Increment of brain temporal perfusion during auditory stimulation. Preliminary study with technetium-99m HMPAO SPET

Tonotopic organization of human auditory cortex analyzed by SPET

Single photon emission tomography (SPET) was used to map blood flow increases in the temporal and parietal cortex of 16 normally-hearing subjects after auditory stimulation. Eight subjects were stimulated with a multifrequency 40 dB HL pure tone at 250, 500, 1000, 2000, 4000 Hz, each frequency varying every 30 s. Single 500 Hz pure tones at 40 dB HL were delivered to the remainder of the subjects. Five bilaterally deaf subjects were used as controls. Marked cerebral flow increase following acoustic stimulation with a significantly prevalent activation of the contralateral temporal cortex was achieved (p < 0.001). According to the tonotopic organization of the human auditory cortex, low monofrequency stimulation activated the most lateral sagittal tomograms (from 48.75 to 56.25 mm laterally to the brain midline) only, while multifrequency stimuli activated all sagittal tomograms (from 18.75 to 56.25 mm). On the basis of these results, it is likely that SPET is able to give real information on the cortical distribution of the auditory frequency range, taking into account the number and position of the activated slices. Further clinical investigations in order to define the relationships among blood flow cortical increases, stimulus intensity and auditory threshold, are in progress.

Assessment of functional hemispheric asymmetry by bilateral simultaneous cerebral blood flow velocity monitoring

The aim of this study was to investigate side-to-side differences of simultaneously measured middle cerebral artery (MCA) blood flow velocities during various hemisphere-specific tasks. Using a transcranial Doppler device, flow velocity changes of 24 healthy, right-handed subjects were monitored simultaneously in the left and right MCA during different hemisphere-specific tasks. Mean flow velocity (MFV) curves were averaged for each individual subject and task. Simultaneously, heart rate, blood pressure and end-tidal carbon dioxide (CO2) were measured in a subgroup of six subjects. When compared with the resting state, all stimuli produced significant (p < 0.001) bilateral MFV increases, ranging from 2.5-9.2%. A lateralization of MFV increases with a significantly (p < 0.001) more pronounced increase in MFV in the hemisphere contralateral to the performing band was observed both during simple sequential finger movements and a complex spatial task. During the complex spatial task, consistently higher MFV increases were observed in the right MCA (p < 0.001), regardless of the side of task performance. Recognition of pictorial material presented as part of a memory task, also resulted in a side-to-side difference of respective MFV increases (right > left, p < 0.001), whereas memorization did not. Whereas bilateral MFV elevations observed during stimulation with white noise were only discrete and not lateralized, exposure to overt speech produced significantly higher (p < 0.001) MFV increases in the left MCA. The time course of the MFV reaction showed a rapid increase with an initial maximum after 4-5 s. Heart rate, blood pressure, and end-tidal CO2 showed only subtle changes during the stimulation periods. In conclusion, the observed side-to-side differences of MFV reaction in the left and right MCA concur with current functional imaging data. Bilateral simultaneous repetitive transcranial Doppler monitoring is a sensitive method to detect cerebral perfusion asymmetries caused by hemisphere-specific activation, and thus may be helpful for noninvasive assessment of hemispheric dominance for language.

Postoperative assessment of cerebral blood flow in subarachnoid haemorrhage by means of 99mTc-HMPAO tomography

Regional hypoperfusion is a very frequent complication of subarachnoid haemorrhage (SAH), being related to vasospasm in the majority of cases. Twenty-six patients who were admitted for SAH underwent follow-up with technetium-99m hexamethylpropylene amine oxime single photon emission tomography (SPECT) 3 and 8 days after surgery. Fifteen patients of these had one more examination 15 days after surgery. The degree of hypoperfusion was quantified using an index of asymmetry which allow the comparison of two symmetrical regions of interest (ROIs) on the transaxial slice which presented the greatest perfusion defect. Comparison of CT data, transcranial Doppler data and clinical signs with the perfusion as quantified by 99mTc-HMPAO SPECT indicates that a difference in counts of less than 10% between the two symmetrical ROIs is of no diagnostic value. Follow-up of the brain perfusion clearly shows that the most pronounced hypoperfusion was observed just after surgery, with progressive normalization at 8 and 15 days after surgery. 99mTc-HMPAO SPECT performed 8 days after surgery allows prediction of the clinical outcome. For these reasons, 99mTc-HMPAO SPECT, which is the only method for follow-up of cerebral perfusion in routine clinical practice, should be the first examination to be performed after surgery in patients with SAH.