Saturation Transfer: CEST, APT & MT
Click on to view the abstract pdf. Click on to view the recorded presentation.
Friday 11 May 2012
Room 210-211  10:30 - 12:30 Moderators: Nikola A. Stikov, Elena Vinogradov

10:30 0736.   Introduction
Elena Vinogradov
10:42 0737.   
Imaging of Glutamate in the Spinal Cord using Chemical Exchange Saturation Transfer (CEST) at 7T
Feliks Kogan1,2, Anup Singh1, Mohammad Haris1, Kejia Cai1, Hari Hariharan1, and Ravinder Reddy1
1Center for Magnetic Resonance and Optical Imaging, University of Pennsylvania, Philadelphia, PA, United States, 2Bioengineering Graduate Group, University of Pennsylvania, Philadelphia, PA, United States

Glutamate (Glu) is the primary neurotransmitter that is responsible for excitatory synaptic transmission in the brain stem and spinal cord and plays a wide role in neuropathology. Glu exhibits a CEST effect which is linearly proportional to the Glu concentration. In this work, we demonstrated that it is feasible to detect the CEST effect from glutamate in the cervical spinal cord at 7T with high spatial resolution. We showed that the spinal cord Glu CEST map demonstrates a distinct gray and white matter distribution pattern. Finally, we demonstrated from phantom data that the majority of the CEST contrast in the spinal cord is due to glutamate.

10:54 0738.   Imaging acute ischemic tissue acidosis with quantitative in vivo amide proton transfer (APT) MRI
Phillip Zhe Sun1, Enfeng Wang1, and Jerry S Cheung1
1Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States

Amide proton transfer (APT) MRI is capable of imaging tissue acidosis during acute stroke. However, magnetization transfer asymmetry (MTRasym) is often calculated for pH-weighted APT contrast, which is subject to a baseline shift (ÄMTR’asym) attributable to the slightly asymmetric magnetization transfer (MT) effect. In this study, we modeled MTRasym as a superposition of pH-dependent APT contrast and a baseline shift ÄMTR’asym (i.e., MTRasym=APTR(pH) + ÄMTR’asym). We found schemic lesion pH was 6.44 ± 0.24, significantly reduced from that of the normal tissue (7.03 ± 0.05), which correlated with tissue perfusion and diffusion rates.

11:06 0739.   Quantitative MR imaging of the Amide-proton transfer, the Nuclear Overhauser effect, and MT asymmetry: a 9.4 T study
Tao Jin1, Xiaopeng Zong1, Ping Wang1, and Seong-Gi Kim1
1Department of Radiology, University of Pittsburgh, Pittsburgh, Pennsylvania, United States

The amide proton transfer (APT) effect has shown great potential in stroke and cancer studies. However, quantitative imaging of the APT effect is still challenging. The magnitude of APT is typically assessed from an MTR asymmetry image, which may have contamination from the conventional MT asymmetry and the Nuclear Overhauser effect (NOE). In this report, the wide spectral separation from the high field of 9.4 T is utilized to obtain quantitative APT and NOE images without using the asymmetry analysis. We found that pure APT contrast is highly sensitive to pH, while the NOE map has little tissue and pH contrast.

11:18 0740.   
Separating CEST from MT asymmetry by simultaneous two-frequency irradiation
Jae-Seung Lee1,2, Ravinder R Regatte1, and Alexej Jerschow2
1Department of Radiology, NYU Langone Medical Center, New York, NY, United States, 2Department of Chemistry, New York University, New York, NY, United States

CEST and MT contrast have enjoyed wide popularity recently in MRI applications. It is often difficult to separate genuine CEST signatures from MT effects, which are asymmetric with respect to the water resonance. We recently developed a method that utilizes simultaneous two-frequency rf irradiation, which can make MT effects independent of irradiation frequencies over a wide range, and thus can suppress MT asymmetry. Based on the results from the simulations as well as experiments, we propose a new strategy to isolate CEST contrast from MT asymmetry contrast by using the two-frequency rf irradiation technique.

11:30 0741.   
Improved Accuracy of Cross-Relaxation Imaging Using On-Resonance MT Effect Correction
Pouria Mossahebi1, Vasily L. Yarnykh2, and Alexey A. Samsonov3
1Biomedical Engineering, University of Wisconsin, Madison, WI, United States, 2Radiology, University of Washington, Seattle, WA, United States, 3Radiology, University of Wisconsin, Madison, WI, United States

This study demonstrates that separate treatment of VFA and MT data in the CRI method causes non-negligible systematic errors in both R1 and cross-relaxation parameters. Our modified CRI data processing approach effectively corrects these errors and does not require any additional measurements, thus maintaining time-efficiency of the original CRI technique.

11:42 0742.   In vivo human kidney pH mapping at 3T using time-interleaved parallel RF transmission CEST
Ivan E Dimitrov1,2, Masaya Takahashi2, Koji Sagiyama2, A. Dean Sherry2,3, and Jochen Keupp4
1Philips Medical Systems, Cleveland, OH, United States, 2Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, United States, 3Chemistry, University of Texas Dallas, Richardson, TX, United States, 4Philips Research Europe, Hamburg, Germany

A parallel transmission CEST method is presented for ratiometric, concentration-independent pH mapping of human kidneys at 3T using clinically approved contrast agent, Iopamidol. The method combines respiratory triggered single-shot turbo-spin echo that runs in time-interleaved mode of an MR scanner with parallel RF transmission system. Triggering was made such that to lower the acquisition duty cycle and SAR by skipping pre-defined number of respiratory cycles to enable longer and more powerful RF saturation. CEST effects of up to 20% were recorded in the pelvic and medullar regions of the kidney, allowing for generating pixel-wise pH maps.

11:54 0743.   Interleaved Parallel Transmission Saturation Scheme for 3D Amide Proton Transfer Imaging of Brain Tumors at 3 Tesla
He Zhu1,2, Jochen Keupp3, Jaishri Blakeley4, Lindsay Blair4, Michael Schar1,5, Peter B. Barker1,2, Peter C.M. van Zijl1,2, and Jinyuan Zhou1,2
1Department of Radiology, Johns Hopkins University, Baltimore, Maryland, United States, 2F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, Maryland, United States, 3Philips Research, Hamburg, Germany, 4Department of Neurology, Johns Hopkins University, Baltimore, Maryland, United States, 5Philips Healthcare, Cleveland, Ohio, United States

Amide proton transfer (APT) image contrast is generated by selective RF labeling of amide protons of cytosolic proteins and peptides in tissue, followed by chemical exchange of this label to water protons. Currently, clinical APT imaging protocols are typically limited by scanner hardware constraints, particularly with respect to the RF amplifier duty cycle. In this study, time-interleaved parallel RF transmission (pTX) was used for 3D APT imaging. The preliminary results show that the use of the pTX-APT approach can maximize APT-MRI effects on clinical scanners and that high-quality 3D APT imaging of human brain tumors can be acquired within a clinically feasible time.

12:06 0744.   
Effect of Saturation Pulse Length on Parallel Transmission Based Amide Proton Transfer (APT) Imaging of Different Brain Tumor Types
Osamu Togao1, Takashi Yoshiura1, Jochen Keupp2, Akio Hiwatashi1, Koji Yamashita1, Kazufumi Kikuchi1, Yuriko Suzuki3, Koji Sagiyama4, Masaya Takahashi4, and Hiroshi Honda1
1Clinical Radiology, Graduate School of Medical Science, Kyushu University, Fukuoka, Fukuoka, Japan, 2Philips Research, Hamburg, Germany, 3Philips Electronics, Japan,4Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, Texas, United States

Amide proton transfer (APT) imaging employs the exchange between protons of free tissue water and the amide groups (-NH) of endogenous mobile proteins and peptides, imaged by a saturation transfer technique. In this imaging technique, the length of RF saturation (Tsat) is an important parameter for sensitivity. A technique based on parallel RF transmission was demonstrated, which allows arbitrarily long RF pulses (~5s) via amplifier alternation in clinical scanners. We evaluated the Tsat dependence of the APT contrast in human brain tumors and to demonstrate the efficacy of long Tsat achieved by the parallel RF transmission based technique.

12:18 0745.   
RF Power Dependence of Human Brain CEST, NOE and Metabolite MT Effects at 7T
Dapeng Liu1, Jinyuan Zhou2,3, Rong Xue1, Zhentao Zuo1, Jing An4, and Danny JJ Wang5
1State Key Lab. of Brain and Cognitive Science, Beijing MRI Center for Brain Research, Institute of Biophysics, Chinese Academy of Sciences, Beijing, China, 2FM Kirby Center, Kennedy Krieger Institute, Baltimore, MD, United States, 3Department of Radiology and Radiological Sciences, Johns Hopkins Medical Institutes, Baltimore, MD, United States, 4Siemens Shenzhen Magnetic Resonance Ltd, Shenzhen, China, 5Neurology, UCLA, Los Angeles, CA, United States

In this work we investigated the RF power dependence of the magnetization transfer (MT) asymmetry effects at 7T. The results suggest that at low B1, the nuclear Overhauser enhancement (NOE) and metabolite MT effects are the dominant source of MT asymmetry while at higher B1, the amide proton transfer (APT) effect becomes stronger and finally reverses the asymmetry plot. Our results suggest that MT asymmetry at 7T with low B1 may provide an approach for in vivo imaging of brain lipid (e.g., myelin).