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34 Cards in this Set
- Front
- Back
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X-ray |
- broad beam of X-rays passes through the head onto X-ray film results in a projection (2D) image of a (3D) object |
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Uses for X-rays |
- skull fractures, bone abnormalities - symmetry of structures can be observed - angiograms show aneurisms, vascular defects |
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Advantages of X-rays |
- very high spatial resolution (0.05mm) - good depiction of the skull |
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Disadvantages of X-rays |
- projection images only - poor intrinsic tissue contrast - use of ionizing radiation - some risk associated with injection of contrast |
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CT scan |
- Computerised Tomography - narrow X-ray beams are projected through the head onto detectors, the source is rotated about the head to acquire many views - a 2D image of the slice is reconstructed using filtered back projection algorithm - an adjacent slice is imaged, building up a 3D image |
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Filtered back projection algorithm |
- calculate the slice of the brain - can reconstruct to an arbitrary good approximation of what the object was |
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Uses for CT |
- tumours, strokes (acute), anatomical defects - good for acute injury |
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advantages of CT |
- tomographic - can resolve grey and white matter; blood, CSF - no worry about projection anymore - high resolution (1mm) |
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disadvantages of CT |
- use of ionising radiation - some risk associated with injection of contrast - anatomical images only |
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Tomography |
- a technique for displaying a representation of a cross section through a human body or other solid object using X-rays or ultrasound |
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PET scan |
- Positron Emission Tomography - decay of certain atoms emits positrons which collide with electrons --> this is an anti-matter annihilation which produces two gamma rays that are 180degrees apart. - detection of grammar rats by crystal photomultipliers reveals position of labelled molecules - image of activity are reconstructed using tomographic techniques |
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positron |
- electron with a positive charge - an anti-electron |
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Gamma ray |
- type of photon with a certain energy |
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advantages of PET scan |
- brain function can be imaged - can see distribution of receptors, blood flow using labelled water etc |
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disadvantages of PET scan |
- anatomical images not obtained - poor resolution (4-8mm) |
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Millisieverts (mSv) |
- probability that you might get cancer or genetic damage - 1Sv = 5% chance of getting cancer |
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MRI |
- Magnetic Resonance Imaging |
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Ground state |
when nuclei are placed in a static magnetic field, they develop a net alignment with the magnetic field and become slightly magnetised |
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Low energy state |
align with the magnetic field |
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high energy state |
align against the magnetic field |
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net magnetisation |
- points in Z direction and is the sum of protons all pointing in the same direction |
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spin-lattice (T1) |
- longitudinal relaxation - rate at which the Z component goes back to the resting state - about 5T1 to get back to zero (5.8 seconds) |
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Spin-spin (T2) |
- Transverse relaxation - measures decay of XY component - faster = decays faster - longer = decays longer - decays a lot faster than T1 grows - caused by things getting out of alignment - important in fMRI |
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frequency of precession |
w = y B0 |
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spin echo/ pulse sequence diagram |
- initial 90degree RF pulse - nuclei move into transverse plan and start spinning
- then a 180degree RF pulse - flips them: this happens at TE (time to the echo) which can be between 2-50ms - take a signal and then repeat |
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altering TR and TE... |
- gives different contrasts to the image |
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Shorter TR and TE... |
- gives more T1 weighting |
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Longer TR and TE... |
- gives more T2 weighting |
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gradient |
- by turning gradient on - creating a projection of the object - do gradient in lots of different directions - get lots of different projections which an then form an image |
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uses of MRI |
- can make the signal sensitive enough to the magnetic properties of the tissue as well as the motion and chemical content - proton density imaging - T1 and T2 weighting - angiograms - diffusion weighted imaging for early stroke - BOLD - In vivo spectroscopy |
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T1 and T2 weighting |
T1 - shows good white/grey matter contrast T2 - shows bright CSF |
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BOLD |
Blood oxygen level dependent imaging |
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Advantages of MRI |
- signal can be dependent on tissue type - ability to change contrast - sensitivity to motion - sensitivity to blood oxygenation - good resolution (<1mm( - non-invasive |
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Disadvantages of MRI |
- Slow --> motion artifacts can destroy BOLD signals - contraindications (pacemakers, implanted devices etc) - claustrophobia due to small bore |
