Simulation and Imaging

imaging for treatment planning

Used to define targets and OARs. Also used to select dosimetric geometric parameters such as isocenter location and treatment beam angles

conventional imaging

includes general radiography and fluoroscopy. a device that mechanically simulates the behavior of a linac. these simulators have a gantry, which can rotate and a couch. this type of imagine during simulation has almost completely disappeared

spatial resolution

ability to display separate images of objects that are close together.

MRI

~ provides the highest spatial resolution.

~ widely used as an adjunct to CT in localizing treatment volumes, particularly the brain

ultrasound

~ based on the propagation and reflection properties of sound at tissue interfaces

~ can be used in treatment planning for prostate brachytherapy implants and for position verification

CT

~ most commonly used for treatment planning

~ spatially accurate and are not subject to distortion

~ display a high resolution and electron density can be derived from the CT number, provided that calibration is applied

4DCT

~ Three spatial dimensions plus a time dimension

~ Used to track anatomical motion associated with respiration

~ Goal is to accurately delineate target volumes and to reduce the amount of normal tissue treated

Prospective gated imaging

Scans require the patient to hold their breath at either maximum inspiration or maximum inspiration or maximum expiration while an axial scan is acquired. The patient then resume breathing, and the couch is moved in. The patient then holds their breath again until that scan is complete.

Retrospective correlation

The patient breathes freely during a slow scan while the patient moves through the scanner. This allows the imaging of all portions of the anatomy through several respiratory cycles. The respiratory is divided into bins. Phase binning divides the respiratory cycle into equal intervals or amplitudes of time. The CT slices are then arranged in groups according to their bin, with each bin representing a different phase of the patient’s breath cycle.

Digitally Reconstructed Radiographs (DRRs)

A plane film radiograph, such as produced with a conventional simulator, provides a beam’s eye view but not 3-D information. CT provides axial slices but not a beam’s view. The data contained in the CT record has information on their linear attenuation coefficient of each voxel and from this data, it is possible to reconstruct a beam’s eye view projection image for any treatment port. This is known as a….

portal images

projection images made using megavoltage (MV) beams projected onto either film or electronic portal imaging devices (EPIDs)

image-guided radiation therapy (IGRT)

~ using KV energy images soft tissue or implanted markers for positioning

~ this imaging provides 3-dimensional target data not captured with conventional MV imaging.

~ the location of the target can be compared with the expected location, and the patient can be moved to align directly with the target

cone-beam computed tomography (CBCT)

A 3D imaging technique used with an OBI (on-board imager), a device on a linear accelerator for IGRT

Simulation

~ The process of positioning a patient for a scan in the most practical and reproducible way possible.

~ Goal is to gather all relevant planning data and to create a patient position that is reproducible on the treatment unit

conventional simulation

~ The process of creating a treatment plan without the use of modern 3-dimensional imaging techniques

~ Involves use of fluoroscopy and radiographs as well as creating manual contours of the patient’s externa anatomy to create an isodose plan

~ the dose inhomogeneity that occurs due to difference in patient tissue anatomy could not be accounted for with this technique