Transcript Medical Scanners

Buxton & District
Science Discussion
Medical Scanners
Marge Rose
16th November 2012
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Introduction
• Confusion – they all look the same
• CT, MR, SPECT, PET, Ultrasound
• A plethora of names
• Why a scan?
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Names
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CT – computed tomography (was CAT)
SPECT – nuclear medicine (was radioisotopes)
MR(I) is based on NMR
PET stands for positron emission tomography
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Why a scan?
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An aid to diagnosis
Localisation
Screening
Assessment of function
Treatment planning and monitoring
Research
Reassurance
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For each modality
We’ll look at
• History
• Importance
• Probe
• Signal – few natural ones
• Detector – match to signal
• What is it detecting?
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EM spectrum
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Ionisation
Certain types of radiation can ionise atoms
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Ultrasound
• Sound is experienced by our ears
• Caused by longitudinal pressure waves
• We can hear from 20 Hz to 20 kHz
• Above 20 kHz - ultrasound
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Ultrasound scan
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1980
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CT scan – uses x-rays
Science Discussion
They were discovered in 1895 by Röntgen.
Here is the very first x-ray – it shows his wife’s hand
and was taken in 1895. The first medical use was
just a few months later in 1896.
X-rays are the most important and widespread of
the modalities we will look at in this talk.
The method of production is essentially
unchanged.
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X-ray tube and image
But x-ray tubes and images have
improved a great deal in over 100 years
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High Voltage Generator
The naked CT
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Cooling heat
exchanger
Cooling oil pump
Detector Array
Detector
Amplifiers &
A/D Converters
X-Ray tube
120-140kV
Collimator
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Tomography
Science Discussion
“Atom” derived from Greek atomos meaning “uncut, indivisible”
“Tomography” is from the Greek tomē meaning “cut” or tomos meaning
“section” and graphein meaning “to write”
X-ray
tube
Reconstruction of the data by Back projection
Patient
Grid
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CT slice through abdomen
Science Discussion
Probe
120kV X-ray
Interaction
Photoelectric, Compton
Property
X-ray attenuation
Image
3D reconstruction from multiple
projections
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SPECT – uses γ rays
Science Discussion
Becquerel discovered radioactivity in 1896
The Curies researched into it and Marie
opened the first Radium Institute in 1914
Radioisotopes were first used in diagnosis
after World War II when radioiodine
became readily available
Rectilinear scanner appeared in 1951
Anger camera was invented in 1957
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Gamma camera detector
Gamma Camera detector
computer
Light guide
Crystal
collimator
Lead shield
(~60) PhotoMultiplier Tubes
+ pre-amp +
ADC (Analog to
Digital
Convertor)
Science Discussion
First Anger camera I ever saw in
use was in 1975
The most common radionuclide
used is still Technetium 99m
despite supply difficulties
Gamma ray energy 140keV
Half life of 6 hours
‘No’ beta emission
Flexible chemistry
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Nuclear Medicine
Science Discussion
X-ray images show anatomy whereas Nuclear Medicine images
show function
Uses unsealed radioactive sources introduced into the patient. Patients
can still be radioactive when they leave the hospital
Gamma cameras are much less common than x-ray machines
Very few Nuclear Medicine tests are diagnostic – generally they are
highly sensitive but are of low specificity
‘Scans’ can comprise of static or dynamic images, whole body, gated
images or SPECT (single photon emission computed tomography)
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Whole body imaging
A type of static imaging –
A whole body bone scan is
a very common example
Science Discussion
Gated images – the MUGA
Series1
Series1
Series1
R-R interval
24
1
2
3
Frame or bin
4
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SPECT studies – Myocardial perfusion scan
Science Discussion
Probe
Gamma emitting isotope
Interaction
Uptake of radiopharmaceutical
Property
Concentration of pharmaceutical
in organ
Image
Spatial distribution of counts
SPECT – 3D
PET – Positron Emission Tomography
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Antimatter
Science Discussion
• Each fundamental particle has an
antimatter equivalent
• Same mass but opposite charge
• Positrons are positive electrons
• Collide with the first electron they come
across to produce annihilation radiation
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Positron annihilation
511 keV
e.g.18F
b+
e-
511 keV
Coincidence
Unit
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PET images
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Normal
Pre-therapy
Post-therapy
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MR scanner
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An MR(I) Scanner
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MR – souped up NMR
Science Discussion
• A hydrogen nucleus has spin
• If placed in a magnetic field, the
nucleus precesses around in the
direction of that field
• Direct in an RF (radiofrequency)
pulse and the nucleus can flip to
the higher energy state, opposing
the field
• When it relaxes back, it gives off an
RF signal which is dependent on
the chemical environment
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Gradient coils
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MR
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• Probe
– EM pulses
• Interaction
– Resonant energy exchange
changes nucleus spin state
• Property
– proton density, proton
microenvironment
• Image
– Map EM signal
– 3D reconstruction
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Artifacts, Hybrid scans
Science Discussion
The End