A gamma camera is an imaging device used in nuclear medicine to create pictures of the inside of the body. Unlike X-rays or CT scans, which primarily show anatomy and structure, the gamma camera visualizes the function of organs and tissues. It achieves this by detecting radiation that originates from within the patient. The camera, sometimes called a scintillation camera, tracks the distribution of a radioactive compound administered to the patient, allowing clinicians to observe biological processes.
How Gamma Cameras Function
The process begins with the administration of a radiotracer, which is a radioactive pharmaceutical designed to target a specific organ or tissue in the body. This compound emits gamma rays as it travels through and accumulates in the target area. The gamma camera detects these emitted high-energy photons to map out where the radiotracer is concentrated.
The first component the gamma rays encounter is the collimator, a thick plate made of a dense material like lead containing thousands of tiny holes. The collimator acts as a filter, ensuring that only gamma rays traveling nearly parallel to the holes can pass through to the detector. This filtering prevents scattered or angled rays from blurring the picture, maintaining spatial resolution.
Once past the collimator, the gamma rays strike the scintillation crystal, typically a large, single crystal of sodium iodide doped with thallium. The crystal converts the energy of the gamma ray photon into a burst of thousands of lower-energy visible light photons, a process known as scintillation. The intensity and location of these faint light flashes are then recorded.
The light pulses are immediately detected by a tightly packed array of Photomultiplier Tubes (PMTs) positioned directly behind the crystal. Each PMT converts the light signal into an electrical pulse and amplifies it significantly. The coordinated output of the PMTs determines the exact location and energy of the original gamma ray event.
These electrical signals are then sent to a computer system for processing. The computer takes the positional and energy information from all the detected gamma rays to construct a two-dimensional image. Over time, the accumulation of thousands of these individual recorded events creates a detailed map showing the concentration of the radiotracer within the body, which reflects the biological function of the organs.
Primary Medical Applications
Gamma camera imaging provides unique insights into physiological function. One common use is in skeletal imaging, often called a bone scan, where the radiotracer accumulates in areas of high bone turnover. This technique is highly sensitive for detecting subtle fractures, infection, or the spread of cancer to the bones, known as metastasis.
Cardiac imaging utilizes the gamma camera to assess blood flow and evaluate the heart’s pumping function. Scans like a Multigated Acquisition (MUGA) scan or a myocardial perfusion stress test help doctors determine the extent of damage from a heart attack or identify areas not receiving adequate blood supply. These functional images can quantify the heart’s efficiency.
The camera is also routinely employed for imaging the endocrine system, particularly the thyroid and parathyroid glands. By using tracers that the glands naturally absorb, doctors can assess gland function, identify nodules, or pinpoint the location of an overactive parathyroid gland. This helps in diagnosing conditions ranging from hyperthyroidism to specific tumors.
Gamma cameras are used in various neurological studies to assess brain function. Tracers can visualize regional cerebral blood flow, which is helpful in evaluating conditions like stroke, epilepsy, and certain types of dementia. The functional information provides context on neurological activity that complements anatomical scans of the brain.
The Patient Experience
A gamma camera procedure begins with preparation, which varies depending on the specific type of scan being performed. Patients may be asked to fast or temporarily stop taking certain medications that could interfere with the radiotracer’s distribution. This ensures the tracer correctly targets the intended organ or tissue for accurate results.
The radiotracer is usually administered through a simple intravenous injection, though some procedures require swallowing the compound or inhaling it as a gas. Following administration, there is often a required waiting period, ranging from a few minutes to several hours. This time allows the tracer to travel and fully accumulate in the area of interest, tailored to the specific radiotracer used.
During the actual imaging, the patient lies still on an examination table while the camera head is positioned close to the body. The camera may remain stationary or slowly move around the patient to capture images from different angles. Sometimes a full rotation is performed for a 3D image known as SPECT. The imaging procedure is painless and typically lasts between 30 and 90 minutes.
The radiotracer dose is carefully calculated to be as low as reasonably achievable while still providing diagnostic quality images. The radiation exposure is minimal and comparable to many other common imaging tests. After the scan, the radiotracer naturally decays and is quickly eliminated from the body, typically within 24 hours.