Spectroscopic imaging through MR Possibilities to be fully tapped for clinical oncology

Functional-anatomic imaging can be achieved as well, by combining Magnetic Resonance Imaging (MRI) and Magnetic Resonance Spectroscopy (MRS) yielding MRSI (Magnetic Resonance Spectroscopic Imaging). The images produced by MRI alone yield anatomical information depicted by the spatial distributions of magnetizations of nuclei from the scanned tissue as a result of the effects of three external magnetic fields, as will be detailed in the first part of this book. One of the key advantages of MRI is superior contrast among tissues compared to CT. Moreover, MRI has multiplanar capabilities, that are absent from CT [7].

Via magnetic resonance, information can also be obtained about many of the chemical constituents of biological material. This is achieved through MRS, which involves the in vivo application of traditional laboratory-based NMR2 techniques. MRS provides this complementary biochemical and physiologic information in the form of spectra.

As yet, however, application of MRS and MRSI in tumor diagnostics, although increasing, has been substantially more limited than PET and PET-CT. This might seem surprising, given that in vitro studies often describe a rich array of spectral characteristics that distinguish malignant from healthy tissues. Therefore, MRSI should be able to identify key biochemical changes, much before the tumor becomes detectable by other functional imaging methods that mainly rely upon single markers that are not entirely sensitive or specific for malignant activity.

1 FDG = 18F-deoxy glucose is a glucose analogue which is phosphorylated by hexokinase (the first enzyme of the glycolytic pathway) to FDG-6-PO4 and remains trapped intra-cellularly, thus reflecting glycolysis throughout the body. Since many tumor cells have a high metabolic rate mainly via the glycolytic pathway, FDG uptake has been used as a marker of malignancy.

2 NMR = nuclear magnetic resonance

Molecular imaging through magnetic resonance could be particularly well suited for screening and repeated monitoring since it entails no exposure to ionizing radiation.

Perhaps part of the reason why, with a few important exceptions, MRS and MRSI are not yet part of the routine armamentarium for the assessment of patients with proven or suspected cancer is historical. Namely, early in vivo spectrometers did not have imaging capabilities, such that MRS and MRI developed along separate paths [8]. Furthermore, as pointed out over a decade ago by Bottomley [9], spectroscopy differs fundamentally from its "sister technology, MR imaging, to which it is inextricably linked, {such} that its failure to materialize clinically with the same speed that MR imaging arrived should be of little surprise ... biochemical information provided by means of spectroscopy has no real clinical antecedents, and we must look to the biochemistry research literature for its interpretation. Clinical MR spectroscopy ... {is} thus ... relatively uncharted territory in the field of radiology" (p. 1).

Increasingly, however, modern systems have localization capabilities that can connect imaging to spectroscopy. Therefore, it can be expected that MRSI will very soon establish these "clinical antecedents", especially in relation to the advances made in our knowledge of the cell biology of cancer.

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