Mi Flaca Me Dejo

HST.187 Physics of Radiation Therapy “Image Guided Radiation Therapy”
Gregory C. Sharp Massachusetts General Hospital April 24, 2007

What is image guidance?
“[The] use of imaging to aid in decisions in the radiotherapy process
– – – – Decision of whether/how to treat Delineation of structures of interest Aid in positioning, verification, and monitoring Assessment and prognosis of outcome”- James Balter, AAPM summer school 2006

What is image guidance?
“[The] use of imaging to aid in decisions in the radiotherapy process
– – – – Decision of whether/how to treat Delineation of structures of interest Aid in positioning, verification, and monitoring Assessment and prognosis of outcome”
- James Balter, AAPM summer school 2006

Source: www.varian.com

Outline
• • • •Workflow (10 min) Device technology (40 min) Geometry and calibration (40 min) (Image processing)

Goals of IGRT

Images: Schlegel & Mahr

Workflow and IGRT

CT Simulation

Treatment planning

Fx’d Treatment
Images: Philips / MGH / Varian / MGH

Simulation

CT: Computed Tomography
• Primary imaging device in Rad Onc! • Anatomic imaging:
– – – – Tumor location Tumor size Criticalstructures Density

Images: philips.com

CT: Computed Tomography
• • • • X-Ray source & detectors Rotate on slip ring 120 or 140 kV x-ray source High resolution
– 1 mm in-plane – 1-5 mm slice width

• 1-16 slices per rotation • 8-60 seconds per scan
Images: http://www.pueblo.gsa.gov/

Lasers & Tattoos
CT-SIM

Images: www.ge.com / www.medtec.com / www.scrippsclinic.com / MGH

TxPlanning

Simulator
• Verify patient setup
– Gantry, couch, lasers just as in treatment – Diagnostic X-Ray instead of Treatment X-Ray

Images: Karla Knott, Varian

Portal Imaging

Images: http://www.ucsf.edu/jpouliot/Tutorial/main.htm

kV Imaging

Images: elekta.com

Ultrasound

Images: nomos.com

Optical Systems

Images: MGH / resonantmedical.com / nomos.com

DeviceTechnology
• • • • • X-ray tubes X-ray generators X-ray imagers Optical imaging (Ultrasound)

X-Ray Tubes

Images: www.osha.gov

X-Ray Tubes

Images: ehs.unc.edu

Energy Spectra

Images: “The physics of radiation therapy,” Faiz Khan

Images: MGH

Images: http://tesladownunder.com

X-Ray Tubes
• Spot size

X-Ray Tubes

http://www.varian.com

http://www.varian.comX-Ray Generators
• Three knobs to turn
– kV – mA –s Potential across anode and cathode Current through anode and cathode Duration of exposure

kV determines x-ray spectrum mAs determines # of photons (at given kVp)

Subject contrast - kVp dependence
Subject (and therefore Image) contrast decreases with increasing kVp.

75 kVp

100 kVp

150 kVp

M. Ter-Pogossian, The PhysicalAspects of Diagnostic Radiology, Harper & Row, 1969 Adaped from` Richard Behrman PhD

Subject contrast - kVp dependence

2 MVp. Primarily Compton interaction

150 kVp

M. Ter-Pogossian, The Physical Aspects of Diagnostic Radiology, Harper & Row, 1969 Adaped from` Richard Behrman PhD

X-Ray Generators

Images: “The physics of radiation therapy,” Faiz Khan

X-Ray Generators
Tube currentImages: “The physics of radiation therapy,” Faiz Khan

X-Ray Generators
Tube voltage + –

Images: “The physics of radiation therapy,” Faiz Khan

X-Ray Generators
Filament current

Images: “The physics of radiation therapy,” Faiz Khan

X-Ray Generators
• Tube current controlled by filament current

http://www.varian.com

Images: MGH

CPU board Interface board Roominterface transformer

Filament control

Power transformer

Images: MGH

HT Tank

Digital X-rays Imaging
• Direct detection
– X-rays -> electric field -> current

• Indirect detection
– X-rays -> visible light -> current

Images: MGH / www.pemed.com

Images: “Principles of Digital Radiography with Large-Area, Electronically Readable Detectors: A Review of the Basics,” Chotas et...