Pregnancy Radiology

Question 1

What are the different types of radiocontrast agent used in VQ scans and how does this affect pregnant patients?

Answer 1

Ventilation isotopes
# 99m Tc-DTPA (Technetium)
- The normal agent that is used
- Excreted by the kidneys and accumulates in the bladder
- Renal impairment can lead to large times take for clearance with higher collective radiation exposure
- Accummulation in the urine leads to foetal exposure
- Good hydration, frequent urination or catheter insertion can reduce foetal exposure
# 133 Xe (Xenon)
- Not excreted in the urine so no build up around the foetus
- minimal absorption, excreted via the lungs
- Much less exposure of radiation to the foetus

Perfusion
# 99m Tc-MAA (Technetium)
- Similar to Tc-DTPA but injected as opp

Question 2

What ionizing scan has the least radiation for the diagnosis of PE in pregnancy?

Answer 2

If going to use an ionizing radiation scan in pregnancy then perfusion only VQ scan has the least radiation
- VQ has injected radioactive material and inhaled radioactive material
- Need perfusion aspect to see the VTE
- Can start with the Q scan first, and if normal then no need to go to the V scan

Question 3

What is considered to be the upper limit of foetal exposure to radiation acceptable during pregnancy?

Answer 3

50mGy

Above this limit the risk of non-stochastic (tissue damage) effects starts to occur
Below this limit stochastic effects occur but are less common

Question 4

What are the usual radiation doses the foetus is exposed to during a CTPA and V/Q scan>?

Answer 4

CTPA
- Dose increases with gestational age (large foetus, closer to chest)
- 0.013-0.026 mGy in early pregnancy
- Up to 0.1mGy in later pregnancy, still well below the 50mGy cut off

V/Q scan
- Using 99m Tc-MAA for the perfusion and 99m Tc-DTPA for the ventilation equates to approx 2.25mGy
- using 133 Xe the dose will be lower to the foetus
- Can use lower dose Tc-MAA/DTPA which can make the dose as low as 1mGy

Question 5

What is the radiation exposure risk to the mother for V/Q and CTPA in pregnancy?

Answer 5

V/Q
- approx 2.2mSv/mGy
- Relatively minimal to the mother
- Theoretical increased breast cancer risk of 3% over background
- Absolute increased total cancer risk 0.04%

CTPA
- Approx 15mSv/mGy
- Theoretical increase in breast cancer risk of 15% over background
- Absolute increased risk for cancer of 0.28%

Question 6

How should the diagnosis of appendicitis be approached in pregnancy?

Answer 6

Clinical 1st
- If surgeons think clinically appendicits then can take directly to theatre at any time

Ultrasound 2nd
- Graded compression
- Gets harder with advancing gestational age
- L) lateral decubitus can help in 3rd trimester

MRI 3rd
- +/- contrast

CT Abdomen last line
- Approx 30mGy exposure on average
- 1:500 foetuses will develop cancer from this

Question 7

What is the issue with using urine pregnancy tests in late pregnancy?

Answer 7

The Hook Effect
- Urine pregnancy tests combine a fixed antibody and a free antibody
- The BHCG subunit binds to the fixed antibody, and then the free antibody binds to this complex
- The tracers binds to this combined antibody-HCG-Antibody complex and turns positive
- If there is too much BHCG then all the free antibody and bound antibodys are bound individually
- This leads a large amount of antibody-HCG complexes, but no antibody-HCG-antibody complexes for the tracer to bind to
- Thus the tracer will not activate and the test will be negative