Comparisons will be limited to advantages and disadvantages of image…

The idea

This statement defines the scope of comparison questions involving CT scanning and alternative imaging techniques. You should be able to weigh up, for a given clinical situation, how well each method resolves fine anatomical detail, what it costs to buy, run and maintain, and what radiation or other safety risks it poses to patients and staff. For example, a CT scan of a suspected fracture gives excellent spatial resolution of bone and is fast, but delivers a relatively high X-ray dose; ultrasound is cheap, portable and radiation-free but cannot penetrate bone or air-filled lung well. MRI avoids ionising radiation and resolves soft tissue superbly, yet is expensive, slow and unsuitable for some implants. Answers should compare techniques on these three criteria rather than describe detector construction or reconstruction algorithms.

What earns the marks

• Identifies image resolution as the ability to distinguish two close objects or fine detail, and links it to the imaging modality's wavelength or physical interaction mechanism.
• Explains that CT uses X-rays and so involves ionising radiation, giving a patient dose that must be justified against clinical benefit.
• Compares cost in terms of equipment purchase, installation shielding, running and maintenance, and staff training, not just the scanner price.
• Discusses safety for both patient and operator, including dose, exposure time, need for contrast agents and any non-ionising alternatives.
• Reaches a justified conclusion for a stated context, such as trauma, soft-tissue injury or pregnancy, rather than listing generic points.
• Uses correct terminology such as spatial resolution, ionising radiation, absorbed dose and contrast, avoiding vague words like 'better' or 'clearer'.

Common mistakes and how to put them right

• Treating resolution as brightness or contrast.

  Put it right by defining resolution as the smallest separation at which two points remain distinguishable.

• Claiming CT is always safest because it is quick.

  Put it right by noting the ionising dose and the need for justification, especially in pregnancy or repeated scans.

• Comparing only purchase price and ignoring running costs, shielding, maintenance and staffing.

  Put it right by treating cost as a total-lifecycle comparison.

Exam tips

• Read the context in the question and tailor the comparison to that patient or scenario rather than writing a generic list.
• Structure each paragraph around one criterion, giving an advantage and a disadvantage for each technique before concluding.
• Use comparative language such as 'higher resolution than', 'lower dose than' and 'more expensive to install than' to make the comparison explicit.