Pulmonary Embolism (PE) is a condition in which a blood clot typically formed within the leg in a Deep Vein Thrombosis (DVT) becomes loose, travels through the system and blocks an artery in the lungs. This can typically cause difficult breathing and coughing up blood. Pregnant women have around five times the risk of developing a PE and it is one of the leading causes of maternal death during pregnancy1.
The most common methods of diagnosis of PE utilise ionising radiation. These are typically standard chest x-ray, a Computed Tomography Pulmonary Angiography (CTPA) or some version of a ventilation- perfusion (V/Q) SPECT CT scan. All exposure to ionising radiation carries a risk commensurate with the level, and for pregnant individuals this also includes a risk to the developing foetus. An analysis of the different doses involved across the imaging modalities has been undertaken, with foetal dose assumed to be equivalent to the dose to the uterus in all cases.
CTPA is the most common examination used to diagnose a PE. The scan of the blood vessels utilises contrast to indicate any potential blockages. The National Diagnostic Reference Levels for this examination are a volume CT dose index (CTDIvol) of 9.1 mGy and a dose length product (DLP) of 310 mGycm. Using the ImPACT CT Patient Dosimetry Calculator, the whole-body effective dose for an exposure can be estimated based on using phantom measurements. For a selection of 10 scanners across various manufacturers, the estimated wholebody effective dose to the patient for a CTPA examination is 6.04±0.21 mSv. From the same data range, the estimated dose to the uterus would be around 0.062±0.003mGy.
Figure 1 – image of a CTPA taken for PE diagnosis2
Lung Perfusion
Perfusion is performed with intravenous injection of a macro aggregated albumin (MAA) labelled with radionuclide Tc99m. MAA localises in the capillaries of the lungs and using SPECT scanner an image is formed. This can be performed using planar imaging or SPECT; The Diagnostic Reference Level3 for Lung Perfusion imaging is 100MBq for planar or 200 MBq for SPECT; the typical effective dose to the patient due to each is 1.1 or 2.2 mSv respectively, with the dose to the uterus estimated at 0.2 or 0.4 mGy respectively. It should be noted that sites which perform perfusion on pregnant patients typically employ a lower LDRL of around 40MBq for this cohort (effective dose 0.44 mSv, dose to uterus 0.08 mGy).
Lung Ventilation
Ventilation is performed by the patient inhaling a radionuclide whilst passing through the SPECT scanner. There are several different radiopharmaceuticals currently utilised for the imaging, with effective doses and doses to uterus summarised in Table 1.
Radiopharmaceutical | Tc99m (DTPA) | Tc99m (technegas) | Krypton (Kr81m) |
Activity (MBq) | 80 | 40 | 6000 |
Effective Dose (mSv) | 0.5 | 0.6 | 0.2 |
Dose to uterus (mGy) | 0.5 | 0.01 | 0.001 |
Table 1 – DRLs for different lung ventilation procedures3
V/Q Scanning
Whilst they scan be performed separately, Ventilation and Perfusion scans are often performed together to achieve a complete diagnosis; whilst perfusion looks at the blood vessels and essentially the organ structure, ventilation is more focussed on the function of the lung. As a dose comparison we will use SPECT perfusion with ventilation using Tc99m technegas.
Discussion and Dose Comparison
Table 2 shows a comparison of the total dose received across different imaging protocols. Whilst a well optimised CT exposure will deliver the lowest dose to the foetus, it should be noted that foetal dose increases greatly as the exposure approaches the uterus. If this is performed on pregnant individuals great care should be taken when selecting the scan range. There are also breast dose implications which should be considered; it is estimated that the breast dose from CTPA is around 22 times higher than the dose delivered from V/Q scanning4. All exposures should be appropriately justified taking into account the risks involved and ensuring a positive net benefit – given this, V/Q scanning may be the preferred methodology.
Modality | NDRL/LDRL | Whole Body Effective Dose (mSv) | Estimated Foetal Dose (mGy) |
CTPA | 310 mGycm | 6.04 | 0.062 |
SPECT V/Q | 200 MBq Tc99m Perfusion 40 MBq Tc99m Ventilation |
2.8 | 0.41 |
Planar V/Q | 100 MBq Tc99m Perfusion 40 MBq Tc99m Ventilation |
1.7 | 0.21 |
Optimised SPECT V/Q | 40 MBq Tc99m Perfusion 40 MBq Tc99m Ventilation |
1.04 | 0.09 |
Table 2 – dose comparison
Presented by Mikey Richardson.
References