TRANSIENT-RESPONSE HARMONIC IMAGING - AN ULTRASOUND TECHNIQUE RELATEDTO BRAIN PERFUSION

Background and Purpose-Gray-scale harmonic imaging is the first method to visualize blood perfusion and capillary blood flow with ultrasound after intravenous contrast agent application. The purpose of the pres ent study was to evaluate the potential of transient response second h armonic imaging (TRsHI) to assess normal echo contrast characteristics in different brain areas by transcranial ultrasound. Methods-In 18 pa tients without cerebrovascular diseases, TRsHI examinations were perfo rmed bilaterally with the use of the transtemporal approach after appl ication of 6.5 mt of a galactose-based microbubble suspension (400 mg/ mL). The transmission rate was once every 4 cardiac cycles. Regional c erebral contrast was visually assessed and then quantified off-line wi th the use of time-intensity curves. In 4 different regions of interes t (ROI) (posterior part of the thalamus [ROIa], anterior part of the t halamus [ROIb], lentiform nucleus [ROIc], and white matter [ROId]), th e following parameters were evaluated: peak intensity, area under the curve (AUC), and time to peak intensity. AUC ratios for ROIc/a, d/a, c /b, and d/b were calculated. Results-In all patients parenchymal contr ast enhancement was visually detectable. One hundred thirty-one charac teristic time-intensity curves (baseline phase, peak contrast intensit y, slow washout phase) were demonstrable in 144 ROIs. In ROIc and ROId characteristic contrast curves could be observed most frequently (68/ 72 examinations), whereas time-intensity curves in ROIa and ROIb could not be evaluated because of inadequate contrast enhancement in 9 of 7 2 examinations. Time to peak intensity varied between 20 and 52 cardia c cycles; in 1 patient it was 88 cardiac cycles. In all individuals AU Cs and in 16 of 18 subjects peak intensity in ROIc and ROId showed a 2 - to 10-fold increase compared with ROIa and ROIb. In no examination d id AUC ratios show a >2-fold side difference irrespective of the ROI. Conclusions-The present study demonstrates for the first time that TRs HI produces accurate contrast in different brain areas and represents an ultrasonic tool related to brain perfusion. Absolute values of quan titative parameters show high variations caused by different temporal bone thicknesses and a complex relationship between echo contrast conc entrations and measurements of optic intensities. Ratios between diffe rent ROIs help to compare contrast enhancement in different brain area s. Furthermore, because of the fact that attenuation of contrast enhan cement in TRsHI depends strictly on the insonation depth, harmonic ima ging studies of brain perfusion cannot be compared directly with other imaging techniques such as positron emission tomography.