Understanding CT Physics for FRCR Part 1: Concepts, Hounsfield Units, Reconstruction, and Dose Explained Simply

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CT physics is one of the highest-yield yet most misunderstood topics in FRCR Part 1 physics.

FRCR candidates often struggle with CT physics because ideas like attenuation, Hounsfield Units, image reconstruction, pitch, and dose are taught as isolated facts rather than a connected system. This leads to memorisation without understanding and poor accuracy in True/False questions.

This guide explains CT physics for FRCR Part 1 in a simple, exam-focused way, covering how CT images are formed, what Hounsfield Units actually mean, how reconstruction works, and which CT dose concepts are commonly tested.

This guide is aligned with the examination framework of the Royal College of Radiologists (RCR).

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Why CT Physics Is So Important for FRCR Part 1

CT physics integrates:

• X-ray attenuation

• image contrast and resolution

• reconstruction algorithms

• radiation dose

FRCR questions rarely test definitions alone. They test cause-and-effect relationships across these areas.

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What Does FRCR Expect You to Know in CT Physics?

For FRCR Part 1, you are expected to understand:

• how CT images are generated

• the principle of attenuation

• how Hounsfield Units are derived

• basic image reconstruction concepts

• CT dose metrics and trade-offs

You are not expected to memorise scanner engineering or advanced mathematics.

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CT Image Formation Explained Simply

CT uses X-rays that pass through the patient.

• An X-ray tube rotates around the patient

• Detectors measure how much the beam is attenuated

• Data from multiple angles are reconstructed into an image

Key FRCR concept:
A CT image is a mathematical reconstruction of attenuation values, not a photograph.

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Attenuation: The Foundation of CT Physics

X-ray attenuation depends on:

• tissue density

• atomic number

• photon energy (kVp)

This explains why:

• air appears black

• fat appears dark

• soft tissue appears grey

• bone appears white

Understanding attenuation is essential for HU, contrast, artefacts, and dose.

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Hounsfield Units (HU) Explained for FRCR

Hounsfield Units quantify attenuation relative to water.

Tissue	Approximate HU
Air	−1000
Water	0
Fat	−80 to −120
Soft tissue	+20 to +60
Bone	+700 and above

FRCR pearl:
The exam tests relative HU relationships, not exact numbers.

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Why Windowing Is Necessary in CT

CT data span a very wide HU range.

• Window width controls contrast

• Window level controls brightness

Windowing allows specific tissues to be visualised appropriately.

FRCR questions often ask why windowing is required, not just what it does.

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Image Reconstruction in CT (Exam-Relevant Depth)

Filtered Back Projection (FBP)

• Fast

• More image noise

Iterative Reconstruction

• Reduces noise

• Allows dose reduction

• Increasingly common in modern CT

FRCR focus:
Iterative reconstruction improves image quality without increasing dose.

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Slice Thickness and Spatial Resolution

• Thin slices → better spatial resolution, more noise

• Thick slices → lower noise, poorer resolution

This trade-off is frequently tested.

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Pitch in CT (Very High-Yield)

Pitch = table movement per rotation ÷ beam width

• High pitch

  • Faster scan

  • Lower dose

  • Reduced image detail

• Low pitch

  • Higher dose

  • Better image quality

Pitch directly affects both dose and image quality.

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CT Dose Concepts You Must Understand

CTDIvol

• Indicator of scanner output

• Unit: mGy

DLP

• CTDIvol × scan length

• Unit: mGy·cm

Common FRCR mistake:
Assuming CTDIvol represents patient dose.

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Common CT Artefacts (High-Yield for FRCR)

Beam Hardening

• Preferential absorption of low-energy photons

• Causes streaks or cupping

Partial Volume Effect

• Multiple tissues in one voxel

• Averaged HU values

Motion Artefact

• Patient movement

• Causes blurring or streaking

Understanding why artefacts occur is more important than recognising appearances.

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CT Physics for FRCR Part 1: At a Glance

Concept	Exam Priority
Attenuation	Very high
Hounsfield Units	Very high
Reconstruction	High
Slice thickness	High
Pitch	Very high
CTDI & DLP	Very high
Artefacts	High

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Common CT Physics Mistakes in FRCR

Common FRCR CT errors include:

• memorising HU values without understanding attenuation

• confusing pitch and slice thickness

• misinterpreting CT dose metrics

• ignoring reconstruction principles

Most errors are conceptual, not factual.

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How to Study CT Physics Effectively for FRCR Part 1

• Focus on cause → effect chains

• Use diagrams to visualise beam geometry

• Practise True/False questions early

• Review explanations, not just answers

• Avoid over-memorisation

Once the logic is clear, CT physics becomes predictable.

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Frequently Asked Questions (FAQ)

Is CT physics heavily tested in FRCR Part 1?

Yes. It is one of the most consistently tested physics topics.

Do I need to memorise HU values?

No. Relative understanding is sufficient.

Is CT dose important?

Yes. CTDI and DLP are high-yield concepts.

Are reconstruction algorithms tested?

Conceptually, yes — not mathematically.

What is the most common CT physics mistake?

Learning facts without understanding attenuation and reconstruction.

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Final Takeaway

CT physics for FRCR Part 1 is not about memorisation.

It is about:

• understanding attenuation

• interpreting HU correctly

• linking image quality to dose

• recognising trade-offs

Candidates who master CT physics conceptually almost always score well.

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Author

Dr B Gayathri Priyadharshinee
FRCR Radiologist & Educator
Dr Gayathri mentors radiology trainees for international exams, focusing on physics clarity, exam logic, and high-yield preparation strategies.

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