OCSE revision notes Flashcards
What is Exposure Index and range for uni x-ray tube?
Range of interest indicated by a green square surrounding the image
* Linear scale - proportional to exposure
* Normal EI range usually 250-350 (may vary depending on manufacturer)
Possible reasons for changes in Exposure Index?
Exposure field recognition error – doesn’t recognise area you have
exposed
* Unexpected material in field
* Collimation margins not detected
* Scatter
If EI is out of range:
what should you do
Look where green square is
* Look for distortion
* Look what DAP says
- If DAP is within range but EI is not, likely to be a processing error.
- If DAP not within range, could be an equipment fault. Note down
exposure factors used, DAP and EI, report to senior radiographer or RPS,
perform QA, take room out of action until issue resolved.
Radiographic Brightness
High density = low brightness, low density = high brightness.
Are you able to see trabecular patterns within the bone?
* Need to say how you would rectify it if the image is too bright or not
bright enough (e.g. windowing, repeats)
What affects Exposure
- Increasing kV and reducing mAs decreases absorbed dose, but not
always DAP (DAP measures the amount of radiation exiting the tube) - There are less x-ray photons so less of the photons produced are
absorbed by the patient - As we increase kV, the contrast decreases – algorithm has to work
harder to produce a diagnostic image, will lead to changes in EI - Both collimation and mAs have a proportional effect on DAP (as long as
no other changes are applied): - Double mAs = double DAP
- Half area of collimation = half DAP
How do you manage absorbed dose?
To maintain image quality, but managed absorbed dose, the following Rule of
10 should be used:
For every 10kVp increased, the mAs should be halved – gives the same overall
exposure and image quality, EI should stay the same, but DAP may change
How does AEC work
- The AEC determines the mAs of the exposure to produce an image with
optimal density. - mAs is pre-set but kV needs to be set.
- Time is the variable factor – determined by the exposure of an ionisation
chamber
Impact of changes with AECs
- Inaccurate centring – area over the detector may not have the same
density as correct anatomy/pathology - Incorrect detector – incorrect anatomy or pathology will be over the
detector, may not have the same density as the correct AOI - kVp too high/low – time will be changed as mAs will change accordingly
When increasing SID from 100cm to 180cm – double the exposure (add 10kV
and double your mAs)
Grids
- Stops scatter from reaching the IR (attenuates some of the primary
beam) but does not stop scatter from reaching the patient. - Consider using a grid for any area over 15cm in depth.
- Justify use of a grid every time – grids increase dose to the patient as
more mAs is needed with a grid than without. - Increase contrast due to less scatter on the IR.
Grid factor is usually 4. Assume this or ask your examiner, but state out loud
that you are assuming this.
Describing “normal”
Just stating ‘no bony injury’ without further detail is not enough for this exam
(possibly is in practice). You need to base your answer on the clinical history.
Some examples of descriptions of “normal”:
* No bony injury. The glenohumeral joint is congruent. The
acromioclavicular joint remains undisrupted.
* No bony injury. Normal bone, soft tissue and joint appearances.
* Normal appearances of heart, mediastinum and lungs. No evidence of
pneumothorax. normal bony anatomy (clinical history: stabbed in right
side of chest)
* Normal appearances of bone and soft tissue. No breaks in bony cortex or
abnormal lucent/opaque areas. Joints aligned normally
Adapted technique scenarios- Wider collimation than normal
say what the normal collimation should
be, say where the obvious deformity appears to be, increasing the AOI will
ensure that the fracture is fully demonstrated on the first image, hopefully
preventing need for repeats
Adapted technique scenarios- Addition (or removal) of a grid
larger volume of tissue being imaged,
increased amount of scatter produced, will adversely affect the resultant
image if a grid is not used
Adapted technique scenarios- angling of the tube
ensure perpendicular relationship between affected
body part and IR
Adapted technique scenarios- Larger SID
if IR cannot be placed in contact with the body part being
imaged, need to increase SID to account for increased magnification.
Standard SID for CXR is 180cm because the heart naturally has space
between it and the IR so need to reduce magnification.
Adapted technique scenarios- Placing affected body part on radiolucent pad
raises affected body part
enabling posterior/anterior aspect to be visualised on the IR
Explaining differences in DAP
if exposure factors used are the same then the only thing changed is collimation.
if one projection has a smaller field of view (e.g AP lumbar spine compared to AP pelvis), reduced are exposed so reduces DAP and therefore reduced DRL will be lower.
DAP measures the beam leaving the tube, not the patient dose
DAP also does not take SID into account - remains the same.
What is exposure index and what are possible reasons for change in EI
-Range of interest indicated by a green square surrounding the image
* Linear scale - proportional to exposure
* Normal EI range usually 250-350 (may vary depending on manufacturer)
- Exposure field recognition error – doesn’t recognise area you have
exposed - Unexpected material in field
- Collimation margins not detected
- Scatter
if EI is out of range, what do you do?
- Look where green square is
- Look for distortion
- Look what DAP says
- If DAP is within range but EI is not, likely to be a processing error.
- If DAP not within range, could be an equipment fault. Note down
exposure factors used, DAP and EI, report to senior radiographer or RPS,
perform QA, take room out of action until issue resolved.
pregnancy pelvis x-ray exposure factors
high kV technique is needed to reduce dose to foetus
increase kV and reduce mAs to get kV over 100
overall image quality and brightness maintained even though contrast is affected but still ALARP
What do you do if there is half the volume of tissue?
half the volume of tissue requires half the exposure (e.g child’s ankle compared to adults ankle)
To halve an exposure - minus 10 kV and 1/2 the mAs
(very low kV gives very good contrast)
What do you do if there is double the volume of tissue?
double the volume of tissue requires double the exposure
add 10KV and double the mAs
For x-raying COPD how will you change the exposure from the standard exposure 85kV 2.5mAs
Lungs will be very low density, reduce the exposure because of effects of cumulative dose 75kV 1.5mAs
For x-raying pleural effusion how will you change the exposure from the standard exposure 85kV 2.5mAs
same exposure, will allow comparison with previous imaging without increasing patient dose. 85kV 2.5mAs
For x-raying surgical emphysema- how will you change the exposure from the standard exposure 85kV 2.5mAs
For x-raying surgical emphysema- reduce kV due to air in soft tissues, requires less penetrating. Keep mAs same, same number of photons to maintain image quality. 75kV 2.5mAs
Grid portrait or landscape?
-look at which direction the grid lines are running
-if anything the tube cranially, placing IR portrait ensures that primary beam will pass through the grid slots rather than being attenuated by them (would lead to grid cut-off)
What do you need to mention for safeguarding scenarios?
Recommended steps
- listen to the patient and show understanding and support
- remain calm and try not to show shock or disbelief
- assess person’s capacity and establish their wishes/views about the safeguarding issue and procedure
- inform the person that you are required to share this information, explaining what will be shared, who with and why
- make a written record of what the person has told you exactly, what you seen and your actions (dont paraphrase)
-follow local safeguarding procedures and incident reporting procedures
-ensure the patient’s privacy and dignity are maintained and that they are safe.
what is pneumocephalus?
air within the cranial vault, may be due to surgery or fractures of the skull
What would a hyperdense middle cerebral artery mean (MCA) mean?
may be noted in strokes and indicates the presence of a large thrombus within the vessel
What is oedema?
build up of fluid causing swelling
Confidentiality notes for situations
Calmly explain that I cant discuss patient’s information and ask the person to wait in an appropriate area while I discuss with the team. (this means not breaching patient confidentiality)
Speak to the referring clinician and confirm the relationship of the visitor with the patient. (this maintains confidentiality)
Explaining that any doses of radiation will be minimum and kept ALARP. (may provide reassurance and a duty to explain this as the radiographer, not disclosing any patient information)
Ensure consent is discussed with medical team and that the examination is in the best interest of the patient. (medical team will give consent for the examination whilst patient is incapacitated. Next of kin legally cannot withhold consent)
Fully document in CRIS or patient notes including your rationale for your decision (meet medico-legal documents and patient can be aware of actions taken whilst they could not give consent)
Confidentiality notes for situations
Calmly explain that I cant discuss patient’s information and ask the person to wait in an appropriate area while I discuss with the team. (this means not breaching patient confidentiality)
Speak to the referring clinician and confirm the relationship of the visitor with the patient. (this maintains confidentiality)
Explaining that any doses of radiation will be minimum and kept ALARP. (may provide reassurance and a duty to explain this as the radiographer, not disclosing any patient information)
Ensure consent is discussed with medical team and that the examination is in the best interest of the patient. (medical team will give consent for the examination whilst patient is incapacitated. Next of kin legally cannot withhold consent)
Fully document in CRIS or patient notes including your rationale for your decision (meet medico-legal documents and patient can be aware of actions taken whilst they could not give consent)
Solving complex clinical scenarios
1) features required of an x-ray room for undertaking these examinations
points to consider
-Close to emergency equipment/A&E
-Lots of space to move around the trolley
-Equipment to image on a trolley e.g free detector, to use a grid?, sponge pads, leg supports
-ceiling suspended tube with sufficient track to move around the trolley- obtain required projections
-any other reasonable answers
Solving complex clinical scenarios
2) How to apply ALARP principles to the examination
points to consider
-ID patient
-Justify examination
-Careful technique to avoid repeats
-accurate collimation
-optimisation of exposure factors - adjust for the patient and techniques used
Solving complex clinical scenarios
3) Patient is fully conscious- how do you obtain informed consent?
points to consider
-explain what you are going to do and how many x-rays you need to take
- ensure they have understood this- give opportunity to ask questions
-confirm with the patient that they are happy for you to carry on
How do you improve the quality of an image already taken without increasing patient dose?
-post-processing/windowing
- to improve brightness and contrast
