Radiation therapy is a cancer treatment method that employs high-energy radiation beams to destroy cancer cells by damaging the ability of these cells to reproduce. Normal tissue sparing is important as it improves the balance of complications and cure.

Barring brain,  motion affects all tumor sites in the thorax and abdomen; even the pelvis motion broadly has two components:

1. Predictable quasi-periodic motion due to respiration (relatively stable from minute-to-minute and day-to-day, and can be predicted to first order by a few seconds).

2. Unpredictable as in motion due to swallowing, peristalsis, organ filling (bladder and bowel).  Patient motion and organ distortion, whether as a result of voluntary patient movement or natural functions such as respiration, can make radiotherapy planning assessment less precise and accurate.

The 4D technology uses the time and space concept that provides four-dimensional, real-time images of the projected organ. While 2D scans simple image of the projected organ.

The 3D technique provides three-dimensional images of the organ by assembling a series of 2D images using a specific computer programme, 4D allows scanning 3-dimensional, real-time images rather than capturing delayed images.

Role of radiation therapy in cancer treatment

Basic 3D medical imaging acquisition and reconstruction principles are based on the assumption that the object being imaged is static over the course of the acquisition. Imaging moving anatomy (such as the thorax and upper abdomen during respiration) violates these principles and results in the presence of artifacts in the reconstructed image.

In radiation therapy applications, the motion pattern impacts target design and delivered a dose, and is therefore necessary information to generate an acceptable treatment plan.

The 4D imaging techniques explicitly developed for radiation therapy applications have emerged to capture this information. The introduction of 4DCT into radiation therapy was quickly followed by 4D MRI, 4D cone beam CT (CBCT), and 4D PET.

The tools used in 4D Imaging

4D-CT Imaging:

Stacked acquisition Requires partial images (e.g., reconstructed slices or partial sets of projections) from separate breathing cycles to be combined into a full volumetric image representing a breathing state.

Stacked acquisition can be acquired prospectively or retrospectively. The 4-D images provide geometry data for treatment which can be used for the spatial margins prediction during the treatment course.

4D PET-CT Imaging:

Captures the movement of your organs and tumor over time, while also recording the metabolism of a tumor. It creates the most complete and accurate imaging data on your tumor and critical organs.

Makes it possible to see the makeup and function of a tumor. 4D PET and 4D CT images fuse together and show how a tumor moves; how breathing affects a tumor, and how the movement of nearby organs affects the tumor.

4D-MRI

It has a superior soft-tissue in contrast to CT and has no risk of radiation exposure. It is flexible in image plane selection and offers a variety of image contrasts