Gantry (medical imaging)

In a medical facility, such as a hospital or clinic, a gantry holds radiation detectors and/or a radiation source used to diagnose or treat a patient's illness. Radiation sources may produce gamma radiation, x-rays, electromagnetic radiation, or magnetic fields depending on the purpose of the device.

The gantry of a computed tomography scanner (CAT) is a ring or cylinder, into which a patient is placed. The x-ray tube and x-ray detector spin rapidly in the gantry, as the patient is moved in and out of the gantry. The CAT scanner produces 3-dimensional x-ray images of the patient.

In the field of medical imaging, the use of proton nuclear magnetic resonance is called magnetic resonance imaging (MRI). An MRI gantry remains fixed, while electro-magnetic field coils vary the magnetic field.

Medical nuclear magnetic resonance imaging works by exciting the nuclei of hydrogen atoms with radio frequency energy, then listening for an echo as the excited nuclei fall back to their normal energy state.

Living things, including the human body, contain large amounts of water and fat, which in turn contain hydrogen atoms. At the center of a hydrogen atom is a nucleus, usually a single proton. Protons have a quantum number called spin. A strong magnetic field within the gantry causes protons to align with the field. A pulse of radio frequency energy of exactly the correct frequency causes some protons to flip. Flipped protons produce a radio signal as they fall back to their normal energy state. These signals are combined to produce an image of where the protons, and therefore, the water or fat molecules are located.

The gantry of an external beam radiotherapy machine moves a radiation source around a patient. A linear accelerator (linac) is built into the top part of the gantry in the photo at the right. The rectangular screen on the right side of the gantry is a cone beam x-ray detector, which is used to help position a patient prior to treatment.

The gantry is supported by a drive stand, which rotates the gantry on a fixed horizontal axis as the linac revolves around a patient. A klystron in the drive stand behind the gantry supplies radio frequency energy to the linac. The linac accelerates a pencil size beam of electrons horizontally. After leaving the linac, the electrons are deflected and focussed downward by magnets, causing the electrons to strike tungsten target. The target stops the electrons, and the sudden deceleration results in bremsstrahlung radiation of X-rays.

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