Clinical applications ofin vivomagnetic resonance spectroscopy in oncology

Abstract

Brief introduction to magnetic resonance spectroscopyMagnetic resonance spectroscopy (MRS) can be used to probe tissue biochemistry in vivo by detecting signals from many metabolites, primarily those which have a low molecular weight and a concentration of a few mmol l −1 . MRS uses the same hardware as magnetic resonance imaging (MRI). Examples of 1 H MR spectra acquired from brain are shown in figure 1. A brief introduction is given here for the benefit of readers not familiar with the basic principles of MRS to provide a basic framework to understand the subsequent clinical applications. For more detail there are a number of good books describing in vivo MRS with helpful chapters on methods and applications (de Graaf 2007, Barker et al 2010.Some atoms (such as 1 H, 13 C, 31 P, 23 Na and 19 F) have nuclei with a magnetic dipole moment. The sum effect of all the magnetic dipoles is known as the bulk magnetisation. In a magnetic field and at thermal equilibrium the bulk magnetisation is aligned with the magnetic field. Application of an alternating magnetic field at the appropriate frequency (see equation (1) below) causes the bulk magnetisation to rotate away from the direction of the static magnetic field. The magnetisation then freely rotates about the static field at this same frequency. In time the bulk magnetisation relaxes back to thermal equilibrium parallel to the static field. If a detector coil is placed close to the sample a small current will be induced in it by the rotating magnetisation. This signal is known as the free induction decay (FID). The spectrum of frequency components (such as shown in figure 1) is obtained by Fourier transform of this signal.The frequency position of a resonance peak in Hz (often known as the Larmor precession frequency) is determined by the following equation:where γ is the gyromagnetic ratio of the nucleus (e.g. 2.675 × 10 8 rad/T/s for 1 H), and B local is the local field at the atomic nucleus. B local is determined primarily by the magnitude of the applied magnetic field (typically 1.5 T or 3 T for clinical MRI scanners), but it is slightly modified by the shielding effect of the electron cloud associated with bonding electrons (the chemical shift

Developments in proton MR spectroscopic imaging of prostate cancer

“…As most prostate MR spectra are acquired with spin-echo type of pulse sequences, acquisition can start at the center of the echo, minimizing first-order phase errors. Fortunately, in contrast to MR spectra of the brain, there is no significant contribution of macromolecular signals to the baseline in MR spectra of the prostate down to echo times of 32 ms [ 18 , 29 ]. Baseline corrections may be needed if the tails of broad water or lipid signals stretch out into the spectral region of interest (about 2.3–4 ppm).…”

“…Because of the use of endorectal coils with an inhomogeneous receive B 1 field by which signal intensity of the coil drops towards the ventral parts of the prostate, it has become custom to calculate signal ratios, avoiding intrinsic spatial variations in signal intensity. Because it is not always feasible to resolve Cho from Cr and Spm signals, in particular at 1.5T and with PRESS sequences, the most often used ratios are (Cho+Cr)/Cit and (Cho+Spm +Cr)/Cit [ 18 ]. With better resolution, e.g., at 3T, it is common to also use Cho/Cr [ 81 ] and Cho/Cit ratios.…”

“…The most severe inhomogeneity occurs at air-tissue transitions such as at the rectum. Commercial MR systems can correct for B 0 inhomogeneities with shim coils, but these usually are not able to handle multiple boundaries with strong susceptibility transitions [ 18 ]. To improve this situation, a range of dedicated technical solutions have been developed, mostly for brain applications [ 202 – 206 ].…”

Interleaved and simultaneous multi‐nuclear magnetic resonance in vivo. Review of principles, applications and potential

“…MRS has demonstrated its usefulness in classifying mass lesions and tumours and in monitoring their therapeutic treatment. 12 The complementary information brought by 1 H MRS alone during an MRI examination increased the rate of correct diagnoses by 15.4%. 164 31 P MRS has also been used, to a lesser extent, for tumour classification and monitoring.…”

“…For example, phosphorus-31 ( 31 P) MRS has been employed as a tool to investigate intracellular pH and metabolism in vivo since its early days [2][3][4] and continuously throughout, particularly in skeletal muscle. 5,6 It has proven useful to study metabolites in liver, 5,7 heart and brain 8 as well as bone mineralization 9,10 and oncology 11,12 ; carbon-13 ( 13 C) can provide information about the metabolism of glucose and glycogen in vivo 4,13,14 ; deuterium-2 ( 2 H) is also suited to evaluate glucose metabolism [15][16][17] and as a tracer 18,19 ; fluorine-19 ( 19 F) for cell tracking, monitoring of fluorinated drugs and as an alternative to hyperpolarized (HP) helium-3 ( 3 He) and xenon-129 ( 129 Xe) gases in functional lung imaging and ventilation studies [20][21][22] or oxygen-17 ( 17 O) to image and quantify the metabolic rate of oxygen consumption and as a tracer of cerebral blood flow (CBF). 23,24 The viability of healthy and tumorous tissue can be studied with sodium-23 ( 23 Na) imaging and spectroscopy, 25 which is also a valuable tool for the diagnosis and research of kidney 25 and cartilage defects.…”