Multiplet resonances in MRS data from normal and cancerous prostate - Signal Processing in MRS with Biomedical Applications - page 333

is presumed to be representative of the entire tumor”. They note that “this
technique provides no information on the extent of the metabolic abnormality
and suffers both from the inaccuracy of conventional MRI to define the exact
extent of solid neoplasms and the considerable tissue heterogeneity within the
tumor mass” (pp. 98, 99). Thus, these authors underscore the need for 3D
spectroscopic imaging for “identification of viable tumor on the basis of both
morphologic and metabolic parameters . . . for targeting and following local
therapy” (p. 100) [216].
The ratio between two magnetic resonance observable metabolites, choline
at 3.2 ppm and citrate (2.62 - 2.68 ppm), has been the cornerstone of appli
cations of in vivo MRS and MRSI for prostate cancer detection. The normal
prostate accumulates and secretes very high amounts of citrate produced by
nonmalignant prostate epithelial cells. Citrate is lowered with malignancy,
since prostate cancer cells oxidize citrate [416]. Citrate helps distinguish be
nign prostatic hypertrophy (BPH) from cancer, since in the former there is
high citrate production, whereas with malignant transformation, citrate is ox
idized rather than synthesized [417]. Based primarily upon the ratio of choline
to citrate, MRSI has substantially improved the accuracy with which prostatic
malignancy and extracapsular extension are detected, as well as helping to dis
tinguish cancerous prostate from benign prostatic hypertrophy [412, 413, 418].
Guidance as to the optimal site for biopsy has been substantially improved
by MRSI, as well.
Wu et al. [419] note that regions of absent or low citrate concentration in
the prostate can be visualized at a resolution of a few mm. They consider
MRSI of the prostate as a potential tool for early diagnosis and screening.
Among 42 patients with PSA > 4 ng/ml and without any prior treatment, a
total of 201 benign sextants and 51 malignant sextants, all confirmed by biop
sies, were analyzed via 3D MRSI by Chen et al. [420]. The ratio of (Choline
+Creatine)/Citrate was assessed in each sextant. The mean normal value
(±standard deviation (SD)) of 0.42±0.19 was used for scoring on a scale
from 1 to 5, with the cutoff score being 5 denoting more than three SD above
the mean. Using that criterion of MRSI for diagnosing prostate malignancy,
the sensitivity and the specificity were 84.3% and 98.0%, respectively. The
accuracy of the detection of prostate cancer was increased through a combina
tion of T 2 weighted MRI and diffusion weighted imaging. Nevertheless, MRSI
demonstrated higher accuracy compared to these other two methods [420].
In another series, the combination of MRI plus MRSI correctly indicated the
peripheral zone sites containing prostate cancer in all 17 of 54 patients with
elevated PSA and negative TRUS in whom biopsies subsequently confirmed
the presence of malignancy [421]. The criteria for judging a site as malignant
were (Choline + Creatine)/Citrate > 0.86 or low intensity signal on MRI.
However, the specificity was only 51.4% [421]. Other studies of MRI plus
MRS among patients with these characteristics indicate a sensitivity ranging
from 73% to 100% and specificity from 51% to 96% [422].
Of thirty six men with elevated PSA who had no malignant voxels on MRSI,