Image Acquisition Flashcards
What is the difference between fine/broad focus?
How is dose affected?
What are the advantages and disadvantages of -
- fine focus
- broad focus
When would you select one over the other?
X-ray tubes have two filament lengths. Short and long cathode filaments. Selecting fine or broad focus determines the area of electron distribution on the target.
Does not affect dose. FF delivers a lower mA over a longer time whereas BF delivers high mA over shorter time.
FINE Ads - * increased spatial resolution * increased detail Dis ads - * Longer exposure time * Higher chance of patient motion * More tube loading
Used for smaller areas such as extremities when you want to visualise bony trabecular detail.
BROAD
Ads -
* Less tube loading/anode wear and tear
* Can use higher exposures - reduces chance of patient movement, less exposure time
Dis ads -
- reduced quality
- increased geometric un-sharpness
Used for imaging thicker body parts and when patients ability to stay still is under consideration
What are density steps when using AEC?
Discuss what changes in the density steps when changing from slim patient to large patient buttons.
Density steps, refer to the circuit resistance during an exposure and allows you to adjust pre set exposure levels, required to terminate the timer circuit. Each density step increase circuit resistance by 25%
Larger patients produce more scatter which causes AEC to shut off too soon. Therefore we increase the CR to allow more primary photons to reach chamber which will increase our image quality.
Opposite happens with small patient - no scatter which can cause overexposure, decreasing image quality.
What is the anode heel effect
Anode heel effect refers to the lower field intensity towards the anode in comparison to the cathode due to lower x-ray emissions from the target material at angles perpendicular to the electron beam.
What is and what is the purpose of the tube filament
A coiled tungsten wire
When the filament is heated it produces thermionic electrons
How does the AEC work?
List 2 advantages of using AEC
During exposure, photons pass through the patient and ionise the air in the chamber, creating an electrical charge.
Once the charge has travelled along the circuit and reaches the timer the exposure is terminated.
The termination only occurs when there is a sufficient amount of radiation delivered.
It helps to provide a consistent signal-to-noise ratio between images, regardless of patient size and density.
AEC systems also help to reduce ‘dose creep’ that can occur due to radiation overexposure by the technologist
What is the bucky factor
The bucky factor is the amount of which your exposure must be increased in order to compensate for xrays absorbed by the grid.
Hence, the Bucky factor reflects the increased radiation dose required from anti-scatter grid use, as any increase in mAs proportionally increases dose.
Bucky factor in nelson is 4
What are the advantages of a rotating anode?
It can withstand high heat loads produced by greater exposure factors
Describe tube current
The stream of electrons between the cathode and the anode is the tube current. The tube current is measured in milliamps, a heat current is applied to the cathode. The higher the temperature of the filament, the larger the number of electrons that leave the cathode and travel to the anode.
What is the name/type of detector used here at the Nelson Hospital.
How do they work?
It is a Samsung S-Detector which is an indirect conversion system with thin film transistor.
Step 1 - Photons reach the SCINTILLATION layer which is made of caesium iodide (rods) and this layer converts photons to light.
CsI improves spat res.
Step 2 - Light photons reach the PHOTO DIODE layer which is compromised of amorphous silicon, this convert light photons into an electric charge
Step 3 - The electrons in the charge are transferred to the TFT layer, which is comprised of an array of DELs. This layer converts electrons into a digital signal which the computer used to create image.
Describe a QA test that is preformed in your department.
Bring II's down from theatre II on bottom, copper sheet over it Set kV - same factors as each month Expose copper plate and record the dose Make sure it is within the acceptable dose range
What material is the anode target made of and why?
Predominantly tungsten - as has high atomic number and a very high melting point.
Describe the maintenance of CR cassettes
Receptor plates erased regularly, carefully cleaning for dust.
Special screen cleaner used, with vertical wiping method.
Quality assurance vs quality control
Why are both important?
QA -
Requirement of all radiology departments as stated in CSP 5. QA is the whole mgmt process and incorporates QC documentation.
Purpose is to ensure optimal diagnostic image quality, ensure the safety of patients, staff and the environment, and to ensure accurate results with a minimal error margin.
QC is to maintain optimal diagnostic image quality by the routine monitoring of equipment, recording deviations and taking corrective action.
Name and describe 4 QC tests
AEC test -
Purpose is to check consistency of the automatic density setting for the reproducibility of images.
LBD test -
Purpose to check the light beam and xray beam co-ordinate in order to ensure centring and to minimise dose.
Bucky alignment -
Purpose to check the bucky alignment for correct centring
Step wedge -
Purpose check equipment for consistency of kV, mA and time.
Grids -
Purpose
How they work
When would you use them
Purpose -
The anti-scatter grid improves image quality by transmitting the majority of the primary x-ray beam and selectively rejecting scattered radiation by attenuating low energy photons in the lead strips.
The grid consists of a series of thin lead strips separated by radiolucent interspaces.
As scattered radiation is increased in “thicker” patients grids are useful in such scenarios to improve image contrast.
Describe 2 types of radiation
Bremsstrahlung -
- 90% of photons produced
- Electron-nucleus interaction
- Continuous energy
Characteristic -
- About 10% of photons produced
- Electron-electron interaction
- Fixed energy
Explain how “reject analysis” is used to improve standards
Reject analysis can see if theres a problem with a particular view or a particular technique, which suggests more training may be needed to target those areas.
Briefly describe the x-ray generator, tube and envelope
- Power or energy supplied by generator
- AC is transformed and rectified to DC
- The capacitor is housed in the generator
- Tube consists of anode + and cathode -
- Cathode has tungsten filament which is heated by a filament circuit
- Vacuum inside the tube in order for the electrons to travel without collision
- Tube is housed in an envelope which acts as an electrical insulator
- Tube also contains lead for shielding and oil for cooling of the anode
Describe 2 types of scatter
Photoelectric absorption -
- Low energy
- X-rays which are majorly absorbed by the patient.
- This type of scatter is more harmful than CS.
- To reduce amount use an ALARA exposure which is not overexposing however is high enough to stop the production of PEA.
Compton scatter -
- Higher energy
- Occurs due to the interaction of the photon with free electrons or loosely bound outer shell electrons.
- Less harmful than PEA
- Use a anti scatter grid to improve image quality and remove CS
What is the importance of checking ID on images and using the correct marker
Ensure correct patient and image to ensure patient does not get inappropriate treatment/wrong diagnosis.
Ensure we have x-rayed the correct limb which may be later treated/operated on
Describe the air gap technique
The air gap decreases the amount of scatter that reaches the imaging plate by being projected away from the imaging plate. Therefore the scatter will not be part of our image. Used in lateral c-spines
What affects the EI number on images aside from kV and mA
Filters Gonad shielding Collimation Centring of body part to plate FFD Prothesis
Grid ratio vs grid frequency
RATIO -
The grid ratio of these grids is 10:1 – which is the ratio of the height of the lead strips vs the distance between two strips
A grid with a higher grid ratio will reject scatter better than a lower grid ratio, due to the limited angle that is allowed by the grid structure. However, a higher ratio grid typically has a higher dose
FREQUENCY -
The grid frequency is a measure of the number of grid lines per unit distance (inches or centimeters), and is in the range of 40 - 50 lines/cm
