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Buxton & District
Science Discussion
Medical Scanners
Marge Rose
16th November 2012
Buxton & District
Science Discussion
• Confusion – they all look the same
• CT, MR, SPECT, PET, Ultrasound
• A plethora of names
• Why a scan?
Introduction
Buxton & District
Science Discussion
• CT – computed tomography (was CAT)• SPECT – nuclear medicine (was radioisotopes) • MR(I) is based on NMR• PET stands for positron emission tomography
Names
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Science Discussion
• An aid to diagnosis• Localisation• Screening• Assessment of function• Treatment planning and monitoring• Research• Reassurance
Why a scan?
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Science Discussion
We’ll look at• History• Importance• Probe• Signal – few natural ones• Detector – match to signal• What is it detecting?
For each modality
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Science DiscussionEM spectrum
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Science DiscussionIonisation
Certain types of radiation can ionise atoms
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Science DiscussionUltrasound• 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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Science DiscussionUltrasound scan
1980
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Science Discussion
CT scan – uses x-rays
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.
They were discovered in 1895 by Röntgen.
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Science DiscussionX-ray tube and image
But x-ray tubes and images have improved a great deal in over 100 years
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Science DiscussionThe naked CT
X-Ray tube120-140kVDetector Array
Detector Amplifiers &A/D Converters
Cooling oil pump
Cooling heatexchanger
High Voltage Generator
Collimator
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Science DiscussionTomography
“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”
Reconstruction of the data by Back projectionX-ray tube
Patient Grid
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Science DiscussionCT slice through abdomen
Probe120kV X-ray
InteractionPhotoelectric, Compton
PropertyX-ray attenuation
Image3D reconstruction from multiple projections
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Science DiscussionSPECT – uses γ rays
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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Science DiscussionGamma camera detector
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
Buxton & District
Science DiscussionNuclear Medicine
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)
Buxton & District
Science DiscussionWhole body imaging
A type of static imaging –
A whole body bone scan is a very common example
R-R interval
24
1 2 3 4Frame or bin
Gated images – the MUGA
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Science DiscussionSPECT studies – Myocardial perfusion scan
ProbeGamma emitting isotope
InteractionUptake of radiopharmaceutical
Property Concentration of pharmaceutical in organ
ImageSpatial distribution of countsSPECT – 3D
PET – Positron Emission Tomography
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Science DiscussionAntimatter
• 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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Science DiscussionPositron annihilation
e.g.18F
511 keV
511 keV
+
e- Coincidence Unit
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Science DiscussionPET images
Normal Pre-therapy Post-therapy
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Science DiscussionMR scanner
An MR(I) Scanner
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Science DiscussionMR – souped up NMR
• 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
• A hydrogen nucleus has spin
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Science DiscussionGradient coils
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Science DiscussionMR
• 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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Science DiscussionArtifacts, Hybrid scans
The End