Understanding CT Physics for FRCR Part 1: Basics, Hounsfield Units, and Image Reconstruction Explained

(https://www.spotters.ai/academy/blog/understanding-ct-physics-basics-hounsfield-units-image-reconstruction-and-frcr)

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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 concepts like Hounsfield Units, image reconstruction, pitch, and dose are taught as isolated facts instead of a connected system. This leads to confusion, over-memorisation, and poor accuracy in True/False questions.

This guide explains CT physics for FRCR Part 1 from first principles — covering how CT images are formed, what Hounsfield Units actually represent, how reconstruction works, and what examiners really test.

This guide is aligned with the Royal College of Radiologists FRCR Part 1 physics syllabus.

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Why CT Physics Is So Important for FRCR Part 1

CT physics is tested heavily because it integrates:

• image formation

• contrast and resolution

• radiation dose

• artefacts

FRCR questions rarely ask definitions in isolation. They test whether you understand cause-and-effect relationships in CT.

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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

• what attenuation means

• how Hounsfield Units work

• basic reconstruction principles

• trade-offs between image quality and dose

You are not expected to memorise engineering equations or scanner hardware detail.

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CT Image Formation Explained Simply

CT uses X-rays that pass through the patient, unlike nuclear medicine where radiation comes from the patient.

• An X-ray tube rotates around the patient

• Detectors measure attenuation of the beam

• Data from multiple angles are reconstructed into an image

Key FRCR concept:
CT images are mathematical reconstructions of attenuation, not photographs.

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X-Ray Attenuation: The Foundation of CT Physics

Different tissues attenuate X-rays differently based on:

• density

• atomic number

• beam energy (kVp)

This explains why:

• air appears black

• fat is darker than soft tissue

• bone appears white

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

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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:
Questions test relative HU values, not exact numbers.

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Why Windowing Works in CT

Windowing adjusts:

• window width → contrast

• window level → brightness

Because CT data have a wide HU range, windowing allows visualisation of specific tissues.

FRCR questions often test whether you understand why windowing is required.

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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

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 resolution - a favourite FRCR concept.

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CT Dose Concepts You Must Understand

CTDIvol

• Scanner output indicator

• Unit: mGy

DLP

• CTDIvol × scan length

• Unit: mGy·cm

Common FRCR mistake:
Confusing CTDIvol with 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 matters more than recognising them.

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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 for FRCR Part 1

• Focus on cause → effect relationships

• Use diagrams to visualise beam geometry

• Practise True/False questions early

• Revise explanations, not just answers

• Avoid over-memorisation

CT physics becomes straightforward once the logic is clear.

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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 very 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 understand 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.