The Development of A Novel Gradient Echo Arterial Spin Tagging Technique for Whole Body MR Perfusion Imaging

• Main Authors: Chai Jyh-Wen, 蔡志文
• Other Authors: Chu Woei-Chyn
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/22753170147702967299

博士 === 國立陽明大學 === 醫學工程研究所 === 90 === [ABSTRACT] Purpose: MR arterial spin-tagging (AST) technique is a noninvasive method that uses endogenous water as a diffusible tracer to measure tissue perfusion. The tagged arterial blood spins flow to the imaging slice, where they are extracted from the intravascular compartment and join the larger spin system in the tissue compartment. The accumulation of the tagged spins in tissue will lead to a decrease in magnetization that is proportional to the amount of tissue perfusion. In recent years, several techniques have been developed for producing more sensitive perfusion image. These AST techniques can be applied to the assessment of cerebral blood flow in brain tumors, Alzheimer’s disease, acute ischemic stroke, functional mapping of brain during task activation, and tissue perfusion of other organs such as kidney and lung. Unfortunately, these techniques need special pulse sequence programming and are susceptible to a number of systematic errors. Moreover, most of them are not implemented in commercial MR systems. In this study, we introduce a simple method - the gradient-echo arterial spin tagging (GREAST) technique to acquiring tissue perfusion information. The pulse sequence is a short TR gradient echo sequence with a spatial presaturation RF pulse as the tagging pulse that is available in most commercial MR scanners. The scan time for each tag and control image could be shortened to less than 20 seconds, which allows one to take a quick look at the tissue perfusion state during routine MR examination without a significant time penalty. In addition to the advantages of simplicity and convenience, GREAST also works on reducing the asymmetric magnetization transfer (MT) and slice profile effects and the sensitivity to some systematic errors. Therefore, it is not only applicable to perform brain perfusion imaging, but also auspicious to measure local perfusion of organs other than the brain. Materials and Methods: The experiments were performed in a GE 1.5T MR system. The GREAST technique consists of a spoiled gradient-echo (SPGR) sequence with a selective presaturation RF pulse for arterial spin tagging. As a short TR SPGR sequence is used, the continuous spin tagging would be appropriate for the arterial blood spins by using a thinner presaturation RF pulse at 2cm proximally to the imaging slice. Serving as the control pulse, another thicker presaturation pulse was applied on the same side but far away from the imaging plane. The phantom studies were performed for evaluating the feasibility in measurement of local perfusion, saturation efficiency of the presaturation RF pulse, and the efficacy in alleviation of the asymmetric magnetization transfer (MT) and slice profile effects, and other systematic errors. The preliminary in vivo experiments were also accomplished in normal and diseased human brains, normal livers and kidneys. Results: In the perfusion phantom studies, the results showed good linear relationship between the signal attenuation caused by the presaturation pulse and flow rates in phantom experiment and effective alleviation of the asymmetric MT and slice profile effects for various orientations of imaging slices. The results from the beef tendon experiment showed that the differences of the mean MTR (magnetization transfer ratio) between the thinner and thicker presaturation slices on the same side of the imaging slice were smaller than those between the thinner presaturation slices on the opposite sides, regardless whether they were axial or sagittal imaging planes and obtained with head or body coils. Using GREAST technique also significantly diminished slice profile effects. Human studies showed good brain perfusion images and tissue perfusion effects of liver and kidney demonstrated by signal intensity changes of the subtraction images between the control and tagged images. Conclusion: The successful demonstrations of perfusion effects on human studies illustrate the diagnostic potential of the proposed method. The GREAST technique is simple, easy to use, quick and totally noninvasive. It is applicable to examine local perfusion in most human organs.