Nuclear Medicine Flashcards
(39 cards)
1
Q
Alpha Decay
A
- Spontaneous emission of alpha particle (4.2He)
- A, Z A-4, Z-2 + alpha particle
- Occurs in heavy nuclides e.g. Radon 226 and Uranium 235
- Poor penetrance (absorbed by skin), but high ionisation along its path (high LET), dangerous when inhaled
- Accounts for most of background radiation dose (inhalation of Radon gas)
2
Q
Beta minus decay
A
- Neutron is converted to a proton, with ejection of electron and antineutrino. _
- (A, Z) (A, Z+1) + (e-) + (v) + energy
3
Q
Beta plus decay
A
- Proton is converted to a neutron, with ejection of a positron (positive electron) from the nucleus, with an neutrino release
- Occurs in nuclides deficient in neutrons
- (A,Z) (A, Z-1) + (e+) + (v) + energy
- Used in PET imaging:
- When positron encounters an electron, it undergoes annihilation reaction where both particles are converted to energy in form of 511keV photons that travel 180 degrees to each other
4
Q
Electron Capture
A
- electron from K-shell is captured by proton to form a neutron + neutrino
- e- vacancy is filled by outer electron release of characteristic radiation
- (A, Z) (A, Z-1) + v + energy
- can compete with beta plus decay, given it results in a neutron + neutrino to try and stabilise the atom
5
Q
Gamma Decay
A
- Emission of gamma rays from nucleus of metastable radionuclide
- (A,Z)m (A, Z) + gamma ray
- 99m Tc is the most important gamma-emitting radionuclide
6
Q
Radioactive decay law
A
Activity = number of atoms at a particular time X decay constant
= N x λ
7
Q
Activity
A
transformations per unit time
8
Q
Bequerel
A
1 transformation per second
9
Q
Curie
A
3.7 x 10^10 transformations per second
10
Q
Physical half life
A
0.693 / λ
Where λ = decay constant of a particular radionuclide
time taken for half the present radionuclide to decay
11
Q
BIological Half life
A
time taken for half the radionuclide present to be cleared from body
12
Q
Effective half life
A
1/Te = 1/Tb + 1/T½
Tb = biological half life
Te = effective half life
T½ = physical half life
13
Q
Construction of scintillation detector
A
- Gamma photons emitted from the radiopharmaceutical in the patient strike the scintillator crystals (NaI)
- Scintillator crystal’s orbital electrons become excited by radiation and emit photon of visible/UV light when they de-excite
- Photomultuplier tubes convert light into a measureable electrical signal
-
2 modes of operation
- current mode: measures total current, and thus cannot distinguish between prompt fluorescence or afterglow (phosphorescence) of previous interactions
- pulse mode: measures the peak currents only, ignoring the afterglow
- Pulse height analyser (part of multi-channel analyser) is used to accept only useful peaks to form an image
14
Q
Gamma camera
A
- Gamma photons emitted by radiopharmaceutical in patients body passes through lead collimator
- Different types of collimators
- Parallel hole (image projected onto scintillator is same size as object)
- Converging: produce magnified image, FOV decreases with distance
- Diverging: produces smaller image, increased FOV with distance
- Pinhole: single hole through which gamma rays pass, used for thyroid imaging
- High sensitivity: big holes, thin septa, more gamma rays detected, less resolution
- High resolution: small holes, thick septa, fewer gamma rays counted, higher resolution
- Different types of collimators
- Gamma rays strike scintillation crystal, exciting the orbital elections which then de-excite, releasing UV or visible light
- Flash of light is detected by photomultiplier tubes, which converts the light into an electrical signal
- Electrical signal is preamplified, digitised via ADC and analysed by multi-channel analyser, which determines mode of operation:
- Pulse mode: uses only the peaks of signal
- Current mode: cannot discriminate between discrete scintillation and afterglow or other interactions in crystal (e.g. compton’s scatter)
- Computer analyses the x and y coordinates and energy of each signal and fills matrix to produce an image
15
Q
SPECT Camera
A
SPECT camera is essentially a gamma camera which is mounted on a gantry which can rotate 180 or 360 deg around a patient OR a annular detection with a rotating collimator which can then produce computed tomographic views of the 3D distribution of tracer in the body.
- Parallel collimators are used.
- A series of images are taken around the patient, with the detectors above to move very close to the patients body
- Images are formed by iterative reconstruction algorithm (matrix size is usually 64x64)
- Improved contrast is major benefit of SPECT because it eliminates the problem of overlapping structures
- QUANTATIVE DATA from SPECT requires correction for scatter and attenuation
16
Q
Performance characteristics of cameras: Uniformity
A
variability of observed count density from uniform source
- Non-uniformity degrades image quality
- Due to:
- Crystal imperfection
- Damaged collimator
- Non-linear response from PMT, AD converter, positional circuitry
17
Q
Performance characteristics of cameras: Resolution
A
- Measured by full width half maximum = minimum distance between 2 objects must be separated to be distinguishable as separate objects
- Intrinsic:
- Thinner crystals
- More PMTs, smaller PMTs
- Extrinsic:
- Collimator design – smaller holes increase resolution, but decrease sensitivity
- Patient to detector distance – smaller = better
Gamma ray energy – higher energy = better resolution but not too high because there will be increased scatter at very high energies
18
Q
Performance characteristics of cameras: Sensitivity
A
- Sensitivity of a system is the fraction of emitted gamma rays from patient that produces counts on an image
-
Collimator efficiency
- Larger holes higher sensitivity but reduce resolution
-
Crystal efficiency
- Thicker crystals increase sensitivity but decrease resolution due to light diffusion
19
Q
Intrinsic spatial resolution
A
- Ability of a imaging system to distinguish between 2 small closely related objects.
- Intrinsic:
- PMT size and number: smaller and increased number of PMTs improve spatial resolution
- NaI crystal thickness: thinner crystals produce better spatial resolution but reduces the efficiency of the camera. Thicker crystals will reduce resolution due to increased scatter of light.
20
Q
Extrinsic Spatial Resolution
A
- Collimator design: smaller holes improve spatial resolution but reduces efficiency
- Detector-patient distance: shorter distance improves spatial resolution
- Gamma ray energy: higher energies improve resolution by producing increase light intensity at the crystal, BUT increases scatter at very high energies.
21
Q
Artefacts
A
- MOTION ARTEFACT: prolonged acquisition times
- Damaged collimators: problems with uniformity
- Damaged crystals: produce defects in image in shape of crack
- Beam hardening artefact: variable attenuation of gamma rays as they exit body à attenuation correction
22
Q
Noise
A
- Unwanted counts which interferes with image interpretation and detection of abnormalities
- Quantum mottle (statistical variation in photons striking detector) is a major factor in SPECT due to the low photon numbers used to form an image. It can be reduced by:
- increasing the acquisition time (more counts)
- increasing administered activity (more counts)
- using high-sensitivity collimator (more counts)
23
Q
Subject contrast
A
- image contrast is high when the tracer localises well within the organ and not anywhere else (e.g. thyroid)
- activity is always present in other organs, and this contributes to background activity
- background activity reduces contrast
24
Q
Image contrast
A
- Scatter reduces contrast
Septal penetration affects contrast (gamma ray should no be able to enter one collimator hole and pass into the tube of another hole)
25
Physical properties of radiopharmaceuticals
* Minimal particulate radiation (i.e. minimal beta particles, alpha particles) that would not contribute to diagnostic image
* Gamma rays should be of high enough energy to leave body but not too high to be difficult to detect (between 100-300 keV)
* Half life should be short enough to limit radiation exposure but long enough to allow adequate time for transportation, preparation and administration.
* Must be able to manufacture in pure form.
26
Chemical properties of radiopharmaceuticals
* Should be sterile, non-toxic and neutral pH to minimise interactions with body processes
* should be easy and relatively cheap to manufacture
* should be able to be attached to different molecules to allow organ-specific imaging
27
Tc99m
* produced from beta negative decay of 99 Molybdenum
* T ½ = 6 hours
* Gamma ray energy ~ 140 keV
* Toxicity = none, but must be produced free of 99 Mo, which is toxic
* Can be attaches to a wide range of biological molecules
* 99 Mo produced in reactor by bombarding 98 Mo with neutrons (reactor contains enriched 235 U which releases energetic neutrons in a controlled manner which are captured by 98 Mo)
28
F-18
(used is PET) is produced in a cyclotron (+ or –ve charged particles (NOT electrons) are accelerated and bombard a target material to produce new material
29
Spatial resolution of SPECT
Spatial resolution of the images:
* measured using capillary tube of 99m Tc
* resolution SPECT is worse than planar NM imaging
30
Contrast resolution of SPECT
* better than planar imaging due to elimination of overlapping structures
31
SPECT artefact
- motion artefect
- partial volume effect if slices too thick
- artefact form detector defects
32
PET scanner
* A PET scanner comprises of a detector ring of scintillation crystals arranged around the patient that are connected to PMTs.
* F-18 labelled FDG injected into the patient produces annihilation reactions that release 2 photons with energies of 511keV at 180 degrees angles to each other.
* These coincident photons should strike the detectors almost simultaneously
* **Two interactions occurring simultaneously forms a true coincidence**
* The true coincidences are detected and are reconstructed using filtered-bac projection (superseded now by iterative reconstruction).
33
PET coincidences
* true coincidence: 2 photons travel at 180 angles to each other and are detected simultaneously
* Scatter coincidence: 2 or more of the photons are scattered in different direction from their original path, but are detected simultaneously
* Random coincidence: 2 photons from different interactions are detected simultaneously, and thus will appear to have originated from along the line between those 2 detected points.
34
PET spatial resolution
* Spatial resolution is **better in the centre than it is at the peripheries:**
* Photons at the centre strike detectors perpendicularly, where peripherally they hit at oblique angles.
35
Beta + decay in PET
* **Beta+ decay of F-18 releases a positron (+), which travels out of the F-18 nucleus and strikes an electron within the tissue, causing an annihilation reaction**
* The annihilation reaction occurs short distance away from the site of beta+ decay, thus spatial resolution of PET imaging is limited by variability of the distance travelled by the positron
* **the annihilation reaction results in photons which travel at 180deg +/- 0.25 degree angles to each other. This variation in angle also limits spatial resolution of PET**
* **the scintillation crystals within the detector also limit resolution**
* previously used NaI, now use BGO, BaF2, CsF
36
PET time of flight
* Time of difference between 2 coincident events can be used to better localise where the annihilation event occurred.
37
Role of low dose CT in PET
* Attenuation of the photons occurs as they travel through tissue out of the body
* Low-dose CT provides information on density to allow correction of the attenuation of photons
* The low-dose CT also allows localisation of tracer activity by superimposing the CT images onto the PET images.
38
Construction and mode of operation of gas filled detectors
1. Chamber within detector is filled with inert gas and lies between 2 electrodes with voltage applied between them
2. Ionising radiation entering the chamber will produce ion pairs (positive ions and electrons)
3. This will induce a current, which can be detected by an ammeter.
Cannot differentiate between different types of radiation.
39
Pulse height analysis
* Pulse height analysis accepts a detected signal only if it is above a certain user-preset threshold and ignores any signals below that threshold. It is used in nuclear medicine gamma cameras imaging to try to differentiate between true signals and signals produced by scatter/noise.
