Development of a new system for measuring skull bone thickness by the pulse compression method

Precise correction for gamma ray attenuation in the skull bone is essential when obtaining quantitative single-photon emission computed tomography (SP ECT) images of the brain. Correction for gamma ray attenuation is approxima tely proportional to the density and thickness of the bone under investigat ion. Therefore, if the acoustic impedance and speed of sound in the bone ar e measurable using ultrasonic techniques. then the density and thickness of the bone sample can be calculated. We propose a method for determining sim ultaneously the thickness of and speed of sound in the skull bone through i n vivo measurements, the principle being that the time delay between two di screte transmission paths will yield the desired information. Thus. it is n ecessary to distinguish between the responses of these two transmission pat hs. The proposed method incorporates the pulse compression method to measur e the time delay between detected transmission paths and reduce dispersion in the transmission line, thus increasing the signal-to-noise (S/N) ratio a nd significantly improving measurement accuracy. Using the proposed pulse c ompression method, the speed of sound in a number of materials was obtained . with the following results: 5 mm-thick poly methyl methacrylate(PMMA) pla te, 2620 +/- 130 m/s, compact bone. 3820 +/- 250 m/s; spongy bone, 1930 +/- 90 m/s. The errors in thickness indicated by these measurements were 5.6%. 7.2% and 12% for the PMMA plate. compact bone and spongy bone, respectivel y. Thus, using a thin transmission line, the proposed method makes it possi ble to determine the thickness of a bone sample with sufficient accuracy. I t is anticipated that this method, which is based on ultrasonic measurement s, will be useful for application in brain SPECT.