Broadband & volumetric thermoacoustic imaging of fresh human prostates using a clinical array

Prostate cancer is difficult to image because contrast mechanisms of current modalities fail to distinguish healthy from cancerous prostatic tissue. Prostate cancer decimates citrate production and therefore overall ionic content of prostatic fluids. Amplitude of thermoacoustic pulses induced by very high frequency (VHF) irradiation increases with ionic content. Superior depth penetration over photo- and microwaveinduced thermoacoustic imaging is achieved at the cost of signal strength precisely because VHF energy loss is low in soft tissue. VHF-induced thermoacoustic imaging must overcome several challenges, including development of ultrasound transducer arrays suitable for clinical use. Sensitive single element ultrasound transducers yield good SNR after low noise amplification (54 dB) and signal averaging. Clinical ultrasound transducer arrays are less sensitive, particularly at frequencies below 1 MHz, which dominate power spectra of VHF-induced thermoacoustic pulses. Volumetric reconstruction of a fresh human prostate specimen in a 4.5 x 5.5 x 5.5 cm
³
volume is displayed. We demonstrate that a clinical ultrasound array is sufficiently sensitive to high frequencies, but not low frequencies, for quantitative VHF-induced thermoacoustic computerized tomography (TCT). Simultaneous acquisitions by a clinical array with 1-4 MHz bandwidth and a single element transducer sensitive to kHz frequencies are compared and contrasted in images of resolution phantoms and a fresh human prostate. Combining images from both single element and phased array transducers retains superior image resolution of the phased array and contrast from the single element transducer. This demonstrates the importance of acquiring kilohertz frequencies for accurate image reconstruction over a large field of view, and will be useful in designing arrays suitable for a clinical prototype.