Fundamental Ultrasonic Properties of Fresh and Formalin Fixed Brain Visualized as Parametric Images

The brain is a complex organ with multiple tissue types and a complicated morphology of lobes, folds, ventricles and other structures. The goal of this study was to create detailed parametric images of brain tissue before and after formalin fixation for four ultrasonic parameters: speed of sound (SOS), frequency slope of attenuation (FSA), integrated backscatter coefficient (IBC) and apparent integrated backscatter (AIB).

Twenty-three, 1-cm thick slices of brain tissue were prepared from the sagittal and coronal planes of nine bovine brains. Ultrasonic measurements were performed using an immersion scanning system equipped with a 5 MHz focused transducer moved in 615 μm steps.

Measured values, reported as mean ± standard deviation (representing variation between specimen means) averaged over all measurements on all specimens of fresh tissue, were (1535 ± 2) m/s for SOS, (0.546 ± 0.037) dB/cm/MHz for FSA, (0.402 ± 0.165) × 10^-3 cm^-1 str^-1 for IBC and (-60.1 ± 1.1) dB for AIB measured relative to a planar glass reflector. Regions of white matter were characterized by higher values of SOS and FSA, and lower values of AIB and IBC. Formalin fixation caused up to a 0.6% increase in SOS, up to a 2% increase in AIB, up to a 20% increase in FSA and up to a 55% increase in IBC averaged over all measurements on all specimens.

Tissue structures and white matter were clearly distinguishable in most parametric images. Formalin fixation produced significant changes in all four ultrasonic parameters.