It has been a few years since researchers have begun exploring the possibility of medical imaging using Cerenkov radiation. Cerenkov radiations are electromagnetic radiations produced when charged particles travel through a medium(dielectric) with speeds greater than the phase velocity of light in that particular medium. Cerenkov Luminescence Imaging (CLI) utilizes such emissions given by the radiotracers injected into the human body to detect tumors and malignant cells.
The latest news describes the success of clinical trials using a prototype of the imaging machine. The setup essentially consists of a Clinical Cerenkov fiberscope with a light-proof enclosure for patient imaging. The studies were published in Nature Biomedical Engineering by the group led by Edwin C. Pratt of Memorial Sloan Kettering Cancer Center, Manhattan, New York.
Cerenkov Luminescence Imaging (CLI)
Every year a large number of people, especially under oncology, undergo nuclear medicine both for diagnosis and treatment. Diagnosis involves the use of radioactive tracers emitting electromagnetic radiations that are captured as two-dimensional images or three-dimensional tomography. Of the available imaging techniques, positron emission tomography (PET) is currently the most acceptable tool.
According to IAEA, a radioactive tracer is a chemical compound in which one or more atoms have been replaced by a radioisotope. Monitoring its radioactive decay, a radiotracer can be used to explore the mechanism of chemical reactions .The commonly used radioisotopes – positron emitters – 18F and 13N as well as beta emitters 32P or 90Y produces measurable Cerenkov radiations when it enters the tissue. Administering these radiotracers to patients induce luminescence in tissues and the images show contrast depending on the cell properties. CLI uses Cerenkov light emitted in tissues to image the body. With advanced camera techniques and the use of high sensitivity charge coupled devices (CCD), CLI can detect even earlier stages of malignancy. Due to the low light level, CLI requires complete removal of surrounding lights to obtain images and the researchers could solve this problem with the help of a light-proof enclosure.
A cost effective alternative
Even when PET is a widely accepted tool for diagnosis, it requires dedicated, sophisticated infrastructure and is still inaccessible to a large section of the population. In low-income and developing nations the costs of PET are far beyond the economic capacity of the common. CLI can be an effective alternative to techniques like PET and single-photon emission computed tomography (SPECT). The portability of CL cameras and the array of low-cost radioisotopes compatible with CLI helps reduce the cost drastically. This paves the way for an affordable nuclear imaging modality.
Fiberscope with lightproof enclosure
In the first clinical trials, the presence of environmental and surrounding lights hindered the imaging using CLI. In the present clinical trials, the group has developed an imaging machine that practically masks all the surrounding light with the help of a light-proof enclosure. The enclosure also helps in lowering the image noise and is equipped for imaging in even patients with extreme obesity or limited mobility and those with fear of enclosed spaces. The machine calls for ex-vivo imaging. The Cerenkov light from the patient can be recorded outside the enclosure thereby easing the patient. The enclosure is connected to a fiberscope that uses optical fibers to record the images. Our tissues generally absorb the blue end of the visible spectrum and ultraviolet radiations. Thus the emitted Cerenkov luminescence will be on the red end of the visible spectrum.
Clinical trials
The group between May 2018 and March 2020 conducted clinical trials on 96 patients, with over 106 imaging sessions, who consented to image with the Cherenkov fibrescope. These trials were carried out using five different radiotracers and most of them provided satisfactory results.
CLI could identify the tumors in above 90% of the patients accurately. It could even identify superficial lesions. The results and images were compared with the standard-of-care imaging techniques like PET, SPECT, and planar imaging. This is the largest of its kind trial to date where participants with different conditions including prostate cancer and lymphoma were imaged and correlated with standard techniques. The 90% correlation led to a verdict that classifies CLI as acceptable with good agreement.
CLI is found to be a more versatile technique than PET for it responds to almost every radioisotope which is not the case with PET. PET is highly specific and requires dedicated, time-consuming, and costly equipment. CLI also detects radiopharmaceuticals like 223RaCl2 for which none of the present techniques is adequate or feasible.
But CLI images do not have the precision and accuracy of PET/SPECT scans. This still doesn’t limit its applicability as CLI is the best choice as an initial diagnostic tool. The low cost of this imaging technique can ensure the facility of nuclear medicine and diagnostics to larger sections of the society to whom these are currently inaccessible. CLI is so versatile that it allows imaging in about 5 to 15 minutes. If anything is found abnormal the patient can be referred for more detailed procedures like PET. This will potentially enable more hospitals to incorporate nuclear medicine. This is crucial for about 70% of deaths due to cancer worldwide happen in low-income nations with limited access to healthcare.
You may also be interested in reading about the advancements in the traditional x-ray imaging techniques: “High-Resolution X-Ray Imaging“
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