Imaging in breast cancer: Magnetic resonance spectroscopy

The first in vivo magnetic resonance spectroscopy (MRS) studies of breast measured resonances from phosphorus atoms (31P). These studies showed that measurable variations in phospholipid metabolism could be detected and used for diagnosing cancer and monitoring the response to treatment (reviewed in [1,2]). More recently, there has been growing interest in breast cancer research using hydrogen (1H)-MRS, because of its higher sensitivity than 31P-MRS. The first breast 1H-MRS reports focused on the diagnostic utility of the water : fat ratio in the breast [3-5], but subsequent studies did not find this ratio to be a useful diagnostic metric [6,7]. However, several studies performed with 1H-MRS noted that a resonance from choline-containing compounds (tCho) was commonly present in malignant lesions but not in benign or normal tissues [4,6-11]. Figure 1 shows a representative example of a localized 1H spectrum of an invasive ductal carcinoma, with the tCho resonance indicated.Ex vivo studies have been performed to identify the different choline compounds giving rise to the tCho resonance at a chemical shift of 3.2 ppm. High-resolution 1H spectra acquired from biopsy tissues have shown that the tCho resonance is actually a superposition of several resonances [12-14]. The primary constituents are those with a trimethylamine moiety, R-(CH2)2-N+-(CH3)3, including free choline, phosphocholine, and glycerophosphocholine. Other metabolites possibly contributing include taurine, glucose, phosphoethanolamine, and myo-inositol [14]. The choline head groups associated with semi-mobile lipids may also contribute. These resonances can be separated in ex vivo studies with high-resolution magnetic resonance (MR) spectrometers, but in vivo these peaks are substantially broadened, and at fields as high as 4 T these resonances are generally indistinguishable. Consequently, the simplified approach used in studies in vivo is to treat the 3.2 ppm spectral peak as a single resonance.Numerous