The award of three recent Nobel prizes, those in medicine (MR imaging), chemistry (aquaporins), and physics (superconductivity) highlights the importance and present interest in the biochemistry, pathophysiology, and imaging of brain function. Advanced magnetic resonance techniques, such as perfusion MR imaging (MRI) and MR spectroscopy (MRS) provide important in vivo physiological and metabolic information from the brain, complementing morphologic findings from conventional MRI in the clinical and research setting (Fig. 1). This article will provide an outline of dynamic susceptibility contrast-enhanced MRI (DSC MRI) and proton MR spectroscopic imaging (MRSI) and their application in the evaluation of brain tumors and multiple sclerosis (MS).
Prostatic magnetic resonance imaging (MRI) and magnetic resonance spectroscopic imaging (MRSI) promise to correct these shortcomings. Currently the primary indication for prostate MRI and MRSI is in the evaluation of men with newly diagnosed prostate cancer with a moderate or high risk of extraprostatic disease, a clinical scenario in which it is often uncertain whether surgery or radiotherapy is the optimal treatment. Data critical to this decision is the size and level of aggressiveness of the tumor, as reflected in its metabolic activity, and the presence and degree of extra-glandular extension of tumor. The comprehensive and detailed evaluation obtained with MRI is not limited to the prostate gland. This imaging technique also allows detection of periprostatic and pelvic pathology. While MRI provides anatomic information, MRSI provides functional information that differentiates normal tissue from tumor and reflects the degree of tumor metabolism. Additional potential roles for MRI and MRSI include tumor detection and treatment follow-up, as detailed below.
Fig. 1: Advanced magnetic resonance techniques, such as diffusion tensor imaging, perfusion MR Imaging and MR spectroscopy can give important in vivo physiological functional and metabolic information from the brain, complementing morphologic findings from conventional MRI in the clinical and research setting.
Dynamic Susceptibility Contrast Enhanced Perfusion MRI (DSC MRI)
Perfusion is the steady-state delivery of blood to tissue through the capillaries. Blood flow delivers oxygen and important nutrients to tissues, but can be abnormal in many diverse pathological processes. For instance, low perfusion might result in cellular ischemia and high perfusion might be associated with hypervascular tumors. Over the past 10 years, there has been increasing interest in the use of contrast agents in concert with MRI to measure hemodynamics. The basic technique is intravenous injection of the contrast agent, gadolinium, and rapid acquisition of images as the contrast agent bolus passes through the blood vessels in the brain. The contrast agent causes a signal change over time which can be analyzed to measure cerebral hemodynamics and blood flow. The most common imaging modality capable of noninvasively measuring brain tissue perfusion is DSC MRI. In DSC MRI, the signal measured is due to the susceptibility T2 or T2* effect induced by the contrast agent remaining in the capillary intravascular space. The signal loss is related to the concentration of contrast, and is proportional to cerebral blood volume in that area.
Fig. 2: 3D 1H-MRS study of a patient with MS and a healthy control.
Top: Axial T2-weighted (TE/TR=90/2500 ms) (a), axial (b), and coronal (c) T1-weighted (TE/TR=14/450 ms) MRI of a 26 yo. female MS patient, superimposed with the MRS volume of interest. Spectra from hypo- (arrows 1 & 4) and iso-intense (2 & 5) lesions as well as two regions of normal-appearing white matter (3 & 6) are shown below the images on common intensity and chemical shift (ppm) scales. Bottom, d–f: Corresponding slices from a matched control. The numbered arrows indicate equivalent regions to a–c for metabolite levels and spectra comparisons. Note lower NAA signal in hypointense lesions 1 and 4.
From Radiology 234: 211–217; 2005
Proton MR Spectroscopy (1H-MRS)
Proton MR Spectroscopy (1H-MRS) has been shown to be a powerful magnetic resonance method for investigating molecular and metabolic characteristics of brain tissues. 1H-MRS can define several chemical correlates of pathological change: neuronal/axonal injury through the N-acetylaspartate (NAA) level and membrane turnover through the choline, inositol, and mobile lipid signals. Unlike traditional single voxel spectroscopy (SVS), multi-voxel spectroscopy (MVS), either 2D or 3D, obtains spectroscopic information from multiple adjacent volumes over a large volume of interest in a single measurement. This yields better resolution and facilitates evaluation of focal as well as diffuse pathologies involving the entire brain. Furthermore, MVS can be combined with MR imaging. Spectral peaks and metabolite maps can be over-laid on gray-scale MR imaging to obtain distributional patterns of specific metabolites. A novel 3D 1H-MRS technique developed by Dr. Oded Gonen and his team at NYU, combines the advantages of chemical shift encoding (CSI) with those of the spatially selective eight-order Hadamard pulse providing metabolic multi-planar maps of large segments of brain parenchyma (Fig. 2). However, despite the utility of this current 3D localized 1H-MRS method, the volume of interest is still restricted to < 500 cm3, located completely within the brain.
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