Michael S. Hansen

Equilibrium contrast cardiovascular magnetic resonance for the measurement of diffuse myocardial fibrosis: preliminary validation in humans.

Background— Diffuse myocardial fibrosis is a final end point in most cardiac diseases. It is missed by the cardiovascular magnetic resonance (CMR) late gadolinium enhancement technique. Currently, quantifying diffuse myocardial fibrosis requires invasive biopsy, with inherent risk and sampling error. We have developed a robust and noninvasive technique, equilibrium contrast CMR (EQ–CMR) to quantify diffuse fibrosis and have validated it against the current gold standard of surgical myocardial biopsy. Methods and Results— The 3 principles of EQ–CMR are a bolus of extracellular gadolinium contrast followed by continuous infusion to achieve equilibrium; a blood sample to measure blood volume of distribution (1−hematocrit); and CMR to measure pre- and postequilibrium T1 (with heart rate correction). The myocardial volume of distribution is calculated, reflecting diffuse myocardial fibrosis. Clinical validation occurred in patients undergoing aortic valve replacement for aortic stenosis or myectomy in hypertrophic cardiomyopathy (n=18 and n=8, respectively). Surgical biopsies were analyzed for picrosirius red fibrosis quantification on histology. The mean histological fibrosis was 20.5±11% in aortic stenosis and 17.1±7.4% in hypertrophic cardiomyopathy. EQ–CMR correlated strongly with biopsy histological fibrosis: aortic stenosis, r2=0.86, Kendall Tau coefficient (T)=0.71, P<0.001; hypertrophic cardiomyopathy, r2=0.62, T=0.52, P=0.08; combined r2=0.80, T=0.67, P<0.001. Conclusions— We have developed and validated a new technique, EQ–CMR, to measure diffuse myocardial fibrosis as an add-on to a standard CMR scan, which allows for the noninvasive quantification of the diffuse fibrosis burden in myocardial diseases.

T1-mapping in the heart: accuracy and precision

The longitudinal relaxation time constant (T1) of the myocardium is altered in various disease states due to increased water content or other changes to the local molecular environment. Changes in both native T1 and T1 following administration of gadolinium (Gd) based contrast agents are considered important biomarkers and multiple methods have been suggested for quantifying myocardial T1 in vivo. Characterization of the native T1 of myocardial tissue may be used to detect and assess various cardiomyopathies while measurement of T1 with extracellular Gd based contrast agents provides additional information about the extracellular volume (ECV) fraction. The latter is particularly valuable for more diffuse diseases that are more challenging to detect using conventional late gadolinium enhancement (LGE). Both T1 and ECV measures have been shown to have important prognostic significance.T1-mapping has the potential to detect and quantify diffuse fibrosis at an early stage provided that the measurements have adequate reproducibility. Inversion recovery methods such as MOLLI have excellent precision and are highly reproducible when using tightly controlled protocols. The MOLLI method is widely available and is relatively mature. The accuracy of inversion recovery techniques is affected significantly by magnetization transfer (MT). Despite this, the estimate of apparent T1 using inversion recovery is a sensitive measure, which has been demonstrated to be a useful tool in characterizing tissue and discriminating disease. Saturation recovery methods have the potential to provide a more accurate measurement of T1 that is less sensitive to MT as well as other factors. Saturation recovery techniques are, however, noisier and somewhat more artifact prone and have not demonstrated the same level of reproducibility at this point in time.This review article focuses on the technical aspects of key T1-mapping methods and imaging protocols and describes their limitations including the factors that influence their accuracy, precision, and reproducibility.

Accelerating cine phase-contrast flow measurements using k-t BLAST and k-t SENSE

Conventional phase‐contrast velocity mapping in the ascending aorta was combined with k‐t BLAST and k‐t SENSE. Up to 5.3‐fold net acceleration was achieved, enabling single breath‐hold acquisitions. A standard phase‐contrast (PC) sequence with interleaved acquisition of the velocity‐encoded segments was modified to collect data in 2 stages, a high‐resolution undersampled and a low‐resolution fully sampled training stage. In addition, a modification of the k‐t reconstruction strategy was tested. This strategy, denoted as “plug‐in,” incorporates data acquired in the training stage into the final reconstruction for improved data consistency, similar to conventional keyhole. “k‐t SENSE plug‐in” was found to provide best image quality and most accurate flow quantification. For this strategy, at least 10 training profiles are required to yield accurate stroke volumes (relative deviation <5%) and good image quality. In vivo 2D cine velocity mapping was performed in 6 healthy volunteers with 30–32 cardiac phases (spatial resolution 1.3 × 1.3 × 8–10 mm3, temporal resolution of 18–38 ms), yielding relative stroke volumes of 106 ± 18% (mean ± 2*SD) and 112 ± 15% for 3.8× and 5.3× net accelerations, respectively. In summary, k‐t BLAST and k‐t SENSE are promising approaches that permit significant scan‐time reduction in PC velocity mapping, thus making high‐resolution breath‐held flow quantification possible. Magn Reson Med, 2005.

Whole-heart cine MRI using real-time respiratory self-gating

Two‐dimensional (2D) breath‐hold cine MRI is used to assess cardiac anatomy and function. However, this technique requires cooperation from the patient, and in some cases the scan planning is complicated. Isotropic nonangulated three‐dimensional (3D) cardiac MR can overcome some of these problems because it requires minimal planning and can be reformatted in any plane. However, current methods, even those that use undersampling techniques, involve breath‐holding for periods that are too long for many patients. Free‐breathing respiratory gating sequences represent a possible solution for realizing 3D cine imaging. A real‐time respiratory self‐gating technique for whole‐heart cine MRI is presented. The technique enables assessment of cardiac anatomy and function with minimum planning or patient cooperation. Nonangulated isotropic 3D data were acquired from five healthy volunteers and then reformatted into 2D clinical views. The respiratory self‐gating technique is shown to improve image quality in free‐breathing scanning. In addition, ventricular volumetric data obtained using the 3D approach were comparable to those acquired with the conventional multislice 2D approach. Magn Reson Med 57:606–613, 2007.

Gadgetron: An open source framework for medical image reconstruction†

This work presents a new open source framework for medical image reconstruction called the “Gadgetron.” The framework implements a flexible system for creating streaming data processing pipelines where data pass through a series of modules or “Gadgets” from raw data to reconstructed images. The data processing pipeline is configured dynamically at run‐time based on an extensible markup language configuration description. The framework promotes reuse and sharing of reconstruction modules and new Gadgets can be added to the Gadgetron framework through a plugin‐like architecture without recompiling the basic framework infrastructure. Gadgets are typically implemented in C/C++, but the framework includes wrapper Gadgets that allow the user to implement new modules in the Python scripting language for rapid prototyping. In addition to the streaming framework infrastructure, the Gadgetron comes with a set of dedicated toolboxes in shared libraries for medical image reconstruction. This includes generic toolboxes for data‐parallel (e.g., GPU‐based) execution of compute‐intensive components. The basic framework architecture is independent of medical imaging modality, but this article focuses on its application to Cartesian and non‐Cartesian parallel magnetic resonance imaging. Magn Reson Med, 2013.

Wall shear rates differ between the normal carotid, femoral, and brachial arteries: An in vivo MRI study

To investigate wall shear rates in vivo in the common carotid, brachial, and superficial femoral arteries using very high resolution magnetic resonance imaging (MRI) phase contrast measurements.

Accelerating the Nonequispaced Fast Fourier Transform on Commodity Graphics Hardware

We present a fast parallel algorithm to compute the nonequispaced fast Fourier transform on commodity graphics hardware (the GPU). We focus particularly on a novel implementation of the convolution step in the transform as it was previously its most time consuming part. We describe the performance for two common sample distributions in medical imaging (radial and spiral trajectories), and for different convolution kernels as these parameters all influence the speed of the algorithm. The GPU-accelerated convolution is up to 85 times faster as our reference, the open source NFFT library on a state-of-the-art 64 bit CPU. The accuracy of the proposed GPU implementation was quantitatively evaluated at the various settings. To illustrate the applicability of the transform in medical imaging, in which it is also known as gridding, we look specifically at non-Cartesian magnetic resonance imaging and reconstruct both a numerical phantom and an in vivo cardiac image.

Improvement in left ventricular filling properties after relief of right ventricle to pulmonary artery conduit obstruction: contribution of septal motion and interventricular mechanical delay.

AIMS To investigate the impact of relief of right ventricle (RV) to pulmonary artery (PA) conduit obstruction on septal motion and ventricular interaction and its functional implications for left ventricular (LV) filling properties. METHODS AND RESULTS In 20 consecutive patients with congenital heart disease and RV to PA conduit obstruction, the following were prospectively assessed before and after percutaneous pulmonary valve implantation (PPVI): the septal curvature and LV volumes throughout the cardiac cycle by magnetic resonance imaging; RV to LV mechanical delay by 2D-echocardiographic strain imaging; and objective exercise capacity. Percutaneous pulmonary valve implantation led to a reduction in RV to LV mechanical delay (127.9 +/- 50.9 vs. 37.7 +/- 35.6 ms; P < 0.001) and less LV septal bowing in early LV diastole (septal curvature: -0.11 +/- 0.11 vs. 0.07 +/- 0.13 cm(-1); P < 0.001). Early LV diastolic filling (first one-third of diastole) increased significantly (17.5 +/- 9.4 to 30.4 +/- 9.4 mL/m(2); P < 0.001). The increase in early LV diastolic filling correlated with the reduction in RV to LV mechanical delay (r = -0.68; P = 0.001) and change in septal curvature (r = 0.71; P < 0.001). In addition, the improvement in peak oxygen uptake (56.0 +/- 16.0 vs. 64.1 +/- 13.7% of predicted; P < 0.001) was associated with the increase in early LV diastolic filling (r = 0.69; P = 0.001). CONCLUSION Relief of RV to PA conduit obstruction significantly improves early LV filling properties. This is attributed to more favourable septal motion and reduction in interventricular mechanical delay.

Cartesian SENSE and k-t SENSE reconstruction using commodity graphics hardware.

This study demonstrates that modern commodity graphics cards (GPUs) can be used to perform fast Cartesian SENSE and k‐t SENSE reconstruction. Specifically, the SENSE inversion is accelerated by up to two orders of magnitude and is no longer the time‐limiting step. The achieved reconstruction times are now well below the acquisition times, thus enabling real‐time, interactive SENSE imaging, even with a large number of receive coils. The fast GPU reconstruction is also beneficial for datasets that are not acquired in real time. We demonstrate that it can be used for interactive adjustment of regularization parameters for k‐t SENSE in the same way that one would adjust window and level settings. This enables a new way of performing imaging reconstruction, where the user chooses the setting of tunable reconstruction parameters, in real time, depending on the context in which the images are interpreted. Magn Reson Med 59:463–468, 2008.

Adiabatic inversion pulses for myocardial T1-mapping

To evaluate the error in T1 estimates using inversion‐recovery‐based T1 mapping due to imperfect inversion and to perform a systematic study of adiabatic inversion pulse designs in order to maximize inversion efficiency for values of transverse relaxation (T2) in the myocardium subject to a peak power constraint.