Abstract
Photoacoustic imaging has broad clinical potential to enhance prostate cancer detection and treatment, yet it is challenged by the lack of minimally invasive, deeply penetrating light delivery methods that provide sufficient visualization of targets (e.g., tumors, contrast agents, brachytherapy seeds). We constructed a side-firing fiber prototype for transurethral photoacoustic imaging of prostates with a dual-array (linear and curvilinear) transrectal ultrasound probe. A method to calculate the surface area and, thereby, estimate the laser fluence at this fiber tip was derived, validated, applied to various design parameters, and used as an input to three-dimensional Monte Carlo simulations. Brachytherapy seeds implanted in phantom, ex vivo, and in vivo canine prostates at radial distances of 5 to 30 mm from the urethra were imaged with the fiber prototype transmitting 1064 nm wavelength light with 2 to 8 mJ pulse energy. Prebeamformed images were displayed in real time at a rate of 3 to 5 frames per second to guide fiber placement and beamformed offline. A conventional delay-and-sum beamformer provided decreasing seed contrast (23 to 9 dB) with increasing urethra-to-target distance, while the short-lag spatial coherence beamformer provided improved and relatively constant seed contrast (28 to 32 dB) regardless of distance, thus improving multitarget visualization in single and combined curvilinear images acquired with the fiber rotating and the probe fixed. The proposed light delivery and beamforming methods promise to improve key prostate cancer detection and treatment strategies.
Highlights
Photoacoustic imaging has the potential to improve prostate cancer screening and treatment procedures,[1,2,3] when visualization with conventional ultrasound (US) is poor due to factors such as suboptimal acoustic contrast between normal and malignant tissues, acoustic clutter, and shadow artifacts.[4,5,6,7,8,9] It is implemented by transmitting nanosecond laser pulses, which cause targets that have higher optical absorption than surrounding tissue to preferentially absorb the light, undergo thermoelastic expansion, and thereby generate pressure transients that are detected by a conventional US probe
We previously proposed and demonstrated the feasibility of an interstitial, transperineal light delivery method to visualize prostate brachytherapy seeds in phantoms, ex vivo liver tissue, and in vivo prostates.[17,28]
Limits to the maximum achievable surface area and energy limit in Figs. 4(a) and 4(b), respectively, include the size of the urethra and the inner diameter of the catheter when present (e.g., 6 and 2 mm, respectively), as well as additional mechanical design constraints and ergonomic considerations, which are beyond the scope of this paper
Summary
Photoacoustic imaging has the potential to improve prostate cancer screening and treatment procedures,[1,2,3] when visualization with conventional ultrasound (US) is poor due to factors such as suboptimal acoustic contrast between normal and malignant tissues, acoustic clutter, and shadow artifacts.[4,5,6,7,8,9] It is implemented by transmitting nanosecond laser pulses, which cause targets that have higher optical absorption than surrounding tissue (e.g., highly vascularized cancerous tumors[10] or metallic brachytherapy seeds11,12) to preferentially absorb the light, undergo thermoelastic expansion, and thereby generate pressure transients that are detected by a conventional US probe. It is minimally invasive, as the optics could be integrated with current transrectal US probes, this approach is limited by poor light penetration through the rectal wall.[30,31] As an alternative, we previously proposed and demonstrated the feasibility of an interstitial, transperineal light delivery method to visualize prostate brachytherapy seeds in phantoms, ex vivo liver tissue, and in vivo prostates.[17,28] This option does not necessarily suffer from poor light penetration, as the light source may be positioned directly next to a target of interest, and it can be well integrated with brachytherapy and biopsy procedures, which already require direct access to prostatic tissue Limitations of this interstitial photoacoustic imaging approach include artifacts caused by a photoacoustic effect at the fiber tip and additional acoustic interactions (e.g., echo clutter).[17]
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