A standard computed tomography (CT) scan uses X-rays to generate thin cross-sectional images, which are compiled into detailed three-dimensional (3D) views. The 4D CT scan incorporates time as the fourth dimension. This specialized procedure captures a sequence of 3D images over a designated period, allowing medical professionals to observe dynamic processes within the body.
What Makes a CT Scan 4D
The distinction of a 4D CT scan is its ability to visualize physiological motion, which is crucial for imaging constantly moving organs. For example, the lungs and tumors shift position with every breath. By tracking these internal dynamics, the scan provides a “movie” instead of a static picture.
This technique is commonly used in radiation oncology for treatment planning, a process called respiratory gating. The 4D data maps the entire path of a tumor through the full breathing cycle. This precision allows oncologists to deliver radiation therapy only when the tumor is in a targetable position, protecting surrounding healthy tissue. Other applications include tracking the washout of contrast agents, such as in parathyroid studies, where elapsed time after injection is the fourth dimension.
The Time Required for Image Acquisition
The actual time spent on the scanner table is generally longer than a standard 3D CT scan. For complex, motion-tracking studies, such as lung tumor planning, image acquisition typically takes between 15 and 45 minutes. This duration is necessary because the scanner must capture multiple, sequential scans to cover the full range of motion.
The process often uses a breathing monitor synchronized with the CT machine, which tags each image with a specific point in the respiratory cycle. The raw data is then sorted into ten different “phases,” representing 0% through 90% of the patient’s breathing pattern. To collect this dataset, the patient must follow specific breathing instructions, which may include holding their breath briefly or breathing steadily throughout the acquisition.
Variables That Affect Scan Duration
Several patient and technical factors cause the image acquisition time to vary. The specific body region being studied is one factor; a parathyroid 4D CT, which tracks contrast agent uptake, might require only five minutes of scanning time. Conversely, a respiratory-gated study of the liver or lung requires a more extensive acquisition period to ensure complete coverage of organ movement.
The patient’s ability to maintain a consistent breathing pattern is another major variable. Irregular or shallow breathing can lead to motion artifacts, blurring the image and requiring the technologist to repeat a portion of the scan. Furthermore, the use of intravenous contrast agents requires precise timing relative to the injection. For example, parathyroid studies require sequential scans during pre-contrast, arterial, and venous phases, adding time to the overall acquisition process.
Total Appointment Time Commitment
The total time a patient spends at the facility is substantially longer than the image acquisition time. Patients should plan for a total appointment commitment ranging from 1.5 to 3 hours from arrival to departure. This duration accounts for all necessary logistical steps surrounding the scan.
Initial steps include check-in, paperwork, and changing into a hospital gown. If intravenous contrast is required, a nurse or technologist must place an IV line, review safety protocols, and allow time for the contrast to be prepared and administered. After image acquisition, the patient may need a short period of post-scan monitoring, especially if contrast was used, before the IV is removed and they are cleared to leave.