Intravoxel incoherent motion (IVIM) imaging is a method the authors developed to visualize microscopic motions of water. In biologic tissues, these motions include molecular diffusion and microcirculation of blood in the capillary network. IVIM images are quantified by an apparent diffusion coefficient (ADC), which integrates the effects of both diffusion and perfusion. The aim of this work was to demonstrate how much perfusion contributes to the ADC and to present a method for obtaining separate images of diffusion and perfusion. Images were obtained at 0.5 T with high-resolution multisection sequences and without the use of contrast material. Results in a phantom made of resin microspheres demonstrated the ability of the method to separately evaluate diffusion and perfusion. The method was then applied in patients with brain and bone tumors and brain ischemia. Clinical results showed significant promise of the method for tissue characterization by perfusion patterns and for functional studies in the evaluation of the microcirculation in physiologic and pathologic conditions, as, for instance, in brain ischemia.

Molecular diffusion and microcirculation in the capillary network result in a distribution of phases in a single voxel in the presence of magnetic field gradients. This distribution produces a spin-echo attenuation. The authors have developed a magnetic resonance (MR) method to image such intravoxel incoherent motions (IVIMs) by using appropriate gradient pulses. Images were generated at 0.5 T in a high-resolution, multisection mode. Diffusion coefficients measured on images of water and acetone phantoms were consistent with published values. Images obtained in the neurologic area from healthy subjects and patients were analyzed in terms of an apparent diffusion coefficient (ADC) incorporating the effect of all IVIMs. Differences were found between various normal and pathologic tissues. The ADC of in vivo water differed from the diffusion coefficient of pure water. Results were assessed in relation to water compartmentation in biologic tissues (restricted diffusion) and tissue perfusion. Nonuniform slow flow of cerebrospinal fluid appeared as a useful feature on IVIM images. Observation of these motions may significantly extend the diagnostic capabilities of MR imaging.

In MRI, the transmit radiofrequency field (B þ 1 ) inhomogeneity can lead to signal intensity variations and quantitative measurement errors. By independently mapping the local B þ 1 variation, the radiofrequency-related signal variations can be corrected for. In this study, we present a new fast B þ 1 mapping method using a slice-selective preconditioning radiofrequency pulse. Immediately after applying a slice-selective preconditioning pulse, a turbo fast low-angle-shot imaging sequence with centric k-space reordering is performed to capture the residual longitudinal magnetization left behind by the slice-selective preconditioning pulse due to B þ 1 variation. Compared to the reference double-angle method, this method is considerably faster. Specifically, the total scan time for the double-angle method is equal to the product of 2 (number of images), the number of phase-encoding lines, and approximately 5T 1 , whereas the slice-selective preconditioning method takes approximately 5T 1 . This method was validated in vitro and in vivo with a 3-T whole-body MRI system. The combined brain and pelvis B þ 1 measurements showed excellent agreement and strong correlation with those by the double-angle method (mean difference 5 0.025; upper and lower 95% limits of agreement were 20.07 and 0.12; R 5 0.93; P < 0.001). This fast B þ 1 mapping method can be used for a variety of applications, including body imaging where fast imaging is desirable. Magn Reson Med 64:439-446,

No abstract

Magnetic resonance elastography (MRE) is an increasingly used method for non-invasive determination of tissue stiffness. MRE has shown its ability to measure in vivo elasticity or viscoelasticity depending on the chosen rheological model. However, few data exist on quantitative comparison of MRE with reference mechanical measurement techniques. MRE has only been validated on soft homogeneous gels under both Hookean elasticity and linear viscoelasticity assumptions, but comparison studies are lacking concerning viscoelastic properties of complex heterogeneous tissues. In this context, the present study aims at comparing an MRE-based method combined with a wave equation inversion algorithm to rotational rheometry. For this purpose, experiments are performed on in vitro porcine brain tissue. The dynamic behavior of shear storage (G') and loss (G ('')) moduli obtained by both rheometry and MRE at different frequency ranges is similar to that of linear viscoelastic properties of brain tissue found in other studies. This continuity between rheometry and MRE results consolidates the quantitative nature of values found by MRE in terms of viscoelastic parameters of soft heterogeneous tissues. Based on these results, the limits of MRE in terms of frequency range are also discussed.

Cirrhosis is an important and growing public health problem, affecting millions of Americans and many more people internationally. A pathological hallmark of the progression to cirrhosis is the development of liver fibrosis, so that monitoring the appearance and progression of liver fibrosis can be used to guide therapy. Here, we report a method to use magnetization-tagged magnetic resonance imaging to measure the cardiac-induced motion and deformation in the liver, as a means for noninvasively assessing liver stiffness, which is related to fibrosis. The initial results show statistically significant differences between healthy and cirrhotic subjects in the direct comparisons of the maximum displacement (mm), and the maximum (P1) and minimum (P2) two-dimensional strains, through the cardiac cycle (3.514 6 0.793, 2.184 6 0.611; 0.116 6 0.043, 0.048 6 0.011; 20.094 6 0.020, 20.041 6 0.015; healthy, cirrhosis, respectively; P < 0.005 for all). There are also significant differences in the displacement-normalized P1 and P2 strains (mm 21 ) (0.030 6 0.008, 0.017 6 0.007; 20.024 6 0.006, 20.013 6 0.004; healthy, cirrhosis, respectively; P < 0.005 for all). Therefore, this noninvasive imaging-based method is a promising means to assess liver stiffness using clinically available imaging tools. Magn Reson Med 65:949-955,

No abstract

Localization of regions with increased uptake of radiotracer in small-animal SPECT is greatly facilitated when using coregistration with anatomic images of the same animal. As MRI has several advantages compared with CT (soft-tissue contrast and lack of ionizing radiation) we developed a SPECT/low-field MRI hybrid device for small-animal imaging. Methods: A small-animal single-pinhole g-camera (pinhole, 1.5 mm in diameter and 12 cm in focal length) adjacent to a dedicated low-field (0.1 T) small MR imager (imaging volume, 10 · 10 · 6 cm 3 ) was used. The animal was placed in a warmed nonmagnetic polymethyl methacrylate imaging cell for MR acquisition, which was followed immediately by SPECT after translation of the imaging cell from one modality to the other. 3-Dimensional T1-weighted sequences were used for MRI. Phantom studies enabled verification of a low attenuation (10%) for 99m Tc and 201 Tl and a very slight increase in Compton scattering due to the radiofrequency coil and polymethyl methacrylate imaging cell. Results: SPECT/ MRI data acquisition and image coregistration of selected examples using different radiotracers for lungs, kidneys, and brain were obtained in 3 nude mice with isotropic spatial resolutions of 0.5 · 0.5 · 0.5 mm 3 for MRI and 1 · 1 · 1 mm 3 for SPECT. The total acquisition time for combined SPECT and MRI lasted 1 h 45 min. Conclusion: A low-magnetic-field strength of 0.1 T is a simple and useful solution for a small-animal dual-imaging device combining pinhole SPECT with the adjacent MR imager. Among small-animal imaging techniques, SPECT provides a unique possibility to follow and measure in vivo, and noninvasively, the biodistribution of a 10 29 molar concentration of a wide range of radiolabeled biomolecules commonly available in nuclear medicine departments (1). However, one essential drawback in SPECT images is the lack of anatomic references related to the tissue uptake of tracer. Therefore, localization of regions with increased uptake of radiotracer is greatly facilitated when using coregistration of SPECT with anatomic images of the same animal, from CT or MRI. Over the last few years, SPECT/ CT dual modality has been widely proposed by manufacturers but MRI presents specific advantages compared with CT, including lack of ionizing radiation, high soft-tissue contrast, and sensitivity to tissue alterations evidenced by specific imaging sequences (2,3). Yet, compared with CT, the use of MRI for coregistration of both functional and structural information has, to our knowledge, essentially served to demonstrate the potential interest of using coregistration of images after data acquisition in separate nuclear and MR rooms. However, as experienced for rat and mice, the strategy of pinhole SPECT followed by MRI in a clinical scanner is a long and complicated task (4-7), requiring a careful transfer of the animal in a specially designed bed equipped with multimodality fiducial markers helping in the coregistration of images (4,5). In addition, separate dual-modality scans (follow...