Changing field size in medical imaging means adjusting how large an area a scanner, probe, or beam captures or targets. The exact method depends on the technology: X-ray and radiation therapy machines use physical collimators, MRI and CT scanners use software parameters tied to gradient strength and reconstruction algorithms, and ultrasound machines let you adjust depth and sector width with on-screen controls. In every case, the goal is the same: see or treat exactly the area you need while minimizing unnecessary exposure or improving image quality.
X-Ray and Radiation Therapy: Collimators
In X-ray imaging and radiation therapy, field size is controlled by collimators, which are adjustable metal plates (called jaws) that sit between the radiation source and the patient. These jaws slide open or closed to create a rectangular beam of the desired dimensions. Most machines have two pairs of jaws oriented at right angles, so you can independently set the width and length of the field.
Modern radiation therapy takes this further with multileaf collimators (MLCs), which are banks of thin metal strips (leaves) that can each move independently. By programming each leaf to a specific position, treatment planners create irregularly shaped fields that conform tightly to a tumor’s outline rather than being limited to simple rectangles. Some manufacturers mount these leaves below the standard jaws as a third layer of shaping, while others replace one set of jaws entirely with the MLC system. If a leaf malfunctions, it can often be manually repositioned so the machine stays in service.
Getting the field size right matters for patient dose. In shoulder X-rays, for example, switching from standard collimation to a smaller rectangular field reduced effective radiation dose by 27.3%. On the other hand, using a diamond-shaped collimation pattern actually increased dose by 45.5% compared to the standard setting, because more tissue outside the target area was exposed. Tighter collimation also improves image contrast by reducing scattered radiation that degrades the picture.
MRI: Field of View Settings
On an MRI scanner, field size is called the field of view (FOV), and it’s adjusted entirely through software. The FOV determines how large a slice of anatomy appears in the final image. It’s controlled by the strength and timing of magnetic field gradients, which are the rapidly switching magnetic fields that encode spatial information into the MRI signal.
Reducing the FOV while keeping the same number of data points (the sampling matrix) improves spatial resolution, because each pixel now represents a smaller piece of tissue. A typical scan might use a 30 cm FOV with a 256-by-128 matrix. Shrinking that FOV to 26 cm with the same matrix gives you finer detail in the lateral direction without changing scan time. The tradeoff: if you make the FOV too small, anatomy outside the field can “wrap” into the image, creating an artifact called aliasing.
CT Scans: Two Types of Field Size
CT scanners have two distinct field-size settings that serve different purposes. The scan field of view (SFOV) is set before scanning and determines how wide the X-ray fan beam spreads and how many detector elements collect data. It essentially defines the physical area being scanned. A larger SFOV is needed for bigger patients or for imaging the full torso, while a smaller SFOV works for a head or extremity.
The display field of view (DFOV) can be adjusted after the scan is complete. It controls how much of the collected data is shown on screen, and it directly affects image sharpness. The pixel size in a CT image equals the DFOV divided by the matrix size (typically 512 by 512). A DFOV of 25 cm gives you smaller pixels and finer detail than a DFOV of 50 cm using the same matrix. Radiologists routinely adjust the DFOV to zoom in on a specific structure, like a single vertebra or a small nodule, without needing to rescan the patient.
Ultrasound: Depth and Sector Width
Ultrasound field size is adjusted with two main controls: depth and sector width. Depth determines how far into the body the sound waves travel before the machine listens for returning echoes. Increasing depth gives a broader anatomical overview but slows the frame rate, because the machine has to wait longer for each echo to return before sending the next pulse. Reducing depth does the opposite: you see less anatomy, but the image updates faster and moving structures like heart valves appear smoother.
Sector width (available on phased-array and convex probes) controls how wide the fan-shaped image spreads. Narrowing the sector concentrates the sound beam into a smaller area, which improves lateral resolution and boosts the frame rate since fewer scan lines need to be generated. This is particularly useful when imaging fast-moving structures where temporal resolution matters. Newer machines with higher processing power handle wide sectors better than older equipment, but narrowing the field still provides a noticeable quality improvement when you need to focus on a specific region.
Adding more focal zones (points where the beam is electronically tightened for sharper detail) also affects the effective field. Each additional focus requires a separate transmit-receive cycle for that depth, which further reduces the frame rate.
Microscopy: Eyepiece and Objective
Under a microscope, the field of view is the circular area you see when you look through the eyepiece, and its size is determined by a simple formula: divide the eyepiece’s field number by the objective magnification. The field number is the diameter (in millimeters) of the visible area at 1x magnification, and it’s usually printed on the eyepiece barrel.
With a field number of 22 mm and a 40x objective, the visible diameter is 0.55 mm, giving a viewable area of about 0.238 square millimeters. Switching to a 10x objective with the same eyepiece expands the field to 2.2 mm across. If your microscope has an additional magnification changer (sometimes called a Bertrand lens or intermediate magnification), you divide by that factor too. The extended formula is: diameter equals the field number divided by the product of objective magnification and any extra magnification factor.
Visual Field Testing in Ophthalmology
In eye exams that map peripheral vision, the “field size” refers to the size of the light stimulus flashed at different locations. Goldmann perimetry uses five standardized stimulus sizes labeled I through V, with size I being the smallest and size V the largest. The choice of stimulus size changes what the test can detect.
Smaller stimuli are better at catching early disease. This is because of a principle called spatial summation: the eye combines light input across a small area (the critical area), and when a stimulus falls within that zone, any damage to retinal cells produces a larger measurable drop in sensitivity. Studies show that stimuli near or within this critical area reveal greater sensitivity loss in conditions like glaucoma, even though test results are slightly more variable. Larger stimuli are more useful for tracking advanced disease, where sensitivity is already significantly reduced and a bigger target produces more reliable measurements over time.