The hazards of lack of co-registration of ictal brain SPECT with MRI: A case report of sinusitis mimicking a brainstem seizure focus
© Butler et al; licensee BioMed Central Ltd. 2004
Received: 01 September 2004
Accepted: 29 November 2004
Published: 29 November 2004
Single photon emission computed tomography (SPECT) following injection of radiotracer during a seizure is known as ictal SPECT. Comparison of an ictal SPECT study to a baseline or interictal study can aid identification of a seizure focus.
A young woman with encephalitis and refractory seizures underwent brain SPECT during a period of frequent seizure-like episodes, and during a seizure-free period. A focal area of increased radiotracer uptake present only when she was experiencing frequent seizure-like episodes was originally localized to the brainstem, but with later computerized co-registration of SPECT to MRI, was found to lie outside the brain, in the region of the sphenoid sinus.
Low-resolution SPECT images present difficulties in interpretation, which can be overcome through co-registration to higher-resolution structural images.
Radiotracers used for brain single photon emitted computed tomography (SPECT) pass the blood-brain barrier and bind intracellularly on their first pass through the circulation, providing a "snapshot" of cerebral perfusion at a particular timepoint. When injected during a focal epileptic seizure, an area of significantly increased radiotracer uptake typically corresponds to the region of maximal abnormal activity, often the seizure focus. This ictal pattern of cerebral blood flow can be compared to an interictal/baseline pattern obtained when the patient is not having a seizure, to provide unique information about the nature and location of a patient's epileptic focus, which can be used to guide therapy [See  for a review of the use of SPECT in epilepsy].
With later co-registration of SPECT data to the patient's MRI using a computerized medical image registration and visualization program,  the area of increased radiotracer uptake was seen to be anterior to the brain, likely in the sphenoid sinus, consistent with sinusitis (Figure 1C). MRI showed prominent maxillary and sphenoidal sinus mucosal thickening also consistent with sinusitis. Chronic nasotracheal intubation is commonly accompanied by sinusitis,  which may resolve with removal of the offending tube, as was performed in this case when the patient underwent tracheotomy during the interval between the first and second SPECT scans.
The nature of this patient's twitching episodes remains uncertain, though they were likely due to epileptic activity involving too small a volume of brain to be detectable by either EEG or SPECT. Her case is described here to illustrate a previously-unreported pitfall in the interpretation of brain SPECT studies, with the goal of emphasizing the importance of image co-registration in nuclear medicine. Diagnostic limitations inherent to low-resolution images can be overcome through use of computer software to accurately co-register brain images of different modalities acquired separately (as described here) or hybrid medical devices such as PET-CT scanners to automatically fuse simultaneously-acquired images. Both methods have been shown to be superior to visual inspection and mental integration of information by experienced radiologists [7, 8].
List of abbreviations
single photon emitted computed tomography
magnetic resonance imaging
The authors thank Hal Blumenfeld, MD, PhD, and Kelly McNally for sharing their knowledge of SPECT analysis and interpretation.
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