MRI screening for acoustic neuroma: a comparison of fast spin echo and contrast enhanced imaging in 1233 patients.

Gadolinium enhanced MRI is the gold standard investigation for the detection of acoustic neuroma. Non-contrast MRI sequences have been suggested as an alternative for screening examinations. In order to determine the utility of fast spin echo imaging, both gadolinium enhanced T1 weighted images and fast spin echo T2 weighted images were acquired in 1233 consecutive patients referred for exclusion of acoustic neuroma. Two radiologists independently recorded their findings. Fast spin echo T2 weighted images were evaluated with respect to the visibility of nerves within the internal auditory canals and allocated a confidence score for the presence or absence of acoustic neuroma. 33 acoustic neuromas were identified. Only 56% were confidently identified on fast spin echo T2 weighted images alone; gadolinium enhanced T1 weighted images were required to confirm the diagnosis in 44% of the cases, including 9 of the 10 intracanalicular tumours. However, when identification of two normal intracanalicular nerves is employed as the criterion of normality, the single fast spin echo T2 weighted sequence excluded acoustic neuroma in 59% of this screened population. It is concluded that an imaging strategy intended to identify small intracanalicular acoustic neuromas cannot rely on fast spin echo T2 weighted imaging alone. Gadolinium enhanced T1 weighted imaging could be restricted to patients where fast spin echo images do not exclude acoustic neuroma but this strategy requires continuous supervision by an experienced radiologist. In most practices the screening examination should continue to include a gadolinium enhanced sequence in order to optimize the detection of small acoustic neuromas.

The design and physical characteristics of a small animal positron emission tomograph.

A small diameter positron emission tomography, designed specifically for small animal studies, was constructed from existing, commercially available, bismuth germanate (BGO) detectors and electronics. The scanner consists of 16 BGO detector blocks arranged to give a tomograph with a diameter of 115 mm and an axial field of view (FOV) of 50 mm. Each block is cut to produce eight (axial) by seven (radial) individual detector elements. The absence of interplane septa enables the acquisition of 3D data sets consisting of 64 sinograms. A 2D data set of 15 sinograms, consisting of eight direct and seven adjacent cross planes, can be extracted from the 3D data set. Images are reconstructed from the 2D sinograms using a conventional filtered backprojection algorithm. Two methods of normalization were investigated, based on either a rotating 68Ge rod source, or a uniform 68Ge plane source, with a uniform cylindrical 18F phantom. Attenuation of the emitted photons was estimated using a rotating 68Ge rod source. The transaxial resolution of the tomograph was measured as 2.3 mm full width at half maximum (FWHM) and 5.6 mm full width at tenth maximum (FWTM) at the centre of the FOV, degrading to 6.6 mm (radial) and 4.4 mm (tangential) FWHM and 10.4 mm (radial) and 14.4 mm (tangential) FWTM at 40.0 mm from the centre of the FOV. The axial slice width was 4.3 mm FWHM, 10.3 mm FWTM at the centre of the transaxial field of view and 4.4 mm FWHM, 10.6 mm FWTM at 20.0 mm from the centre of the FOV. A scatter fraction of 31.0% was measured at 250-850 keV, for an 18F line source centred in a 60 mm diameter, water-filled phantom, reducing to 20.4% and 13.8% as the lower energy discrimination was increased to 380 keV and 450 keV, respectively. The count rate performance was measured using a noise equivalent count rate method, and the linearity of the dead time correction was confirmed over the count rates encountered during routine scanning. In 2D mode, the absolute sensitivity of the tomograph was measured as 9948 counts s-1 MBq-1 at 250-850 keV, 8284 counts s-1 MBq-1 at 380-850 keV and 6280 counts s-1 MBq-1 at 450-850 keV.

Retropharyngeal haematoma: a diagnosis for concern?

Three cases of retropharyngeal haematoma are described. In one case there was underlying aneurysm of the carotid artery which could have led to significant morbidity if undetected.