Undersampled projection reconstruction (PR) is investigated as an alternative method for MRA (MR angiography). In conventional 3D Fourier transform (FT) MRA, resolution in the phase‐encoding direction is proportional to acquisition time. Since the PR resolution in all directions is determined by the readout resolution, independent of the number of projections (Np), high resolution can be generated rapidly. However, artifacts increase for reduced Np. In X‐ray CT, undersampling artifacts from bright objects like bone can dominate other tissue. In MRA, where bright, contrast‐filled vessels dominate, artifacts are often acceptable and the greater resolution per unit time provided by undersampled PR can be realized. The resolution increase is limited by SNR reduction associated with reduced voxel size. The hybrid 3D sequence acquires fractional echo projections in the kx–ky plane and phase encodings in kz. PR resolution and artifact characteristics are demonstrated in a phantom and in contrast‐enhanced volunteer studies. Magn Reson Med 43:91–101, 2000. © 2000 Wiley‐Liss, Inc.

Purpose To determine the precision and accuracy of hepatic fat-fraction measured with a chemical shift-based MRI fat-water separation method, using single-voxel MR spectroscopy (MRS) as a reference standard. Materials and Methods In 42 patients, two repeated measurements were made using a T1-independent, T2∗-corrected chemical shift-based fat-water separation method with multi-peak spectral modeling of fat, and T2-corrected single voxel MR spectroscopy. Precision was assessed through calculation of Bland-Altman plots and concordance correlation intervals. Accuracy was assessed through linear regression between MRI and MRS. Sensitivity and specificity of MRI fat-fractions for diagnosis of steatosis using MRS as a reference standard were also calculated. Results Statistical analysis demonstrated excellent precision of MRI and MRS fat-fractions, indicated by 95% confidence intervals (units of absolute percent) of [−2.66%,2.64%] for single MRI ROI measurements, [−0.81%,0.80%] for averaged MRI ROI, and [−2.70%,2.87%] for single-voxel MRS. Linear regression between MRI and MRS indicated that the MRI method is highly accurate. Sensitivity and specificity for detection of steatosis using averaged MRI ROI were 100% and 94%, respectively. The relationship between hepatic fat-fraction and body mass index was examined. Conclusion Fat-fraction measured with T1-independent T2∗-corrected MRI and multi-peak spectral modeling of fat is a highly precise and accurate method of quantifying hepatic steatosis.

The proton resonance frequency (PRF) shift provides a means of measuring temperature changes during minimally invasive thermotherapy. However, conventional PRF thermometry relies on the subtraction of baseline images, which makes it sensitive to tissue motion and frequency drift during the course of treatment. In this study, a new method is presented that eliminates these problems by estimating the background phase from each acquired image phase. In this referenceless method, a polynomial is fit to the background phase outside the heated region in a weighted least-squares fit. Extrapolation of the polynomial to the heated region serves as the background phase estimate, which is then subtracted from the actual phase. Minimally invasive thermal therapy is a promising treatment for a variety of cancers. It is desirable to monitor temperature during such a procedure by magnetic resonance proton resonance frequency (PRF) shift thermometry (1,2) because it provides quantitative temperature information in near real time. This method uses changes in the phase of gradient-recalled echo (GRE) images to estimate the relative temperature change ⌬T, as given bywhere ␣ ϭ -0.01 ppm/°C is the PRF change coefficient for aqueous tissue, ␥ is the gyromagnetic ratio, B 0 is the main magnetic field, TE is the echo time, and baseline is the initial phase before heating. In conventional PRF shift thermometry, phase images acquired prior to heating (i.e., baseline or reference images) are subtracted from phase images acquired during heating. However, when tissue motion is present, images acquired during heating are not registered to the baseline images, and the background magnetic field changes nonuniformly, resulting in erroneous baseline phase elimination and inaccurate temperature measurements. Many of the target areas for thermotherapy are in the abdomen, where motion is ubiquitous. For example, motion of the liver has an average amplitude of 1.3 cm during normal breathing (3). Because thermal therapy treatments require several minutes to perform, they cannot be performed in a single breath-hold. Furthermore, it is difficult to use multiple breath-holds, because reproducible breathholding is hard to achieve. Even without respiratory motion, displacement between images can occur. Thermal coagulation leads to structural changes and deformation of the tissue (4,5), which can even be observed ex vivo without any other contribution to motion. This heating-induced tissue motion is frequently not a simple displacement, and, unlike respiration, it cannot be corrected by a reregistration scheme. Swelling caused by the formation of edema can also contribute to tissue displacement (6), as can changes in bowel-filling and the state of muscles. For example, we have measured shifts in the canine prostate of Ͼ5 mm over the course of a 48-min ablation experiment.In recent studies, investigators have used conventional respiratory gating in animals under general anesthesia and mechanical respiration (7,8) to overcome the problem of motion in thermal...

A technique is presented for the acquisition of temperature maps in the presence of variable respiratory motion using the proton resonance frequency (PRF) shift. The technique uses respiratory triggering, diaphragm position determination with a navigator echo, and the collection of multiple baseline images to generate temperature maps. Laser ablations were performed in an ex vivo liver phantom undergoing variable simulated respiratory motion and in vivo in four porcine livers, demonstrating a reduction of artifacts in the computed temperature maps compared with conventional single baseline techniques, both uncorrected and corrected for motion. Interstitial thermal therapy techniques can be effective methods for treatment of malignancies. Techniques for monitoring therapy under MR guidance include diffusionweighted MR imaging, measurement of temperature-dependent T 1 changes, and use of the temperature-dependent proton resonance frequency (PRF) shift. Of these, the PRF method is desirable in temperature monitoring for its ability to quantify temperature changes (1). Unfortunately, using the PRF method in the liver or other organs that experience respiratory motion is challenging, since the phase of a pretreatment baseline image is normally subtracted from that of the images acquired during heating. Motion between the acquisition of these images produces artifacts in the temperature maps. Because treatment times for interstitial therapies are several minutes long and reproducible breathholding by patients is often unreliable and difficult, imaging must occur during free-breathing if general anesthesia is not used. Artifacts resulting from motion can be reduced with rapid imaging and triggering, but the baseline and treatment images must still be registered for proper subtraction. Another challenge for MR thermometry is that the background magnetic field is altered nonuniformly during breathing, with spatially nonlinear frequency shifts. In addition, respiratory motion can be variable between respiratory cycles during free-breathing.Recent studies involving thermal therapies in the liver have used several approaches for monitoring tissue damage during heating in regions that undergo respiratory motion. Vogl and colleagues (2,3) performed a significant number of ablations with internally cooled laser applicators under MR guidance with T 1 -weighted images. Other investigators have used T 1 -based temperature mapping strategies (4 -6), and while qualitatively depicting the heated tissue, this technique can be quantitatively inaccurate due to effects caused by tissue changes during coagulation (7,8). Ablations in the kidney have also been performed with T 1 -based thermal mapping (9) and T 2 -weighted imaging (10).Previously, investigators have combined motion detection with navigator echoes to obtain PRF temperature maps in phantoms for both brief displacements of ex vivo tissue (11) and simulated, computer-controlled nonvariable motion in phantoms (12,13). Others have described methods for PRF temperature mapping of...

R*2 mapping has important applications in MRI, including functional imaging, tracking of super-paramagnetic particles, and measurement of tissue iron levels. However, R*2 measurements can be confounded by several effects, particularly the presence of fat and macroscopic B0 field variations. Fat introduces additional modulations in the signal. Macroscopic field variations introduce additional dephasing that results in accelerated signal decay. These effects produce systematic errors in the resulting R*2 maps and make the estimated R*2 values dependent on the acquisition parameters. In this study, we develop a complex-reconstruction, confounder-corrected R*2 mapping technique, which addresses the presence of fat and macroscopic field variations for both 2D and 3D acquisitions. This technique extends previous chemical shift-encoded methods for R*2, fat and water mapping by measuring and correcting for the effect of macroscopic field variations in the acquired signal. The proposed method is tested on several 2D and 3D phantom and in vivo liver, cardiac, and brain datasets.