Ultrasonic properties of fresh and formalin fixed brain tissue

Recent advances in transcranial ultrasound have generated increased interest in the ultrasonic properties of brain. For in vitro laboratory studies of brain tissue, it is convenient to use specimens preserved by formalin fixation. The goal of the present study was to investigate the effect of formalin fixation on the ultrasonic properties of brain. 1-cm thick slices of tissue were prepared from 9 bovine brains. A total of 28 slices from two anatomic planes (16 sagittal, 12 coronal) were studied. Tissue specimens were scanned in phosphate buffered saline at room temperature with a 5 MHz transducer. Scans were performed on fresh tissue and repeated one month after formalin fixation. The speed of sound (SOS) and frequency slope of attenuation (FSA) of ultrasonic pulses propagated through the tissue were determined at every measurement site. The measured values were averaged over all sites on all specimens to obtain the following results expressed as mean ± standard deviation. SOS was (1522 ± 3) m/s before fixation and (1538 ± 12) m/s after fixation. FSA was (0.438 ± 0.064) dB/cm/MHz before fixation and (0.493 ± 0.113) dB/cm/MHz after fixation. Thus, formalin fixation produced approximately a 1% increase in SOS and a 13% increase in FSA.