nuc med Flashcards
Parallel hole type
most common
Allows only radiation parallel to the septa to strike the crystal
Low energy collimator.
Is used to image energies of up to 200 Kev.
Has thin septa
Can therefore have more smaller holes giving better resolution
Medium energy collimator
Has thicker septa so that the more penetrating gamma rays are absorbed
Has fewer holes than a low energy collimator giving poorer resolution
High Resolution Collimator
Has a large number of long holes thus restricting the angle of view of each hole.
Gives better resolution as each point is imaged by fewer holes.
Loses sensitivity as less radiation reaches the crystal.
High Sensitivity Collimator
Has fewer short holes, each hole having a wide angle of view.
Gives poor resolution as each point is imaged by more holes
Has higher sensitivity as more radiation reaches the crystal.
Other Types of Collimator
Diverging Hole
Converging Hole
Pin Hole
The Crystal
Sodium Iodide activated with Thallium.
Photons striking crystal interact by Compton or PE
Produce a scintillation: blue/UV light
Light Guide
Provides optical coupling between crystal and PM tubes
Photo Multiplier Tubes
Several PM tubes will detect each scintillation
Produce small electrical signal from each scintillation
Amplify it maintaining proportionality
The sum of all the signals remains proportional to the energy of the incident photon (z pulse)
Pulse Height Analyser
Remember the z pulse is now proportional to the ENERGY of the incident photon.
PHA “looks” at the z pulse corresponding to each scintillation
Rejects the pulse if outside set limits (window)
Is a filter for scatter
Is not perfect
ADC
x, y and z signals are analogue
ADC processes the signal into digital form
Suitable for computer processing
Choice of collimator
Hi Resolution……..Bone scans, SPECT
- Hi Sensitivity…….Dynamic/ short frame times
Information density
Information density- number of photons collected over region of interest
Computer matrix
Computer matrix
-number of pixels in the image-fewer pixels = coarse image-no point in more than 256 x 256 pixels
Spatial Resolution
Ability to image two adjacent point sources as separate points -Dependent on: Intrinsic resolution (Ri) Collimator (Rc) Scatter component (Rsc)
Scatter
Surrounding tissue produces Compton scatter
Some scattered photons will reach crystal.
Some will be within the 20% energy window.
Degrades the image
Worse at increasing depth
SPECT problem and solution
Problem-background activity in surrounding tissues and the increased amount of scatter limits the resolution and target to background ratio of the organ being imaged.
Solution-SPECT (Single Photon Emission Computed Tomography)
Advantages of SPECT
Increased “pick up” of deep lesions
Localisation
Correlate with CT
Estimate volumes
Disadvantages of SPECT
Requires special equipment Careful setting up Strict QC Patient Co-operation Acquisition times 15-30 minutes
Field uniformity
Linearity:
Field uniformity
Uniform response over whole field of view
Intrinsic (without collimator) Extrinsic (with collimator)
Linearity:Ability of a gamma camera to image a straight line in x and y planes
Intrinsic Uniformity
Remove collimator Point source at least 5 x FOV from camera Collect 5-10 million counts Inspect images for defects Use software to calculate uniformity Do at least weekly
Extrinsic Uniformity
Place “flood phantom” on collimator
Collect 5-10 million counts
Inspect images for defects
Use software to calculate uniformity
Linearity
Point source at least 5 x FOV from camera Place bar phantom on camera face Acquire image Inspect Check lines are not distorted Check resolution
