Nuclear Medicine Flashcards
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
Q
Physical properties of radiopharmaceuticals
A
- 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
Q
Chemical properties of radiopharmaceuticals
A
- 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
Q
Tc99m
A
- 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
Q
F-18
A
(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
Q
Spatial resolution of SPECT
A
Spatial resolution of the images:
- measured using capillary tube of 99m Tc
- resolution SPECT is worse than planar NM imaging
30
Q
Contrast resolution of SPECT
A
- better than planar imaging due to elimination of overlapping structures
31
Q
SPECT artefact
A
- motion artefect
- partial volume effect if slices too thick
- artefact form detector defects
32
Q
PET scanner
A
- 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
Q
PET coincidences
A
- 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
Q
PET spatial resolution
A
- 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
Q
Beta + decay in PET
A
-
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
Q
PET time of flight
A
- Time of difference between 2 coincident events can be used to better localise where the annihilation event occurred.
37
Q
Role of low dose CT in PET
A
- 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
Q
Construction and mode of operation of gas filled detectors
A
- Chamber within detector is filled with inert gas and lies between 2 electrodes with voltage applied between them
- Ionising radiation entering the chamber will produce ion pairs (positive ions and electrons)
- This will induce a current, which can be detected by an ammeter.
Cannot differentiate between different types of radiation.
39
Q
Pulse height analysis
A
- 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.
