|New Contrasts: CEST, et al.|
|Imaging Acute Ischemic Tissue Acidosis Using a
Relaxation-Compensated Multi-Slice Amide Proton Transfer (APT) MRI
Phillip Zhe Sun1, Yoshihiro Murata2, Jie Lu2, 3, Xiao Ying Wang2, Eng H. Lo2, Alma Gregory Sorensen1
1A. A. Martinos Center for Biomedical Imaging, Charlestown, Massachusetts, USA; 2Massachusetts General Hospital, Charlestown, Massachusetts, USA; 3XuanWu Hospital, Beijing, People's Republic of China
Amide proton transfer (APT) imaging is a variant of chemical exchange saturation transfer (CEST) imaging that has been shown capable of detecting acute ischemic tissue acidosis, and may serve as a surrogate metabolic imaging marker complementary to perfusion and diffusion MRI. Given that pathologies such as stroke and cancer are spatially heterogeneous, APT imaging with sufficient volume coverage is necessary in order to fully evaluate the diagnostic power. Using a 2-pool exchange model, we showed that the CEST contrast decrease after RF irradiation is governed by its intrinsic relaxation constant of bulk water. Here, we propose a fast volumetric APT imaging approach that acquires multi-slice CEST images immediately after a single long CW RF irradiation, and the relaxation-induced loss of APT contrast is compensated during post-processing. The proposed technique is verified by numerical simulation and validated with a tissue-like dual pH phantom. When translated to image acute animal stroke, the proposed technique detected heterogeneous distribution of PWI, pH and DWI lesions, permitting future study to fully elucidate the diagnostic value of pH-weighted APT MRI.
Simultaneous 19F and 1H-CEST Technique for Improved
Accuracy and Efficiency in Quantitative CEST Measurements
Jochen Keupp1, Shelton D. Caruthers2, 3, Dirk Burdinski4, Sander Langereis4, Jeroen A. Pikkemaat4, Rolf Lamerichs4, Holger Gruell4, Samuel A. Wickline2, Gergory M. Lanza2, Patrick M. Winter2
1Philips Research Europe, Hamburg, Germany; 2Washington University, St. Louis, Missouri, USA; 3Philips Medical Systems, Andover, Massachusetts, USA; 4Philips Research Europe, Eindhoven, Netherlands
Responsive chemical-shift saturation transfer (CEST) agents have been developed for mapping of diagnostic parameters, like local pH for oncology applications. Because the 1H-CEST signal strength depends as well on the physiological parameters as on agent concentration, a common challenge is the determination of the local concentration for calibration. If a CEST agent is provided, which is additionally labeled by a fluorine marker, the calibration can be performed via 19F-MRI. In this work, a simultaneous turbo-spin-echo imaging sequence is investigated, which allows to acquire 1H-CEST and 19F images at the same time, offering improved time efficiency and precision of the calibration.
|Correction for Artifacts Induced by B0
and B1 Field Inhomogeneities in Chemical Exchange Saturation Transfer (CEST)
Phillip Zhe Sun1, Christian T. Farrar1, Gregory Sorensen1
1A. A. Martinos Center for Biomedical Imaging, Charlestown, Massachusetts, USA
Chemical exchange saturation transfer (CEST) imaging provides an indirect detection mechanism that allows quantification of certain labile groups unobservable using conventional MRI. However, CEST contrast is often only a few percent, and therefore, it is important to optimize experimental conditions for reliable and quantitative CEST imaging. In particular, CEST imaging is sensitive to B0 and B1 field, while on the other hand; field inhomogeneities persist despite recent advances in magnet technologies, especially for in vivo imaging at high fields. Consequently, correction algorithms that can compensate for field inhomogeneity-induced measurement errors in CEST imaging might be very useful. In this study, the dependence of CEST contrast on field distribution was solved and a correction algorithm was developed to compensate for field inhomogeneity-induced CEST imaging artifacts. In addition, the proposed algorithm was verified with both numerical simulation and experimental measurements, and showed nearly complete correction of CEST imaging measurement errors caused by moderate field inhomogeneity.
Magnetic Resonance Contrast Based on Relaxation Along
a Fictitious Field (RAFF)
Timo Liimatainen1, Michael Garwood1, Dennis J. Sorce1, Shalom Michaeli1
1University of Minnesota, Minneapolis, Minnesota, USA
A new contrast based on the frequency swept pulses with sine and cosine amplitude and frequency modulations was developed, referred to as relaxation along a fictitious field (RAFF). Increase of the relaxation rate with increased viscosity was smaller with RAFF than with adiabatic T1ρ and T2ρ. In a fast exchanging system, RAFF relaxation rate was found to be in the same range as adiabatic T2ρ. Relaxation in the dipolar and exchanging systems suggests that the RAFF method appears to slow down dipolar mechanisms as compared to T1ρ and T2ρ, exhibits sensitivity to exchange processes, and produces artifact free relaxation maps.
Activated MR Contrast Agent by a Dual Contrast
Technique and Their Application
Yoshinori Kato1, Arvind P. Pathak1, Dmitri Artemov1
1Johns Hopkins University School of Medicine, Baltimore, USA
We developed the activated MR contrast agent using the dual contrast technique. The concept behind this strategy is that strong negative signal enhancement due to the T2/T2* effects of iron oxides dominates the positive T1 contrast generated by a Gd-based contrast agent when these agents are in close proximity, such as within an intact nanocarrier encapsulating GdDTPA/SPIO, and positive T1 contrast becomes evident upon release of Gd-based contrast agent from the carrier once the distance between Gd-based contrast agents and SPIO molecules is beyond the T2/T2* enhancement range. We corroborated the feasibility of this technique in vitro and in vivo.
Calculation of Susceptibility Maps from Phase Image
Andreas Schäfer1, Samuel Wharton1, Richard Bowtell1
1University of Nottingham, Nottingham, UK
The phase of gradient echo images carries useful anatomical information resulting from field perturbations due to the variation of susceptibility across tissues. Generating quantitative information from phase images is not straightforward, because of the non-local relationship between field perturbations and susceptibility. Here we show how this problem may be overcome via the calculation of a 3D map of the magnetic susceptibility from phase data. The iterative approach which is proposed takes account of the sphere of Lorentz and external field sources. Its operation is demonstrated via successful application to simulated brain images and phase maps measured from a structured phantom.
High Resolution Human Brain
Susceptibility Maps Calculated from 7 Tesla MRI Phase Data
Karin Shmueli1, Peter van Gelderen1, Tie-Qiang Li1, Jeff H. Duyn1
1National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, USA
The phase in susceptibility-weighted MRI shows excellent contrast but varies with tissue orientation and is affected beyond areas of altered susceptibility. Magnetic susceptibility is an intrinsic tissue property, closely reflecting tissue composition. Recently-proposed Fourier Transform methods for calculating susceptibilities from phase data were applied to high-resolution 7 Tesla phase images of the human brain (ex- and in-vivo). Some cortical layers were most conspicuous in the susceptibility maps, having consistent contrast independent of their orientation relative to B0, unlike in the phase images where contrast varied with orientation. 3D rather than 2D susceptibility calculations showed most promise for revealing fine-scale brain structure.
Multiple Orientation Acquisition to Invert Dipole
Field for Quantitative Susceptibility Mapping
Tian Liu1, 2, Pascal Spincemaille2, Ludovic de Rochefort2, Bryan Michael Kressler1, 2, Yi Wang1, 2
1Cornell University, Ithaca, New York, USA; 2Weill-Cornell Medical College, New York, New York, USA
Quantification of susceptibility is practically challenging because the model of this linear system is ill-conditioned. A multiple-orientation measurement is implemented to stabilize this problem. Two reconstruction techniques based on multiple orientation measurement are presented. Both methods provide linearity. Streaking artifacts are eliminated in the reconstruction.
Multi-Field Behavior of Relaxivity in an Iron-Rich
Nilesh R. Ghugre1, 2, Pippa Storey, Cynthia K. Rigsby2, 3, Alexis A. Thompson2, 3, Christine L. Carqueville3, Thomas D. Coates1, John C. Wood1
1Childrens Hospital Los Angeles, Los Angeles, California , USA; 2Northwestern University, Chicago, Illinois, USA; 3Childrens Memorial Hospital, Chicago, Illinois, USA
At 1.5T, relaxivities R2 and R2* can accurately predict hepatic iron concentration in iron overload syndromes. With increasing popularity of 3T scanners, there is need to translate relaxivity-iron relationships to higher field strengths. In this regard, we followed a computational approach by generating realistic liver anatomies and simulating R2 and R2* imaging experiments. We also performed R2 and R2* imaging in the livers of 16 patients at 1.5T and 3T to validate the model. Results demonstrated that R2* scaled linearly with field strength while R2 had a curvilinear relationship. A model-based approach will eliminate the need to recalibrate in patients for changes in sequence type, sequence parameters and imaging conditions.
Role of Anatomic Liver Compartments in Relaxivity-Iron
Nilesh R. Ghugre1, 2, Ignacio Gonzalez1, Ellen Butensky2, Roland Fischer2, 3, Roger Williams2, Paul Harmatz2, Thomas D. Coates1, John C. Wood1
1Childrens Hospital Los Angeles, Los Angeles, California , USA; 2Childrens Hospital and Research Center at Oakland, Oakland, California , USA; 3University Clinic Hamburg-Eppendorf, Hamburg, Germany
R2(1/T2) calibration curve has been established for predicting hepatic iron concentration(HIC) with clinical accuracy. However, there is indication that this relationship is affected by nature of iron overload(transfusion-dependent, hereditary) as well as by type of chelation therapy(subcutaneous, oral). Differences can be attributed to altered iron distribution within two liver compartments viz. hepatocyte and sinusoid. We characterized the relative iron loading in the two compartments(43 patients) and employed a computational approach to systematically evaluate their relative effects on R2. For low HIC, R2 dropped by 15% due to static refocusing while at higher HIC, effects were less apparent. A model-based approach will eliminate the need to recalibrate in patients for changes in therapy and syndrome-type.