A pituitary abscess showing high signal intensity on diffusion-weighted imaging
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- Takayasu, T., Yamasaki, F., Tominaga, A. et al. Neurosurg Rev (2006) 29: 246. doi:10.1007/s10143-006-0021-0
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The utility of diffusion-weighted imaging (DWI) for the diagnosis of intracranial abscesses has already been established. However, the use of DWI for pituitary abscesses has not been previously reported. We present a case of postoperative pituitary abscess in which T1-weighted and T2-weighted magnetic resonance imaging (MRI) revealed a supra-sellar cystic mass, with the cyst contents showing high intensity on DWI. This case suggests that DWI is useful for the diagnosis of pituitary abscesses.
KeywordsPituitary abscessMagnetic resonance imagingDiffusion weighted imagingT2 shine-through effect
The utility of diffusion-weighted imaging (DWI) for the diagnosis of intracranial abscess has now been thoroughly established [3, 4, 6–8]. However, to the best of our knowledge, DWI for pituitary abscesses has not been investigated. We present a case of postoperative pituitary abscess that showed a hyperintense signal on DWI.
DWI provides valuable information on the diffusion properties of water molecules in tissue. To date, DWI has been widely applied in clinical practice, such as for the diagnosis of brain infarctions , and tumors [5, 9, 11, 12, 15]. DWI has also been used for the differential diagnosis between brain abscesses and necrotic brain tumors. In previously reported cases, DWI has shown brain abscesses to have high intensity and a low ADC value [3, 4, 6–8].
Previous studies have suggested that the high consistency and viscosity of pus may explain its restricted diffusion [3, 4, 6–8]. The presence of inflammatory cells, bacteria, necrotic tissue, proteinaceous exudate , and other large molecules, like fibrinogen [3, 8], may play a key role in restricting the diffusion of pus. Ebisu et al.  reported a case of a brain abscess with high signal intensity on DWI both in vivo and in vitro, and concluded that the pus structure itself contributed to the low ADC of the abscess. Our surgical findings of thick consistency and viscosity of the pus are in agreement with those of previous studies.
However, increased signal intensity of a rim-enhancing brain mass on DWI is not specific for brain abscesses. Several studies have indicated that metastatic brain tumors and radiation necrosis with pus-like content can show high intensity on DWI and low ADCs [5, 9, 13]. These studies suggested that the restricted diffusion was due to sterile liquefaction necrosis . These findings suggest that restricted diffusion depends on the properties of the mass fluid.
The high signal intensity in DWI reflects not only low ADC but also prolonged T2. In the present case, the ADC value was 1.262±0.137×10−3 mm2/s, which was higher than the previously reported values of typical abscesses or normal brain [3, 4, 7, 8]. Thus, the T2 shine-through effect may have contributed to the increased signal intensity in DWI in the present case. This T2 shine-through effect has been reported in studies of cerebral infarction and epidermoid tumors [1, 2], and they can be seen in intrasellar or parasellar lesions. Pituitary apoplexy showing high intensity on DWI and low ADC has been reported , and previous studies have demonstrated that epidermoid tumors show high intensity on DWI and can be seen in parasellar lesions [2, 12]. Thus, pituitary abscess can occasionally be confused with these DWI high-mass lesions.
Conventional MRI findings would enable differentiation of these from pituitary abscesses. For instance, the epidermoid may extend without dislocation of the chiasma. Pituitary apoplexy will show high signal intensity on T1-weighted imaging (T1WI) due to the presence of hemorrhagic byproducts (methemoglobin). In addition, the patient’s background and progress, and laboratory data would also be helpful for making a differential diagnosis between pituitary abscess and pituitary apoplexy and/or epidermoid tumor.
The limitation of single-shot echo-planar technique in our study is degradation because of the paramagnetic susceptibility effects of the skull base. Multishot echo-planar imaging-based or fast-spin echo-based diffusion technique will reduce this spatial distortion. Moreover, parallel imaging can greatly compensate for this susceptibility distortion.
In the present case, preoperative laboratory data did not show any obvious signs of infection, the DWI findings and clinical course helped establish the differential diagnosis, and early surgical intervention was recommended. Thus, we emphasize that DWI appears to be a powerful tool for diagnosis of pituitary abscess, especially in cases of post trans-sphenoidal surgery.
We report a case of postoperative pituitary abscess that showed high signal intensity on DWI. Diffusion-weighted imaging appears useful for the diagnosis of pituitary abscesses, and should be performed in cases of suspected pituitary abscess.