Approximately 8,000 pituitary tumors are diagnosed each year in the United States, causing a variety of hormonal complications, compressing critical nerves and arteries at the base of the brain, and creating a potential for vision loss. Transsphenoidal surgery is the most common method for removal of pituitary tumors to reverse endocrine problems and restore normal hormone balance. This is a minimally-invasive procedure in which instruments such as a drill for sphenoidal bone removal and a curette for tumor resection are passed through the nostrils and nasal septum to access the sphenoid sinus and resect the tumor. In endoscopic endonasal transsphenoidal surgery, an endoscope is inserted through an incision at the rear of the nasal septum, for visualization of the surgical field, as depicted in FIG. 1.
Injury to the carotid arteries, which are located behind the sphenoid on either side of the pea-sized pituitary gland, is a significant complication of transsphenoidal surgery that causes severe blood loss, stroke, or death. It may be treated with emergency interventions, albeit with a high risk of irreversible neurological damage. This complication occurs most frequently with novice surgeons who have performed fewer than 200-500 of these surgeries and thus are not sufficiently familiar with potential variations in the anatomy surrounding the pituitary gland. In addition, this procedure is particularly challenging in pediatric patients who are born with small nasal cavities that mainly develop into their full size after puberty. Approximately 75% of hospitals in the country treat 2-25 cases annually, excluding high-volume institutions like the Johns Hopkins Hospital (a pioneering institution of transsphenoidal surgeries), where neurosurgeons treat 100-150 cases per year. Thus, there are generally limited opportunities for novice surgeons to gain necessary caseload experience.
The availability of imaging methods for localizing blood vessels during endonasal surgery would assist with reducing the occurrence of carotid artery injury. Intraoperative CT may be used for guidance of the bony anatomy surrounding the pituitary tumor, however, it does a poor job of visualizing blood vessels and incurs the risks associated with radiation exposure. Magnetic resonance angiography is another option, but it is costly and patients with pacemakers or metal implants are not suitable candidates for this approach. In addition, these imaging modalities are not quite real-time as one volumetric reconstruction could take as long as 20 minutes. Transcranial ultrasound is a potential option, but it requires low transmit frequencies for skull penetration, which translates to poor spatial resolution and a necessity for expert sonographers to interpret images.
Real-time photoacoustic imaging is a faster, safer, less expensive option which generates images by emitting nanosecond light pulses from a laser. When the light irradiates a target, such as bone or vessels, the target absorbs the light, according to its optical absorption spectrum. Optical absorption causes thermoelastic expansion and generates acoustic waves that are detectable with an ultrasound transducer. Photoacoustic imaging is advantageous over conventional ultrasound imaging because there is less acoustic interaction with the skull. The acoustic waves are only required to pass through the skull one time, rather than twice as in pulse-echo ultrasound and as a result, the waves are less susceptible to the sound scattering and aberrations that occur when they encounter the skull.
One challenge with conventional photoacoustic imaging methods is the diminishing light penetration, signal-to-noise ratios, and signal contrast as local laser fluence decreases. This is particularly challenging for transcranial photoacoustic imaging, given the expected poor signal-to-noise ratios due to light obstruction and sound scattering caused by the presence of sphenoidal and temporal bones, respectively.
It would therefore be advantageous to provide a safe, effective method and system for transcranial photoacoustic imaging.