Automation of lab work, more efficient diagnostic testing, and electronic medical records have transformed the speed and accuracy of medical diagnoses. A vital part of this advancement has been medical imaging. By pinpointing the bleed, tumor, or site of infection, modern medical imaging confirms a physician’s suspicion based on clinical evidence, and in many cases identifies problems that are not detectable by physical examination alone. Four innovative medical imaging modalities are reviewed here: optical coherence Doppler tomography, capsule endoscopy, a single-chip catheter-based device, and positron emission tomography.
Optical coherence Doppler tomography (ODT) is a way to see blood vessels in real-time without piercing the skin. The arteries carry oxygenated blood to our body, and an interruption in this rich blood supply (whether due to a cholesterol plaque growing on the inner wall, a blood clot from the heart clogging the artery, stiffening of the artery due to uncontrolled high blood pressure, or weakness of the artery’s wall because of diabetes) can wreak damage instantly in the case of a stroke, or slowly in the case of decades of uncontrolled hypertension. The adage that prevention is the best medicine rings true today. If physicians could study the health of their patients’ blood vessels in an inexpensive and efficient way (for example, in the office), then there would be a potential to intervene earlier to possibly prevent the heart attack, stroke, or amputated foot. ODT is a technology that can acquire tomographic images (“slices”) of biological tissues at the level of micrometers (one millionth of a meter – the width of human hair is 17 micrometers). Just as ultrasound uses sound waves that strike the target and are deflected back to a receiver, ODT uses light waves. ODT is based on optical coherence tomography (OCT), which uses a reference light and back-scattered light from the tissue to visualize it. The Doppler aspect comes from the scattering of light as blood cells move towards or away from the detection site. The technology has been applied to human health in the early 2000’s, to scan the retina as surveillance for a known complication of diabetes or for the diagnosis of optic nerve injury seen in multiple sclerosis. Recent studies in animals have demonstrated the ability of ODT to visualize blood vessels under the skull. In time, these newer applications will be validated in humans and then applied to the non-invasive visualization of other organs. The maintenance of OCT and ODT devices is important to their proper functioning and should be performed by a trained clinical engineer or technologist.
Not all imaging is non-invasive. The digestive system consists of the esophagus, stomach, small intestine, and large intestine. An upper endoscopy or colonoscopy can be used to look for abnormalities in the stomach/esophagus and colon, respectively. However, sometimes a patient is bleeding and the source is not identified on these two endoscopic approaches. It would be difficult to advance a semi-rigid camera to the middle of the digestive tract. This lack of access led to the innovation of capsule endoscopy, which was approved for use in humans by the Food and Drug Administration (FDA) in 2001. The capsule is the size of a large pill yet contains all the necessary hardware (lens, LEDs, semiconductor imager, battery, transmitter, and antenna) to safely travel through the digestive system, taking snapshots of the small intestine along the journey. Just as pictures from outer space being relayed to NASA on earth, the capsule endoscope transmits the pictures via radio telemetry to a recorder that the patient wears on his/her belt. Within 72 hours, the capsule exits the body. The benefits of this invasive yet safe technology include: that the patient can be at home (saves costs to the healthcare system), doesn’t need to extremely modify his/her activities (patient flexibility), and is able to get a diagnosis of a disease of the small intestine that was previously not possible. The capsule costs approximately $500, which is less expensive than a standard colonoscopy. This LED many technologists, engineers, and physicians to investigate follow-up technologies that may be able to serve as substitutes for the traditional colonoscopy or CT scan of the abdomen. As a result, the FDA approved the PillCam® COLON 2 Capsule Endoscopy System in January 2014.
Capsule endoscopy takes advantage of one of the body’s natural conduits, the digestive system. Cardiologists would benefit from being able to visualize small blood vessels from the inside as well. This would enable to them to understand patient’s risk factors for cardiovascular disease and intervene preemptively in cases of eminent danger (for example, a fatal heart attack due to a gradually narrowing left anterior descending coronary artery, the so-called “widow maker,” that finally seals off). Just as gastroenterologists use the GI system as their atlas, cardiologists use the complex highway of blood vessels to navigate their way to the heart. Using the gold-standard technique of coronary angiography to tunnel a catheter (with a camera located at the tip) from the femoral artery to the coronary arteries on the surface of the heart, the blood flow through these critical vessels can be visualized. A breakthrough technology published this year may lead to real-time 3D images from within the heart and blood vessels. Researchers at the Georgia Institute of Technology have developed a 1.4 millimeter chip that contains ultrasound transducers and on-board processing hardware that, when attached to the angiography catheter, could provide such real-time images. This technology is in the very early stages and next needs to be studied in animal models before it can be tested in humans. Thus, this is yet another imaging modality that may become commonplace in the future and save thousands of lives through earlier detection of subtler abnormalities.
Positron emission tomography (PET) is a nuclear medicine study in which radioisotope-labeled tracers are injected into the bloodstream and their decay signal (positron emission) is detected by the PET scanner, a gamma ray detector. Unlike MRI and CT scans, which provide static anatomic images, PET scans provide functional imaging. In other words, they reveal areas of the body that are consuming a disproportionately increased amount of the radiotracer. PET is most commonly used in the field of cancer and FDG, a radio-labeled form of glucose, is most often used. Because tumor cells are rapidly dividing and need to consume a lot of energy (in the form of glucose) to grow and form the cancer, the PET scan of a patient with a lung tumor would “light up” in the area of the lung containing the tumor. This area may not be seen on anatomical MRI or CT imaging alone. Thus, the combination of the structural and functional information can improve the accuracy of a cancer diagnosis. Based on its size, the brain consumes more glucose than other organs. Therefore, using FDG-PET to diagnose brain tumors is difficult because it is not clear whether the increased radiotracer uptake is due to cancer or just an area of the brain that is active. New radiotracers have been developed to help diagnose and monitor patients with brain tumors. For example, tracers based on the amino acid dopamine are now being studied in human clinical trials. Because dopamine is not used up at such a high rate by the brain as is glucose, it may prove to be a more specific way of distinguishing brain cancer from the normal background brain activity.
In summary, these four imaging methods provide a small insight into the exciting role of innovation and engineering in the field of medicine for the betterment of human health. Although they are in varying stages of development and clinical application, they represent the potential for innovation when biomedical technology is applied to the human body.
References
ODT
Capsule Endoscopy
Single-chip catheter-based device
PET
