Saturday 6 August 2011

Role of Diffusion imaging in Cholesteatoma evaluation

Right side mastoid show an expansile T2 hyper intense soft tissue with restricted diffusion on Dw images which is characteristic of a Cholesteatoma. There is an obvious erosion and destruction of tegmen and dural plate with adjacent focal vasogenic odema in right cerebellar hemisphere. Opposite mastoid also show similar but smaller lesion.

Imaging diagnosis : Bilateral Cholesteatoma with typical restricted diffusion.

Role of Diffusion Weighted MR Imaging in Cholesteatoma Evaluation

CT has long been used for evaluation of CSOM and detection of Cholesteatomas. However, this modality often lacks specificity, particularly in cases with an absence of obvious bony erosion. 
MRI with diffusion weighted imaging (DWI) is now becoming the first choice for diagnosing the presence of Cholesteatomas.
Cholesteatomas demonstrate typical restricted diffusion on Dw images where the granulation tissue does not is a diagnostic clue and the main differentiating point where CT often gives equivocal results. The mechanism responsible for high signal on DWI remains somewhat uncertain, but is thought to represent either T2 shine-through alone or in combination with true restricted diffusion.

Main differential diagnosis include Cholesterol granuloma (high T1 signal, no enhancement, no restriction diffusion). In the post operative setting, the differential for a soft tissue attenuating middle ear mass includes recurrent cholesteatoma (increased signal on DWI, no enhancement post contrast, high T1 signal) Vs Granulation tissue (intermediate T1 signal, enhancement post contrast, low signal on DWI and Scarring (low T1 and T2 signal, low signal on DWI)


(Half-Fourier acquisition single-shot turbo spin-echo diffusion weighted imaging)
Although it remains controversial whether the bright signal of cholesteatoma is a result of restricted diffusion or a T2 shine-through effect, DWI has been increasingly used in the evaluation of pre as well as postoperative residual cholesteatomas. 
Several recent reports have focused mainly on two DWI techniques for detecting cholesteatoma: EPI DWI and non-EPI (single-shot turbo spin echo [SSTSE] DWI, also called HASTE) DWI. Initial studies primarily used the EPI DWI technique, but susceptibility artifacts caused by field inhomogeneities at the air-bone interface of the temporal bone were more pronounced with this technique. This limitation is shown reduced in several studies with the use of HASTE DWI. In HASTE, the reduction in susceptibility artifacts can be explained by the use of 180° radio-frequency refocusing pulses for each measured echo. 
Most studies have reported a very high diagnostic performance for HASTE DWI that often reaches 100% for all four statistical measures. De Foer et al. have reported that the HASTE DWI detection limit for a cholesteatoma is as low as 2 mm; EPI DWI had previously set the limit of detection at 5 mm.
So the HASTE Dwi technique is the most reliable for detection of Cholesteatoma among the two. 

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