Medical Physics

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Medical Physics

• Paul Kane
• School of Health Sciences
• University of Ulster

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Aims for this Session

• Understand production and uses of Ultrasound
• Understand the basic principles of Magnetic
  Resonance Imaging (MR)
• Be able to compare MR to some other modalities

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Electromagnetic Spectrum
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Ultrasound

• Sound at a frequency greater than the upper limit
  of human hearing 20 kilohertz
• This quality has useful applications and some
  dubious applications
• Mobile Phones
• Pest Control
• 007!

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Medical Ultrasound

• Can be used to digitally produce images of soft
  tissue
• Non-Ionising
• Non-Invasive
• Some risks

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So how does it work?

• Submarine Movies?
• Waves of sound are produced
• Echoes from the waves are detected
• Data from echo is interpreted and image is
  produced

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How it works, the geeky stuff

• Sound wave is produced by a phased array of
  piezoelectric transducers
• Generate sound in range 1 13MHz
• Sound is partially reflected and produces an echo
• Echo returns to probe and transducers are excited
  again, energy converted back to electrical pulse
• Characteristics of echo allow image formation

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Image Formation

• 3 things interpreted by the scanners processor
• Direction of echo
• Strength of echo
• How long echo took to return
• Scanner then knows which pixel to light up and
  to what intensity

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Image Manipulation
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Acoustic Impedance

• Different media will attenuate sound waves to
  different extent, known as impedance
• Bone is hard to penetrate
• Gas also
• Why coupling gel is required
• Frequency choice is a compromise between low
  frequency for penetration and high frequency for
  improved resolution
• Good image production is a skill and results in
  good manipulation of frequency and placement of
  probe

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Ultrasound Uses

• Obstetrics
• Muscle and Soft Tissue
• Use of colour
• Doppler Imaging
• Military Applications

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Ultrasound Limitations

• Brain imaging
• Lung Imaging
• Aspects of Gastro Intestinal Tract
• Examinations of deep structures

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Magnetic Resonance
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Magnetic Resonance

• Uses equipment producing magnetic fields from 0.5
  Tesla to 3 Tesla
• Earths magnetic field is 50 microtesla
• Cost of equipment is approx 700k per Tesla

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Slices
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Obtaining the Data

• All atoms have a magnetic field or orientation
• When placed in a strong magnetic field the atomic
  nuclei align with that field
• Pulses of Radio Frequency applied parallel to
  magnetic field
• Aligned nuclei assume high energy state
• Relaxation of that state produces resonance which
  can be detected
• Location and intensity will translate into pixel
  location and intensity

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Image Manipulation and Enhancement

• Two time frames T1 and T2 relate to percentages
  of nuclei which have resumed normal state
• T1 improves resolution
• T2 improves sensitivity
• Contrast media can be introduced
• fMR can monitor specific physiology

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T1 and T2
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Using Data Manipulation
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Image Manipulation
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MR Versus CT

• MR
• Digital manipulation
• Non-ionising
• Non-invasive
• Good for soft tissue
• Can image physiology
• Technique can be altered to account for data
  required
• Expensive

• CT
• Digital manipulation
• Ionising Xrays
• Non-invasive
• Good for range of tissue types, soft tissue to a
  lesser extent
• Can only image physiology when used with other
  modalities
• Variation in technique possible
• Relatively Cheap

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Thank you for listening

• QUESTIONS?