Computed Tomography

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However, A third generation CT scanner is particularly vulnerable to ring artefacts because the same physical .... Use bow tie filter. (Only Alternative, appropriate.

Computed Tomography

Basic Principle

• Images taken all round patient • Data reconstructed • Tomogram – slice through patient

X-ray source • • • • •

High loading 120-140kV Heavily filtered Use effective energy Compton scatter dominates attenuation • Measures electron density

First Generation

Effort was immediately directed towards overcoming the main limitation of 1st generation scanners, i.e. SPEED (~4 minutes per section) and therefore MOTION ARTIFACTS However, this configuration had significant ADVANTAGES:

- easy calibration, no need to “balance” multiple detectors (and therefore free from ring artifacts - will discuss later)

-low cost; - High scatter rejection due to beam collimation better than any other CT generation

-parallel-beal reconstruction (easier and freer from artifacts than cone-beam reconstructions- more later)

NB This was an instrument that was initially used to scan heads only, took several minutes to acquire a 13 mm slice, in matrices of only 80x80 3mm x 3 mm pixels; why did it become so successful? (e.g. Hounsfield/Cormack Nobel prize in 1979)

1) increased contrast resolution 2) decreased structural noise

Second Generation

Narrow fan beam (~10o) – fewer rotations It is sufficient to use a rotation angle comparable to that covered by the fan beam, i.e. the multiple detectors are used to cover different ANGLES not “horizontal” positions

The gantry still needs to

Translate and rotate

However this was sufficient to reduce the acquisition time form 4 minutes to

20s per slice: if the patients can hold their breath for 20s than the problem of motion artifacts is partly solved.

Third Generation

Now the gantry only needs to rotate, which among other things means it can do so continuously - slice time ~ 4s

• However, A third generation CT scanner is particularly vulnerable to ring artefacts because the same physical detector channel measures all the rays that form a ring.

“Fourth” Generation

Detectors all round gantry, and therefore Only the source rotates (which for example means that more than one source can be used)

as we don’t have the same detector pixel collecting data from different angles, this configuration to a good extent

avoids ring artifacts; moreover, since every detector at same time is receiving nonattenuated beam, it is possible to calibrate the detector in real time.

Detectors • • • • •

Fast (no lag) High absorption efficiency Small compact High stability Large dynamic range

Xenon ionisation chamber • X-rays ionise gas • Electric field attracts ions • Charge collected proportional to x-ray intensity

Problem : Quantum efficiency of gas low •Use high Z gas – Xenon •Gas at high pressures ~ 25 atmospheres •Make detector long ~ 20 cm •Final efficiency 60-70% currently replaced with

Solid state detectors:

–Scintillator + photodiode –Higher absorption efficiency (may have dead space between pixels)

Helical / Spiral Scanning


table travel per rotation pitch = slice width


called “table feed” TF

• Higher pitch lowers radiation dose, but at the expense of partial volume effect (loss of detail).

• No single axial slice irradiated. • Interpolate! • 2 rotations needed to reconstruct slice To have a feel for it: state-of-the-art scanners take ~ 800-1500 projections per rotation with ~ 600-1200 sampled data points per projection.


βi z1



Circular CT: at every bed position zi, I have projections at all angles 0< β I ESTIMATE THE PROJECTION AT POSITION z1 USING THE ONES AVAILABLE:

1 proj " i (za ) + da proj " i (z1 ) = 1 + da

1 proj " i (zb ) db proj " i (za ) + da proj " i (zb ) db = 1 da + db db

Multislice CT

Multislice CT TF pitch = Tx Total x-ray beam Thickness Tx

! If the same pitch as in single-slice helical CT is used, the exposure time can be reduced by a factor approximately equal to the number of detector rows IT DOES THOUGH INTRODUCE A PROBLEM: apart from the central one, slices are seen from an angle, which causes artifacts when these are reconstructed.

Multislice CT • 4-slice, 16-slice, 64-slice (sometimes obtained with 32 detector lines + longitudinal focal spot wobble, producing interlaced slices)

• Faster: (current state of the art: below 1s per rotation, faster system 330 ms, partial scans with dual-source systems ->100 ms)

• Allows heart imaging

• Thick slices have lower noise • Thin slices reduce partial volume effects and allow off axis image creation with isotropic resolution.

CT Numbers • µ for each pixel converted to CT number • Allows greater range of gray levels to be displayed

µ ! µ water CT number = K µ water • K=1000, then CT numbers in Hounsfield units

CT numbers • By definition CT number for water = 0 Tissue Air Lung Fat Grey matter White matter Muscle Bone (trabecular) Bone (cortical)

CT number -1000 -300 -90 30 40 50 300-500 600-3,000

Windows and Levels • Used to display different CT numbers • Window = width of CT numbers • Level = centre of window

• If CT number below window • If CT number above window • If within window, shade of


Window width


CT number

Example L=50 W=200

L=500 W=1700

L=50 W=400

L=-50 W=400

Image Quality • Image contrast • Spatial resolution – Line pair test object – Acrylic bars

• Noise – Tube mA – Scan time – Slice thickness

• Artifacts

QA of CT systems • Use phantoms to measure PSF etc. ● Noise ● Size dependence ● Spatial resolution ● Contrast scale ● Sensitivity ● Alignment ● Absorbed dose ● Linearity ● Slice thickness

Artifacts A. B. C. D. E. F. G.

Ring Motion artifacts Spectral Streak Partial Volume Cone beam Noise

A. Ring Artifact • 3rd generation • Detector response non- uniform • Concentric rings in image • Removed by averaging

Ring artifacts can be identified and removed by software algorithms

B. Motion Artifact

C. Spectral Artifacts • Beam not monochromatic • Beam hardened across patient • Average energy higher in the centre • CT numbers higher at edge • Cupping effect

Beam hardening

• 120kV beam • 7mm Al filtration

CT number lower in centre

Thick bone

Correction of Spectral Artifacts: • Use bow tie filter

(Only Alternative, appropriate beam filters+software solutions)

D. Streaking • High density objects • Cause streaks • E.g. bullet, surgical pin, replacement hip

E. Partial Volume Effects

2D Pixel

Slice width 3D Voxel • Voxel 3-D • Occupied by more than one tissue type • CT number intermediate

Can mimic pathology

F. Cone beam artifacts • Partial volume • Problem in multislice CT

G. Noise • Statistical noise dominates • Reduced with more photons: – Increase kV – Increase mA – Increase rotation time – Increase slice thickness

• Measured with phantom

H.Other artifacts • Patient outside scan field • Dark bands between dense objects • Scatter

Dose in CT • Higher than in DR • Typically – Chest CT – Head CT – Full body CT

• Chest CT → background 1 year in Cornwall

Number of procedures


Effective doses in CT and radiographic examinations CT Effective examination dose (mSv)

Radiographic examination

Effective dose (mSv)







Chest PA










Ba swallow


Ba enema


CT Dose index (CTDI) 1 CTDI = n"z

# D(z)dz

(n number of detector rows, Δz width of 1 detector row -> nΔz total detector width)

•Dose to a depth in a scanned volume !for a complete series of slices •Measure with a long ionisation chamber inserted into a phantom (14 slices) •Large contribution from scatter

Radiosensitive tissues in the field although they are not the area of interest for the procedure Lens of the eye


Breast tissue


Head CT • Original application – Trauma – Stroke – Degenerative diseases – Infections (e.g. meningitis) – Cancer

• Linear fracture • Why isn’t the white/grey matter visible?

• Subdural hematoma

• Depressed skull fracture • Bleeding


Hemorraghic (burst vessel)

Ischemic (blood clot)

Hydrocephalus • CSF over produced and under-absorbed • Most common in children

Tumour • Density in image • Post contrast agent administration

Chest CT • CT of the chest is used to: – Detect and evaluate the extent of tumours – Assess whether tumours are responding to treatment – Plan radiation therapy – Diagnose lung diseases – Evaluate injury to the chest, including the blood vessels, lungs, ribs and spine

• Lung CT window –1700 level - -600 • Mediastinal window – 400 level- - 50 • Bone CT window –2500 level - 1000


Angiogram • Uses contrast agent • Image blood vessels

Comparison with DR

Spatial Resolution Contrast resolution Dose (chest) View

Diagnostic Radiology Computed Tomography (CT) (DR) ~10 lp/mm 1 lp/mm 5%

0.5 %

0.02 mSv

8 mSv