27.4 Gamma Camera Flashcards
What is the Point of even using a gamma camera and tracer compared to x rays / cat scans?
IMPORTANT
Gamma camera and tracer can produce images this time that show the FUNCTION AND PROCESS of the body RATHER THAN ITS ANATOMY ALONE
-such as the function of the kidney , brain , lungs etc
What is a medical tracer and how does ti work
By defintion = a chemical compound with some of its atoms replaced by a RADIOACTIVE ISOPTOPE (in our case technetium 99m)
It works like this, if we want to model kidney function, we find a compound that’s ONLY ABSEOBED by the kidney, and replace and atom with technetium
Now , using gamma camera, we can see if the function of kidney is good or not alone! Can see if blood flow is good whatver, by comparing to a HEALTHY kidney scan using same technique!
What must be the properties of a medical tracer ? (2) VERY IMPORTSNT
1) must be preferably gamma emitting
- as this is the LEAST IONISING and most penetrating so can acc reach the camera
- beta and alpha too ionising and dangerous, and alpha won’t even reach the camera
2) must have a short enough half life
- such that it stays long enough for the image to be captured (too short Is too quick)
- AND the patient doesn’t have to wait that long before it’s safe for other humans too to be around them
- and a small half life = means HIGH ACTIVITY ( due to ln (2) / t half) and (A = kN) which means ONLY A SMALL AMOUNT OF RADIOACTIVE SOURCE IS NEEDED in body to produce a clear image and in a short amount of time !
Again why is ti so Importsnt for tracer to have shortish half life?
3 Huge reasons
1) short half life = long enough to be in body for image to be captured
2) but short enough so that high dosage of radiation isn’t in patient after long periods of time ( most of it has decayed only small amounts left which is ntd)
3) BUT SMALL HALF LIFE = HIGH ACTIVITY
- this means SMALL DOSAGE OF SOURCE is all that’s needed to produce a CLEAR PICTURE !
this benefit great as don’t need to take something big !
What medical tracer radioactive isotope used in gamers scans?
Technetium -99M
isotope of technetium and NOT regular T 99 but the meta stable variant
It has a half life of 6 hours
When it decays it releases gamma photon, that’s whole point of this
So how is technetium 99m used to make a medical tracer?
Based on function want to model, it’s joined with some chemical compound with atom replaced for technetium
Now about gamma camera
- what are components of it?
Gamma camera =
1) collimator
2) scintillator
3) photomultiplier tube
4) computer
And then screen
1) First gamma photons emitted from body are passed through the collimator
What is it and why we need it
Collimator are beams of lead which are parallel to each other and normal to the ground
- these ensure that ONLY gamma photons that are ALSO PARALLEL to the beams are let through
- any coming from some angle are ABSORBED
-this means we know EXACTLY where our gamma came from, directly underneath it, and jot from some other place.
This means the emission of gamma photon is DIRECTLY BELOW the point at which they meet with the scintsllor !
- means we can model the image accurately rather than a blur
2) next the gamma photon that made it through the collimator reaches the scintillator?
What’s the purpose of this
Any gamma photon that reached the scintillator will cause electrons here to excite and de excited, producing thousands of VISIBLE LIGHT PHOTONS
However Only about 1/10 gamma that hit scintitlalotr will do this
3) what happens with the photons of visible light after they been produced in scintillator
They reach an array of PHOTOMULTIPLIER TUBES
- these aim to convert a SINGLE photon of light into an ELECTRICAL VOLTAGE, that the computer can then process
What does the computer finally do with all voltages produced by photomultiplier tubes
Processed these to form an image identifying where the medical tracer was when it emitted these, and thus can see what’s wrong with organ.
Checks different signal strength
All steps of gamma camera
1) gamma photons are emitted by the medical tracer
2) these meet the collimator, a collection of lead tubes all parallel to each other, and absorbs any gamma photon that hits the collimator at an angle, thus only letting parallel gamma through. This ensured the position of where the gamma photon came from is accurately defined
3) Now non absorbed parallel gamma photons meet the scintillator . 1 in 10 of gamma successfully manage to interact and excite and dexcite electrons to produce THOUSANDS of visible light photons
4) these end up meeting the photomultipertubes. They manage to convert Each visible light ray into an electrical voltage .
5) all these voltages are sent to the computer which processed them to find out exactly where the medical travers where. As a result, an image showing the concentration of the medical tracer relative to its position is shown on the screen,
This allows you to analyse the function of organs rather than direct anatomy
Why must Technetoum 99m be produced on a hospital site only?
Because it has 6 hour half life, thus must be in hospital such that it can be Used to model before it’s conc dies out
What is a job of s lead shield cover around the whole camera?
Stops the scintillator/ photomultiplier from receiving Any RANDOM BACKGROUND RADIAITON and amplifying this instead
How does image produce din gamma differ to x ray
What’s the advantage to this?
X ray is a 2d shadow photograph
Whereas gamma is dynamic image thst can change in a situation due to image of emission points
Advantage = can monitor rapidly changing situations
