ct simulator

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Computed Tomography Simulator

CT simulation is a process of determining the exact location, shape and size of a tumor. The main aim of CT simulation is to help the radiotherapy team in the treatment planning process. A CT simulator consists of a CT scanner with flat table top, laser positioning and marking system, virtual simulation software and various hardcopy output devices.

A CT scanner and virtual simulation system

They are made up of:

a)     X-ray tube

The X-ray tube rotates inside the detector ring through a full 360º with a wide fan beam producing a single beam. The X-ray beam is then absorbed by the body to produce data that can be manipulated to demonstrate various body structures.

b)     Collimator and attenuator

They are made from a suitable X-ray absorbing material like a tungsten or lead. They are located immediately in front of the detectors to protect them from scattered X-rays.

c)      Patient support table

It is located at the rear of the CT gantry. It works in cooperation with a trolley. It has a slightly inclined table top holding the stretcher carrying a patient. It has some locking means that can be locked after positioning a patient in the desired position.

d)     Computer workstation

It consists of high quality display screen and a simulation software package.

e)     External patient marking (positioning lasers)

They are used for marking for patients.

Types of CT simulator scanners

  • Convectional and spiral CT: They acquire data one slice at a time.
  • Multislice scanners CT: Multislice scanners CT acquires data from multiple slices simultaneously. 

During CT simulation, a patient is placed on treatment table, the radiation therapy technologists will determine whether the patient will lie in a prone position, supine or by side. Immobilization devices can be used to position the patient in a comfortable proper position. The patient is marked with some small tattoo marks to serve as reference points during treatment planning and treatment delivery, enabling the radiotherapy team to reproduce the treatment in each treatment session. Contrast might also be used to enhance the visibility of organs. Data is gathered using detailed CT images in the transverse plane. DRRs document is also processed before data is transferred to the treatment planning system.

How a CT simulator works

It transmits X-rays through tissue to create a graphic display of the tumor and surrounding normal tissue. It is connected to a high-speed, high capacity computer that enables the radiotherapy technologist to understand the relationship between healthy and cancerous tissues. The images acquired are then saved, recorded, analyzed and used to design an optimal treatment plan.

Quality Assurance of CT simulators

Quality assurance is done to assure safety of patients, staff and public. It can also be done to assure proper equipment and program operation. It should include:-

Specification of tests

  • Tolerance limits
  • Test frequency
  • Corrective actions
  • Quality assurance assignments

Components of Quality Assurance includes:

  • CT dosimetry
  • Radiation and patient safety such as exposure limits.
  • Evaluation of image quality such as, contrast, spatial integrity, artifacts and slice thickness.
  • Evaluation of electro-mechanical components, example; table indexing, localization laser accuracy and x-ray generation.

According recommendations and guidelines of AAPM TG66, tests should be done on the following equipment:

  • CT scanners. Tests on CT scanners includes, bore size and patient size.
  • Virtual simulation software such as, beam geometry, target localization and virtual field size.
  • Image registration
  • Overall CT-simulation process such as, transfer of images and isocenter localization.

An equipment should perform within the specifications agreed on by the vendor and user. Quality assurance can be done daily, weekly, monthly or annually. 

REFERENCES

  • Emory University - Radiation Oncology. 2018. CT Simulation. Retrieved from radiationoncology.emory.edu on 12/24/2018.
  • Memorial Hermann. 2018. CT Simulator. Retrieved from cancer.memorialhermann.org on 12/21/2018.
  • M. Saiful Huq, Ph.D., FAAPM, FInstP. CT Simulator
  • Wikipedia. 2018. CT Simulation.

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