Fast multislice pH‐weighted chemical exchange saturation transfer (CEST) MRI with Unevenly segmented RF irradiation

Chemical exchange saturation transfer (CEST) MRI is a versatile imaging technique for measuring microenvironment properties via dilute CEST labile groups. Conventionally, CEST MRI is implemented with a long radiofrequency irradiation module, followed by fast image acquisition to obtain the steady state CEST contrast. Nevertheless, the sensitivity, scan time, and spatial coverage of the conventional CEST MRI method may not be optimal. Our study proposed a segmented radiofrequency labeling scheme that includes a long primary radiofrequency irradiation module to generate the steady state CEST contrast and repetitive short secondary radiofrequency irradiation module immediately after the image acquisition so as to maintain the steady state CEST contrast for multislice acquisition and signal averaging. The proposed CEST MRI method was validated experimentally with a tissue‐like pH phantom and optimized for the maximal contrast‐to‐noise ratio. In addition, the proposed sequence was evaluated for imaging ischemic acidosis via pH‐weighted endogenous amide proton transfer MRI, which showed similar contrast as conventional amide proton transfer MRI. In sum, a fast multislice relaxation self‐compensated CEST MRI sequence was developed, with significantly improved sensitivity and suitable for in vivo applications. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Investigation of optimizing and translating pH‐sensitive pulsed‐chemical exchange saturation transfer (CEST) imaging to a 3T clinical scanner

Chemical exchange saturation transfer (CEST) MRI provides a sensitive detection mechanism that allows characterization of dilute labile protons usually undetectable by conventional MRI. Particularly, amide proton transfer (APT) imaging, a variant of CEST MRI, has been shown capable of detecting ischemic acidosis, and may serve as a surrogate metabolic imaging marker. For preclinical CEST imaging, continuous‐wave (CW) radiofrequency (RF) irradiation is often applied so that the steady state CEST contrast can be reached. On clinical scanners, however, specific absorption rate (SAR) limit and hardware preclude the use of CW irradiation, and instead require an irradiation scheme of repetitive RF pulses (pulsed‐CEST imaging). In this work, CW‐ and pulsed‐CEST MRI were systematically compared using a tissue‐like pH phantom on an imager capable of both CW and pulsed RF irradiation schemes. The results showed that the maximally obtainable pulsed‐CEST contrast is approximately 95% of CW‐CEST contrast, and their optimal RF irradiation powers are equal. Moreover, the pulsed‐CEST sequence was translated to a 3 Tesla clinical scanner and detected pH contrast from the labile creatine amine groups (1.9 ppm). Furthermore, pilot endogenous APT imaging of normal human volunteers was demonstrated, warranting future APT MRI of stroke patients to elucidate its diagnostic value. Magn Reson Med 60:834–841, 2008. © 2008 Wiley‐Liss, Inc.     

PET/CT versus MRI for diagnosis,staging, and follow‐up of lung cancer

Positron emission tomography / computed tomography (PET/CT), with its metabolic data of ¹⁸F‐fluorodeoxyglucose (FDG) cellular uptake in addition to morphologic CT data, is an established technique for staging of lung cancer and has higher sensitivity and accuracy for lung nodule characterization than conventional approaches. Its strength extends outside the chest, with unknown metastases detected or suspected metastases excluded in a significant number of patients. Lastly, PET/CT is used in the assessment of therapy response. Magnetic resonance imaging (MRI) in the chest has been difficult to establish, but with the advent of new sequences is starting to become an increasingly useful alternative to conventional approaches. Diffusion‐weighted MRI (DWI) is useful for distinguishing benign and malignant pulmonary nodules, has high sensitivity and specificity for nodal staging, and is helpful for evaluating an early response to systemic chemotherapy. Whole‐body MRI/PET promises to contribute additional information with its higher soft‐tissue contrast and much less radiation exposure than PET/CT and has become feasible for fast imaging and can be used for cancer staging in patients with a malignant condition. J. Magn. Reson. Imaging 2015;42:247–260.     

Comparison of diffusion‐weighted MRI and 2‐[fluorine‐18]‐fluoro‐2‐deoxy‐D‐glucose positron emission tomography (FDG‐PET) for detecting primary colorectal cancer and regional lymph node metastases

Purpose

To examine the usefulness of diffusion‐weighted MRI (DW‐MRI) for the detection of both primary colorectal cancer and regional lymph node metastases, and compare its performance with 2‐[fluorine‐18]‐fluoro‐2‐deoxy‐D‐glucose positron emission tomography (FDG‐PET) in the same patients.

Materials and Methods

We studied 25 patients with known colorectal cancer. All underwent both DW‐MRI and FDG‐PET studies. The images were retrospectively assessed by visual inspection and the imaging findings were compared with histopathological findings on surgical specimens.

Results

Of the 27 primary colorectal lesions surgically excised in 25 patients, 23 (85.2%) were true‐positive on both DW‐MRI and FDG‐PET. Two cancers were false‐negative on DW‐MRI but true‐positive on FDG‐PET, and two were false‐negative on both DW‐MRI and FDG‐PET. With respect to the detectability of metastatic lymph nodes, DW‐MRI and FDG‐PET manifested a sensitivity of 80% (8/10) and 30.0% (3/10), a specificity of 76.9% (10/13) and 100% (13/13), and an accuracy of 78.3% (18/23) and 69.6% (16/23), respectively.

Conclusion

DW‐MRI is inferior to FDG‐PET for the detection of primary lesions, but superior for the detection of lymph node metastases. J. Magn. Reson. Imaging 2009;29:336–340. © 2009 Wiley‐Liss, Inc.