Digital Signal Processing Reference
In-Depth Information
glioma and radiation injury with the appearance of new contrastenhancing
lesions has been facilitated by ratios of choline to creatine and choline to NAA,
particularly with the addition of diffusion weighted imaging [276]. Weybright
et al. [277] performed MRSI in twentynine consecutive patients who had a
new contrastenhancing lesion in the vicinity of a previously diagnosed and
treated brain neoplasm. These authors [277] found that the ratios of choline
to creatine and of choline to NAA were significantly higher in tumor than in
radiation injury. In turn, the ratios of choline to creatine and of choline to
NAA were significantly higher in radiation injury than in normalappearing
white matter. However, these metabolite ratios showed substantial overlap,
so that the distinction among recurrent tumor, radiation injury and normal
appearing white matter was not absolute. Smith et al. [278] have recently
reported that among thirtythree patients who had a new contrast enhancing
lesion after RT for primary brain tumors, an elevated choline to NAA ratio
predicted tumor recurrence with a sensitivity of 85% and specificity of 69%.
The importance of an increase in choline to NAA ratio in an area of contrast
enhancement on postoperative followup MRS as an indicator of tumor recur
rence as opposed to postradiation lesions was also reported in a study of nine
patients with high grade gliomas [279]. Sankar et al. [280] used MRSI to assess
response to tamoxifen chemotherapy among sixteen patients treated for recur
rent highgrade glioma. The metabolite intensities were found to be stable in
all responders, but changed prior to disease progression. Choline, lipid, as well
as ratios of choline to NAA and lactate to NAA were significantly increased,
while creatine was significantly decreased, compared to stabilized levels. In
one patient who continued to respond to tamoxifen at the conclusion of the
trial, metabolite levels remained stable. These authors conclude: “charac
teristic global intratumoral metabolic changes, detectable on serial (proton)
MRSI studies, occur in response to chemotherapy for malignant glioma and
may predict imminent treatment failure before actual clinical and radiolog
ical disease progression”. They emphasize that their study was motivated
by the aim of “early prediction of imminent failure during chemotherapy for
malignant glioma which has the potential to guide proactive alterations in
treatment before frank tumor progression” (p. 63) [280]. A case report of a
patient with low grade glioma treated with temozolomide chemotherapy and
with several followup MRSI examinations revealed that choline levels within
the tumor paralleled shrinkage of the tumor [281]. In contrast, however, mono
voxel MRS assessment of NAA to choline ratios provided very poor sensitivity
(28%) for assessing tumor progression under chemotherapy and/or RT in a
small series of patients treated for low grade gliomas [282].
8.1.6.2
Predicting likelihood of proliferation and relapse of brain
tumors in adults
In a prospective study among eightytwo patients with grade II gliomas, a
comparison was made between spectroscopic and histological data including
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