Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use of specific MRI sequences as well as software that generates images from the resulting data, that uses the diffusion of water molecules to generate contrast in MR images.
It allows the mapping of the diffusion process of molecules, mainly water, in biological tissues, in vivo and non-invasively. Molecular diffusion in tissues is not free, but reflects interactions with many obstacles, such as macromolecules, fibers, and membranes. Water molecule diffusion patterns can therefore reveal microscopic details about tissue architecture, either normal or in a diseased state. A special kind of DWI, diffusion tensor imaging (DTI), has been used extensively to map white matter tractography in the brain.
DWI exploits the random motion of water molecules. The extent of tissue cellularity and the presence of intact cell membrane help determine the impedance of water molecule diffusion. This impedance of water molecules diffusion can be quantitatively assessed using the apparent diffusion coefficient (ADC) value.
There are numerous applications of DWI/DTI in brain tumors: (1) determination of grade and histologic subtype, (2) evaluation of peritumoral edema and assessment of pathways of tumor infiltration, (3) quantitative measurement and monitoring of the response to therapy, and (4) discrimination between necrosis and tumor recurrence (eg, radiation and chemotherapy).
Differences in diffusion properties of low- and high-grade tumors are caused by several factors including different tumor cellularity and nucleus to cytoplasm ratio. In contrast to high-grade tumors, low-grade tumors are characterized by hypocellularity, low nucleus to cytoplasm ratio, and large extracellular spaces, which is typically represented by high MD/ADC values 1) 2).
Studies involving coronary artery bypass graft surgery, carotid endarterectomy, or interventional surgery have demonstrated new small ischemic brain lesions using DWI.
Normally water protons have the ability to diffuse extracellularly and loose signal. High intensity on DWI indicates restriction of the ability of water protons to diffuse extracellularly. Restricted diffusion is seen in abscesses, epidermoid cysts and acute infarction (due to cytotoxic edema).
Some tumors: most tumors are dark on DWI, but highly cellular tumors may have decreased diffusion (bright on DWI) (e.g. epidermoids, some meningiomas…)
In cerebral abscesses the diffusion is probably restricted due to the viscosity of pus, resulting in a high signal on DWI.
In most tumors there is no restricted diffusion - even in necrotic or cystic components. This results in a normal, low signal on DWI.
MRI of 72-year-old woman admitted because of right hemiparesis. MRI was performed 7 days after onset. T1-weighted imaging revealed multiple low-intensity areas around the ventricle, and both T2-weighted imaging and FLAIR showed an area of severe periventricular hyperintensity with suspected multiple high-intensity lesions. DWI showed a high-intensity area that coincided with clinical features on the left corona radiata.
DWI or MRA conducted immediately after Aneurysm clipping may be affected by artifacts resulting from the surgical procedure, such as intracranial air or motion artifacts from the patient.
DWI was performed using two-dimensional, single-shot, spin-echo, echo planar imaging of the entire brain with the following parameters: echo time (TE), 50; repetition time (TR), infinite; B, 1000 s/mm2; field of view (FOV), 24 × 24 cm; flip angle, 90°; imaging matrix, 128 × 128; slice thickness, 5.5 mm with a 1.5-mm gap; and number of slices, 20. Three- dimensional T1 fast field echo time-of-flight MRA of the circle of Willis was performed using the following parameters: flip angle, 18°; TR, 25 ms; TE, 3.5 ms; slice thickness, 1.2 mm; FOV, 20 × 20; matrix size, 512 × 205; number of slices, 132– 160; slice gap, 0.6 mm.
Any new hyperintensities observed using postoperative DWI were interpreted as new ischemic lesions that developed after aneurysm clipping 3).
Hyperintense lesions around the resection cavity on magnetic resonance diffusion-weighted imaging (MR-DWI) frequently appear after brain tumor surgery due to the damage of surrounding brain. The putative connection between the lesion and the prognosis for patients with glioblastoma (GBM) was explored in sixty-one patients with newly diagnosed GBM. Postoperative MRI was performed within 2 weeks after the initial surgery.
The cases into two groups depending on whether DWI hyperintense lesions were observed or not [DWI(+) group and DWI(-) group]. Progression-free survival (PFS) and overall survival (OS) were compared between the two groups. Forty-two patients were identified. The various extents of hyperintense lesions around the resection cavity were observed in 28/42 (66.7 %) cases. In the DWI(+) and DWI(-) groups, median PFS was 10.0 [95 % confidence interval (CI) 8.4-11.5] and 6.7 (95 % CI 4.9-8.5) months, respectively (p = 0.042), and median OS was 18.0 (95 % CI 12.2-23.8) and 17.0 (95 % CI 15.7-18.3) months, respectively (p = 0.254). On multivariate analysis, the presence of DWI hyperintense lesion was more likely to be an independent predictor for 6-month PFS (p = 0.019; HR, 0.038; 95 % CI 0.002-0.582). Tumor recurrence appeared outside the former DWI hyperintense lesion. Hyperintense lesions surrounding the resected GBM on MR-DWI might be a favorable prognostic factor in patients with GBM 4).