21 Jun 2019
University of Toronto develops dual-wavelength variant of PA radar technique.
Spotting the plaques most likely to rupture and cause thrombosis is particularly important, but imaging them with techniques such as intravascular OCT and ultrasound has been hampered by the penetration depths available and the image quality achieved.
A team at the University of Toronto has developed a potential alternative, by modifying the photoacoustic radar (PAR) technique so as make it particularly sensitive and specific to imaging of cholesterol. The work was reported in Journal of Biomedical Optics.
Photoacoustic techniques commonly use a high-power pulsed near-IR laser to excite a subsurface target, generating a brief ultrasound pulse that can be used to image organic material. PAR is a variant approach based on frequency-modulated chirped optical excitation, using low-power continuous-wave (CW) lasers instead.
Frequency-domain signal processing based on radar pulse compression techniques can then deliver depth-resolved images of tissue chromophores, potentially with high signal-to-noise ratios at low power irradiation, and micrometer axial resolution.
In 2014 The Toronto team reported the development of single-frequency wavelength-modulated differential PA spectroscopy utilizing a CW optical source, with the laser intensity modulated by a frequency-swept waveform. That platform was designed to assist early-stage breast cancer detection and monitoring of tissue hypoxia.
The new research extends this approach and describes a platform using two low-power continuous-wave optical sources at 1210 and 970 nanometers in a differential manner. The first wavelength corresponds to a significant vibrational overtone in cholesterol molecules, while the second is a main peak of normal arterial tissues.
Applying PAR theory to the results leads to an imaging modality that is both sensitive and specific to the cholesterol in the blood vessels.
Waveform engineering possibilities
"Intravascular differential PAR (IV-DPAR) inherits general traits and characteristics of photoacoustic radar in terms of signal generation and processing," commented the team in its published paper. "The novelty is the use of a second wavelength in real time with identical chirp modulation at a specific optical phase difference."
Trials on an atherosclerotic artery phantom prepared from porcine tissues found that the team's multi-frequency IV-DPAR system could readily be coregistered with a conventional intravascular ultrasound platform, sharing the same transducer and instrumentation for their signal acquisition process.
The new method proved able to detect cholesterol clusters much more effectively than the ultrasound alone, which lacks sensitivity and specificity to those molecules, and the two coregistered modalities could produce a complete cross-sectional image of the phantom with accurate location and depth-profile information of two arbitrary plaque models.
"Using compact, low cost and low power CW lasers as optical sources, IV-DPAR can be suitable for clinical uses," noted the team. "Due to high pulse-repetition frequency of CW lasers, IV-DPAR is much more likely to make dynamic PA imaging practical with reduced motion artifacts compared to its pulse-based counterparts. IV-DPAR is an excellent example of the many waveform engineering possibilities for further advances in atherosclerosis imaging, beyond the physical limitations of the conventional PAR and pulse-based PA counterparts."