Biomedical Engineering Reference
In-Depth Information
25.1
Introduction
Magnetic resonance imaging (MRI), a sensitive morphological imaging technique
free from ionizing radiation is indispensable for timely cancer detection, but often
has insufficient specificity. Magnetic resonance spectroscopy (MRS) can enhance
specificity by elucidating the metabolic features of malignancy. Since molecular
changes often precede morphologic alterations, sensitivity can be further improved
by MRS. Molecular imaging can be accomplished by combining MRS and MRI
yielding magnetic resonance spectroscopic imaging (MRSI). Rather than selecting a
single voxel to encompass a specific volume, a spectrum is obtained at each point of
selected grids thereby providing volumetric coverage. In this chapter, we highlight
the achievements as well as current limitations of molecular imaging through
MRS and MRSI for radiation oncology. The main limitations are noise corruption
of encoded time signals and their processing by exclusive reliance on the fast
Fourier transform (FFT). We then present certain novel possibilities for optimization
through advanced signal processing methods, notably quantum-mechanical spectral
analysis for metabolite quantifications via the fast Pade transform (FPT) [
1
,
2
].
25.2
Achievements of MRSI in radiation oncology
Molecular imaging has been vital to radiation oncology. By helping to define
complex target geometries and surrounding healthy tissue, volumetric imaging
was the pivotal spur for advances such as Intensity Modulated Radiation Therapy
(IMRT) [
3
]. While MRS and MRSI have been used more widely in cancer
diagnostics [
4
], their main applications within radiation oncology have been for
brain tumors and prostate cancer. The achievements of MRS and MRSI in these two
areas of radiation oncology are now briefly summarized.
25.2.1
Brain tumors
Clearly, in radiation neuro-oncology, the most delicate clinical decisions are made,
requiring maximal information of the highest possible reliability. In no other domain
of oncology have MRS and MRSI become so widely incorporated into clinical
practice. Currently, ratios of certain metabolites have mainly been used. These
include: choline (Cho) a marker of membrane damage and cellular proliferation
whose resonant frequency is at
ppm (parts per million) in relation to
Nitrogen-Acetyl Aspartate (NAA) an indicator of viable neurons, resonating at
3:2
2:0
ppm,ortocreatine(Cr)at
ppm, a marker of cerebral energy metabolism.
Incorporating MRSI into RT planning for primary brain tumors can improve
control, while reducing complications. Traditionally, the clinical target volume for
3:0
