable function were characterized and the results showed good reproducibility of the techniques

Non-invasive evaluation of myocardial perfusion in humans using arterial spin labeling magnetic resonance imaging

Las arterias coronarias son las encargadas de suministrar constantemente oxígeno y nutrientes al corazón. Su estrechamiento u obstrucción causan isquemia y, si no se tratan, provocan la enfermedad isquémica del corazón (CAD), una de las enfermedades cardiovasculares con mayor tasa de mortalidad en Estados Unidos y Europa. La cuantificación de la perfusión miocárdica es un factor clave para el diagnóstico y evaluación de CAD. La perfusión miocárdica se evalúa en resonancia magnética con una técnica de imagen llamada first-pass que requiere de la inyección de un material de contraste basado en Gadolinio, el cual limita la repetibilidad de la técnica y restringe su uso en pacientes con una función renal anormal. Arterial Spin Labeling (ASL) es una técnica de imagen por resonancia magnética que permite cuantificar la perfusión sin necesidad de inyectar un agente de contraste, a través del marcado electromagnético del agua contenida en la sangre arterial, actuando como trazador endógeno. El principal inconveniente de esta técnica es que la magnitud de este trazador es pequeña, debido a la reducida cantidad de sangre marcada por volumen de tejido, y decae con el tiempo de relajación T1. Por tanto, la relación señal/ruido es relativamente baja. Además, su aplicación en el corazón es complicada debido al continuo movimiento y al tortuoso camino que realiza la sangre arterial a través del corazón, lo cual dificulta un marcado preciso. En esta tesis, la técnica ASL (adquisición y procesado de datos) fue optimizada para su utilización en voluntarios sanos y pacientes. En particular, la técnica ASL fue implementada, optimizada y validada en el corazón de sujetos sanos bajo condiciones de reposo y estrés leve mediante el levantamiento pasivo de piernas. Distintas estrategias fueron evaluadas para minimizar el movimiento cardíaco y respiratorio durante la adquisición y el post-procesado de las imágenes de perfusión. Finalmente, la aplicación de la secuencia ASL en el corazón fue evaluada en un grupo de pacientes con sospecha de CAD en condiciones de reposo y vasodilatación farmacológica. La validación de la secuencia miocárdica ASL en voluntarios sanos mostró una mayor eficiencia en el tiempo de adquisición con la utilización de un TR corto de aproximadamente cuatro segundos, el cual proporcionó medidas de reproducibilidad razonables (coeficientes de variación entre sujeto de 17% intrasesión y 37% intersesión). La comparación entre las distintas estrategias de respiración y técnicas de corrección de movimiento mostró la superioridad de la respiración libre en combinación con el método de registro pairwise , resultando en una mayor precisión tras el registro de imágenes sintéticas y mejores resultados de reproducibilidad intrasesión (coeficientes de variación entre sujeto de 14%) junto con una menor variabilidad entre sujetos en la perfusión tras el registro de imágenes reales. Las estrategias de apnea y respiración sincronizada combinadas con el algoritmo de detección de movimiento mostraron resultados similares, pero su aplicación práctica requiere colaboración, lo cual puede ser problemático en pacientes con sospecha de CAD. El estudio clínico ASL demostró la capacidad de la técnica ASL para detectar hiperemia debida a la vasodilatación, para identificar diferencias significativas en la reserva de perfusión miocárdica (MPR) entre segmentos normales e hipoperfundidos y correlaciones de MPR entre ASL y first-pass semicuantitativo, lo que en conjunto sugieren el potencial clínico de la técnica ASL para detectar defectos de perfusión sin el uso de un contraste exógeno

Reduction of motion effects in myocardial arterial spin labeling

Purpose To evaluate the accuracy and reproducibility of myocardial blood flow measurements obtained under different breathing strategies and motion correction techniques with arterial spin labeling. Methods A prospective cardiac arterial spin labeling study was performed in 12 volunteers at 3 Tesla. Perfusion images were acquired twice under breath-hold, synchronized-breathing, and free-breathing. Motion detection based on the temporal intensity variation of a myocardial voxel, as well as image registration based on pairwise and groupwise approaches, were applied and evaluated in synthetic and in vivo data. A region of interest was drawn over the mean perfusion-weighted image for quantification. Original breath-hold datasets, analyzed with individual regions of interest for each perfusion-weighted image, were considered as reference values. Results Perfusion measurements in the reference breath-hold datasets were in line with those reported in literature. In original datasets, prior to motion correction, myocardial blood flow quantification was significantly overestimated due to contamination of the myocardial perfusion with the high intensity signal of blood pool. These effects were minimized with motion detection or registration. Synthetic data showed that accuracy of the perfusion measurements was higher with the use of registration, in particular after the pairwise approach, which probed to be more robust to motion. Conclusion Satisfactory results were obtained for the free-breathing strategy after pairwise registration, with higher accuracy and robustness (in synthetic datasets) and higher intrasession reproducibility together with lower myocardial blood flow variability across subjects (in in vivo datasets). Breath-hold and synchronized-breathing after motion correction provided similar results, but these breathing strategies can be difficult to perform by patients

Optimization of pseudo‐continuous arterial spin labeling for renal perfusion imaging

Purpose: To evaluate labeling efficiency of pseudo-continuous arterial spin labeling (PCASL) and to find the gradient parameters that increase PCASL robustness for renal perfusion measurements. Methods: Aortic blood flow was characterized in 3 groups: young healthy volunteers (YHV1), chronic kidney disease (CKD) patients (CKDP), and healthy controls (HCO). PCASL inversion efficiency was evaluated through numeric simulations considering the measured pulsatile flow velocity profiles and off-resonance effects for a wide range of gradient parameters, and the results were assessed in vivo. The most robust PCASL implementation was used to measure renal blood flow (RBF) in CKDP and HCO. Results: Aortic blood velocities reached peak values of 120 cm/s in YHV1, whereas for elderly subjects values were lower by approximately a factor of 2. Simulations and experiments showed that by reducing the gradient average (Gave ) and the selective to average gradient ratio (Gmax /Gave ), labeling efficiency was maximized and PCASL robustness to off-resonance was improved. The study in CKDP and HCO showed significant differences in RBF between groups. Conclusion: An efficient and robust PCASL scheme for renal applications requires a Gmax /Gave ratio of 6-7 and a Gave value that depends on the aortic blood flow velocities (0.5 mT/m being appropriate for CKDP and HCO)

Assessment of splenic switch-off with arterial spin labeling in adenosine perfusion cardiac MRI

Background: In patients with suspected coronary artery disease (CAD), myocardial perfusion is assessed under rest and pharmacological stress to identify ischemia. Splenic switch-off, defined as the stress to rest splenic perfusion attenuation in response to adenosine, has been proposed as an indicator of stress adequacy. Its occurrence has been previously assessed in first-pass perfusion images, but the use of noncontrast techniques would be highly beneficial. Purpose: To explore the ability of pseudo-continuous arterial spin labeling (PCASL) to identify splenic switch-off in patients with suspected CAD. Study type: Prospective. Population: Five healthy volunteers (age 24.8 ± 3.8 years) and 32 patients (age 66.4 ± 8.2 years) with suspected CAD. Field strength/sequence: A 1.5-T/PCASL (spin-echo) and first-pass imaging (gradient-echo). Assessment: In healthy subjects, multi-delay PCASL data (500-2000 msec) were acquired to quantify splenic blood flow (SBF) and determine the adequate postlabeling delay (PLD) for single-delay acquisitions (PLD > arterial transit time). In patients, single-delay PCASL (1200 msec) and first-pass perfusion images were acquired under rest and adenosine conditions. PCASL data were used to compute SBF maps and SBF stress-to-rest ratios. Three observers classified patients into "switch-off" and "failed switch-off" groups by visually comparing rest-stress perfusion data acquired with PCASL and first-pass, independently. First-pass categories were used as reference to evaluate the accuracy of quantitative classification. Statistical tests: Wilcoxon signed-rank, Pearson correlation, kappa, percentage agreement, Generalized Linear Mixed Model, Mann-Whitney, Pearson Chi-squared, receiver operating characteristic, area-under-the-curve (AUC) and confusion matrix. Significance: P value < 0.05. Results: A total of 27 patients (84.4%) experienced splenic switch-off according to first-pass categories. Comparison of PCASL-derived SBF maps during stress and rest allowed assessment of splenic switch-off, reflected in a reduction of SBF values during stress. SBF stress-to-rest ratios showed a 97% accuracy (sensitivity = 80%, specificity = 100%, AUC = 85.2%). Data conclusion: This study could demonstrate the feasibility of PCASL to identify splenic switch-off during adenosine perfusion MRI, both by qualitative and quantitative assessments. Evidence level: 2 TECHNICAL EFFICACY: 2

Multiparametric renal magnetic resonance imaging: A reproducibility study in renal allografts with stable function

Monitoring renal allograft function after transplantation is key for the early detection of allograft impairment, which in turn can contribute to preventing the loss of the allograft. Multiparametric renal MRI (mpMRI) is a promising noninvasive technique to assess and characterize renal physiopathology; however, few studies have employed mpMRI in renal allografts with stable function (maintained function over a long time period). The purposes of the current study were to evaluate the reproducibility of mpMRI in transplant patients and to characterize normal values of the measured parameters, and to estimate the labeling efficiency of Pseudo-Continuous Arterial Spin Labeling (PCASL) in the infrarenal aorta using numerical simulations considering experimental measurements of aortic blood flow profiles. The subjects were 20 transplant patients with stable kidney function, maintained over 1 year. The MRI protocol consisted of PCASL, intravoxel incoherent motion, and T1 inversion recovery. Phase contrast was used to measure aortic blood flow. Renal blood flow (RBF), diffusion coefficient (D), pseudo-diffusion coefficient (D*), flowing fraction ( f ), and T1 maps were calculated and mean values were measured in the cortex and medulla. The labeling efficiency of PCASL was estimated from simulation of Bloch equations. Reproducibility was assessed with the within-subject coefficient of variation, intraclass correlation coefficient, and Bland-Altman analysis. Correlations were evaluated using the Pearson correlation coefficient. The significance level was p less than 0.05. Cortical reproducibility was very good for T1, D, and RBF, moderate for f , and low for D*, while medullary reproducibility was good for T1 and D. Significant correlations in the cortex between RBF and f (r = 0.66), RBF and eGFR (r = 0.64), and D* and eGFR (r = -0.57) were found. Normal values of the measured parameters employing the mpMRI protocol in kidney transplant patients with stable function were characterized and the results showed good reproducibility of the techniques

Clinical utility of intraoperative arterial spin labeling for resection control in brain tumor surgery at 3 T

Resection control in brain tumor surgery can be achieved in real time with intraoperative MRI (iMRI). Arterial spin labeling (ASL), a technique that measures cerebral blood flow (CBF) non-invasively without the use of intravenous contrast agents, can be performed intraoperatively, providing morpho-physiological information. This study aimed to evaluate the feasibility, image quality and potential to depict residual tumor of a pseudo-continuous ASL (PCASL) sequence at 3 T. Seventeen patients with brain tumors, primary (16) or metastatic (1), undergoing resection surgery with iMRI monitoring, were prospectively recruited (nine men, age 56 ± 16.6 years). A PCASL sequence with long labeling duration (3000 ms) and postlabeling delay (2000 ms) was added to the conventional protocol, which consisted of pre- and postcontrast 3D T1 -weighted (T1w) images, optional 3D-FLAIR, and diffusion. Three observers independently assessed the image quality (four-point scale) of PCASL-derived CBF maps. In those with diagnostic quality (Scores 2-4) they evaluated the presence of residual tumor using the conventional sequences first, and the CBF maps afterwards (three-point scale). Inter-observer agreement for image quality and the presence of residual tumor was assessed using Fleiss kappa statistics. The intraoperative CBF ratio of the surgical margins (i.e., perilesional CBF values normalized to contralateral gray matter CBF) was compared with preoperative CBF ratio within the tumor (Wilcoxon's test). Diagnostic ASL image quality was observed in 94.1% of patients (interobserver Fleiss ¿ = 0.76). PCASL showed additional foci suggestive of high-grade residual component in three patients, and a hyperperfused area extending outside the enhancing component in one patient. Interobserver agreement was almost perfect in the evaluation of residual tumor with the conventional sequences (Fleiss ¿ = 0.92) and substantial for PCASL (Fleiss ¿ = 0.80). No significant differences were found between pre and intraoperative CBF ratios (p = 0.578) in patients with residual tumor (n = 7). iMRI-PCASL perfusion is feasible at 3 T and is useful for the intraoperative assessment of residual tumor, providing in some cases additional information to the conventional sequences