Other: In vivo imaging

* Patients with distal rectal lesions (benign or malignant tumors within 15cm of the anal verge) will be enrolled for the in vivo imaging portion of the study~* Participation will include an intraoperative, in vivo evaluation of the tumor with a novel endorectal photoacoustic ultrasound probe. Following the induction of anesthesia, patients will undergo a 20-minute endorectal imaging evaluation performed by their colorectal surgeon. After imaging, the patient will then undergo standard-of-care surgical resection of the rectum. The resection specimen may then be imaged ex vivo as deemed necessary.~* For this portion of the study, enrollment will be limited to a total of 86 participants

Experimental: Reader Team

3 surgeons and 1 radiologist will be initially trained to grade lesions using the PAM/US image from the first set of 30 patients (Group 1). They will then use the PAM/US images and machine learning model to grade lesions on the images obtained from the second set of 56 patients (Group 2).