X-ray imaging teacher notes

X-ray imaging teacher notes

• Copyright Institute of Physics 2012 Page 1

Teaching Medical Physics

• X-ray imaging
• Curriculum links
• Electromagnetic spectrum
• Background radiation
• Introduction
• X-ray imaging utilises the ability of high frequency electromagnetic waves to
• pass through soft parts of the human body largely unimpeded. For medical
• applications, X-rays are usually generated in vacuum tubes by bombarding a metal
• target with high-speed electrons and images produced by passing the resulting
• radiation through the patient’s body on to a photographic plate or digital recorder to
• produce a radiograph, or by rotating both source and detector around the patient’s
• body to produce a “slice” image by computerised tomography (CT). Although CT
• scans expose the patient to higher doses of ionising radiation the slice images
• produced make it possible to see the structures of the body in 3D.
• Lesson notes

Radiography

In X-ray radiography images are produced by casting an Xray shadow onto a photographic film or digital detector:

• Like gamma rays, X-rays can travel through soft tissues in
• body with little attenuation and are only “stopped” by high
• density tissues such as bone.
• Radiograph:
• Fully exposed areas of film/detector appear black.
• Dense objects block more X-rays and so appear
• white.
• Soft tissues like fat and muscle result in
• intermediate exposure and so appear grey.
• X-rays
• In the high- frequency/short-wavelength part of the
• electromagnetic spectrum the distinction between X-rays
• and gamma-rays is made by the origin of the waves: X-rays
• are emitted (by definition) by electrons outside the nucleus,
• while gamma rays are emitted by the nucleus.
• For most medical application X-rays are produced using
• evacuated tubes in which the electrons are accelerated up
• to high speed using large voltages. The X-rays are produced

when the electrons hits a metal target.Copyright Institute of Physics 2012 Page 2

Teaching Medical Physics

• X-ray imaging
• Computerised tomography (CT)
• X-ray source and detector rotated around body as patient
• moves through scanner
• The data collected can be processed using a computer to
• produce a slice or 3D image.
• Compared to radiography, CT imaging better for:
• imaging of soft tissues
• differentiating between overlying structures in the
• body.
• PLAY: Scan of person moving through scanner
• (head to legs). Lungs and other major organs can be
• seen.
• X-ray dose
• CT scans require exposing the patient to higher dose of
• ionising radiation than radiographs which increases patient
• risk of cancer. Consequently, the additional risk associated
• with a CT scan must be weighed up against benefits of
• enhanced diagnostic capabilities (e.g. the ability to
• manipulate the data to produce 3D simulations)
• Exposure is measured in milliSievert (mSv) and is often
• expressed in equivalent background-radiation exposure
• time. Over a lifetime, medical X-rays contribute
• approximately 20 % of background level for the average
• person in UK.
• Chapter 3: launch chapter 3 of schools lecture 2011
• on X-rays.
• Chapter 5: launch chapter 5 of schools lecture 2011

on CT scanning.Copyright Institute of Physics 2012 Page 3

Teaching Medical Physics

• X-ray imaging
• Worksheet Mark scheme
• 1.
• black; white
• Any one from:
• Images can be produced and checked immediately;
• OR
• There is no need for a developing film and disposing of expensive chemicals;
• OR
• Image can be manipulated to get the clearest possible picture;
• OR
• Image can be stored easily.
• 2.
• X-rays can pass through (soft tissues in) the body/have different frequency
• (high-speed) electrons (hit) metal target
• ultraviolet/UV [accept gamma-rays]
• 3.
• (X-rays cause) ionisation/changes in cells/damage DNA/ cancer.
• (CT scans) better at mapping soft tissues/ differentiating between overlying
• structures in the body/making 3D images
• Radon gas/ground/food/drink/cosmic rays
• Calculation of daily or annual background count rate:
• 0.0057 (mSv/day) or 2.1 (mSv/year)
• Answer:
• 246 days/0.67 years

Total 10 marks