Magnetic resonance in neuro-oncology: Achievements and challenges - Signal Processing in MRS with Biomedical Applications - page 252

as subacute hemorrhagic stroke, inflammation, infection, and a number de
generative brain disorders [210, 211]. The MRI patterns of bilateral thalamic
astrocytomas seen in children are very similar to those of neurometabolic dis
orders and of encephalitis [212]. Not all brain tumors appear T 2 hyperintense.
Meningiomas, for example, are often isointense to the surrounding brain tis
sue, when viewed on MRI without contrast [208]. Small brain metastases may
not appear at all on T 2 weighted imaging [210]. Moreover, the peritumoral
region often appears unchanged on MRI, but represents an “uncertain” zone
where tumor may be present in patients with cerebral glioma [213]. Gadolin
ium contrast is used in MRI workups of suspected brain tumors, with the
majority of brain tumors showing contrast enhancement [208]. However, since
disruption of the blood brain barrier is not unique to brain tumors, contrast
enhancement on MRI is not pathognomonic for malignancy. Cerebral infarcts,
brain abscesses, multiple sclerosis (acute plaques) also show contrast enhance
ment with gadolinium [210]. While the pattern of contrast enhancement is
often useful in identifying tumors, this is not always the case. For example,
ringlike enhancement, which is considered characteristic of brain abscesses,
can also be seen with cystic brain tumors [214].
It should also be emphasized that malignant lesions from areas that are
T 2 hyperintense, but outside the contrastenhancing region have been fre
quently reported [215]. Although the contrast enhancing volume is usually
considered to define the region of tumor, Vigneron et al. [216], note that
the contrastenhancing region does not always reflect the actual extent of the
tumor. The authors from Ref. [216] point out that this can be due to tumor
which does not show contrast enhancement or to necrosis which does take
up contrast. Thus, a lesion may appear larger on the T 2 weighted imaging
because it encompasses not only the tumor, but also “nonspecific effects such
as edema and inflammation. These factors produce considerable uncertainty
concerning the reliability of MRI techniques for defining the treatment target
and evaluating therapeutic success” (p. 89).
Another way of attaining contrast is via diffusionweighted imaging (DWI),
based upon the molecular motion of water. This technique has been helpful
in providing earlier detection of pathology, including brain tumors. Diffusion
weighted imaging can also help distinguish malignancy from secondary effects
of treatment on the tumor. However, the findings on DWI are not pathog
nomonic for neoplasia [209, 217, 218]. The role of MRI versus stereotactic
biopsy in brain tumor diagnostics has been described succinctly by Howe and
Opstad [207]. They emphasize that accurate diagnosis is vital for optimum
clinical management of patients with intracranial tumors. Accessible tumors
are generally surgically resected, but there is “a balance between removing
as much tumor tissue as possible, whilst maintaining vital brain functions”
(p.123). Thus, radiation therapy (RT) is frequently also used to treat resid
ual tumor. Magnetic resonance imaging is widely applied to determine tumor
extent for surgical and RT planning, as well as for posttherapy monitoring of
tumor recurrence or progression to higher grade. The initial diagnosis of an