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Radiological investigations and radiation exposure 

Radiological investigations and radiation exposure

Stephen Chapman

, Grace Robinson

, John Stradling

, Sophie West

, and John Wrightson

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date: 29 November 2021

Table A8.1 Common radiological investigations used in respiratory practice (continued on  Radiological investigations and radiation exposure p. [link])


Plain CXR (AMBER), one view

Staging chest and abdo CT



Low-dose CT

V/Q scan 99mTcMAA and 133Xe

PET scan


Head CT


Best technique for plain chest radiography

For example, for staging lung cancer, usually with IV contrast to identify vascularity of structures

For diffuse lung disease, giving good resolution at level of s pulmonary lobule

For PE and visualization of pulmonary vasculature

Used in lung cancer screening

For identifying perfusion defects without accompanying ventilation defects, as in PE

For detection of malignant deposits and increasingly for areas of inflammation

Better detection of malignant tissue invasion

For example, for brain metastases


Multiple beam equalization improves contrast by varying beam intensity, depending on tissue density

Commonly, 5mm slices at 5mm intervals, with thinner slices reconstructed from same data. Whole lung scanned. 200–400mA beam intensity

1.25mm at 10mm intervals, i.e. only about 10% of the lung scanned

72mm slices at 2mm intervals

0.6mm slices at 0.6mm intervals, i.e. whole lung scanned at approx ¼ standard beam intensity (50mA)

IV radiolabelled albumin macro-aggregates that lodge in the pulmonary arterioles to image vasculature, and inhaled xenon gas to image ventilated areas

Radiolabelled glucose (18 F-FDG) – uptake proportional to metabolic activity

5 or 10mm slices at 5 or 10mm intervals, i.e. whole brain scanned

Radiation dose mSy or mGy (x 100 for mrad)


Variable, 74–8 (higher in USA where 400mA more common). Larger doses of contrast medium scatter more radiation into nearby tissues


  • 4

  • Probable higher effective dose due to contrast medium scattering of radiation

Variable, 71





Radiation dose (time equivalent to background radiation in the UK)

5 days


4 months


4 months

7 months



8 months

Radiation dose (equivalent to numbers of CXRs)











Poorer resolution, so s pulmonary lobule not visualized

8.5mm gaps, so early cancers may be missed

Same as ordinary CT

Lower beam intensity produces lower resolution

No structural information or the ability to make alternative diagnoses

Usually combined with CT, so add on up to another 6mSy

Lower resolution

Different departments/countries will use different protocols, e.g. some will always do a standard CT as well as an HRCT in case a malignant nodule is missed.

Radiation dose estimates are fraught with many assumptions, and there is significant uncertainty in some areas.

1mSy is the dose of absorbed radiation produced by exposure to 1mGy of radiation.

1mSy = 100mrad absorbed dose (mSy includes quality factor (type of radiation and nature of tissue), but, for X-rays and most tissues, mSy and mGy are numerically identical (for alpha emitters, 1mGy causes 20mSy)).

Background radiation ~3mSy/y (mainly from radiation in the home and varies across the country); transatlantic flight = extra 0.03mSy.