Week 3 - Helical + Multislice CT

State the limitations of sequential CT

• Greater scan time means the number of slices scanned within a single-breath hold is reduced

• After each rotation the gantry must stop and spin back to original starting position to prevent snapping of cables

• Not suitable for dynamic studies

Explain how slip rings work

Involve stationary component and rotating component. Stationary conducting brushes which are metal bristles keep in constant contact with the rotating conducting rings of the gantry, allowing for current flow between the 2 without the need for cabling.

Explain the benefit of slip rings

No cabling means no stopping and starting per rotation, therefore allowing for continuous rotation giving rise to helical CT scanners.

Explain the difference in data acquisition from sequential and helical CT

• Sequential - data is acquired in slices from stopping, starting and translating

• Helical - data is acquired in a target volume from continuous rotation and patient translation

Explain why heat dissipation is more difficult in helical CT when compared to sequential CT

In sequential CT the x-ray tube stops operating between 360 rotations and patient translation, providing the tube with a cooling period. Whereas in helical CT, the x-ray tube is continuously operating, therefore excellent heat dissipation is required.

State how high centrifugal force and heat production can be managed in helical CT

By using graphite instead of metal such as copper. The gantry components will be lighter, reducing centrifugal force. And graphite has excellent heat storage capacity in which it can be dissipated between scans.

State which generations of CT scanners employ helical CT

• 3rd generation

• 4th generation

State the typical speed of patient bed translation

40mm/s

State the minimum patient bed translation required for a CT scanner

180cm

State the maximum variation in table positioning

0.25mm

Explain why in helical CT the data is considered to be half real and half fake

Data is acquired in a spiral-like fashion in which there are gaps between each rotation. Therefore the data in these gaps are synthesised via interpolation from projection data on either side at the same angle.

Explain why helical CT is more computationally intensive than sequential CT

In sequential CT the computer system collects the acquired data and undergoes image reconstruction. Whereas in helical CT, the computer system must complete interpolation to synthesise gaps in data as well as undergo image reconstruction afterwards.

Explain pitch

A value that represents the tightness of the helix traced out during CT procedure.

State the formula and units of pitch

Patient bed translation per 360 degree rotation/axial collimation or beam width

No units

Differentiate between axial collimation/beam width and slice thickness

• Axial collimation/beam width is the width of irradiated tissue after axial collimation

• Slice thickness is the width of the final reconstructed image

Explain pitch at 0.5

The patient couch is moving half the distance of the slice thickness per 360 degree rotation. Meaning there is overlap and no interpolation required, increasing image resolution however increasing scanning time and patient dose.

Explain pitch at 1

The patient couch is moving the exact distance as the slice thickness per 360 degree rotation. Meaning there is no overlap and no gaps.

Explain pitch at 2

The patient couch is moving double the distance of the slice thickness per 360 degree rotation. Meaning there are gaps between spiral rotations, reducing scan time and patient dose however reducing resolution.

State patient dose in relation to pitch

Patient dose is proportional to = 1/pitch

Define increment

The distance between the centre of adjacent slice planes during image viewing.

Explain how increment and slice thickness relate

• Increment < Slice Thickness = overlapping data in slices increases resolution in which small lesions on the border of slices are more likely to be detected

• Increment > Slice Thickness = gaps of data missing in slices

Define scan time

The time taken for acquisition of entire data set

Define volume coverage

The axial distance/translation covered during acquisition

State the formula for volume coverage

VC = (beam width x pitch x total imaging time)/gantry rotation time

Explain how a multi-slice helical CT scanner works

The axial dimension of the x-ray beam is widened in order to reach multiple rows of detectors, acquiring multiple slices simultaneously in the time that it would take 1 slice in a single-slice helical scanner.

Explain why x-ray output is more efficient in a multi-slice helical CT scanner

The axial collimation of the beam is wider, meaning less of the energy used to produce the x-ray beam is wasted from collimation. Additionally, multiple slices are acquired at once, shortening scanning times and therefore operation time of the x-ray tube.

Define Z-axis

The long-axis of the patient

Differentiate how slice thickness is determined in single-slice versus multi-slice CT scanners

• Single slice - determined by axial beam collimation

• Multi-slice - axial dimension of beam has been widened therefore cannot be used to determine slice thickness, instead detector width is used.

Differentiate between collimator and detector pitch

• Collimator pitch is used in single-slice helical CT scanners in which it is the ratio of patient bed translation per 360 degree rotation to the axial collimation

• Detector pitch is used in multi-slice helical CT scanners in which it is the ratio of patient bed translation per 360 degree rotation to single detector width

State the formula for the relationship between collimator and detector pitch

Detector Pitch = Collimator Pitch x number of slices per 360 degree rotation

Explain how the values of collimator and detector pitch differ

For collimator pitch of 1, detector pitch is the number of rows of detectors, eg. 4 slice scanner = 4.

State the type of detector used in multi-slice helical CT scanners

Solid-state scintillation detectors

Explain why multi-slice helical CT is based on third-generation scanners

As there can be up to a thousand detector elements in each row, therefore the amount of detectors required to completely encircle the patient would be way too high, creating high unnecessary cost.

Explain binning/adaptive detector arrays

The process of summing signals from detectors in adjacent rows so that the detector elements act like one large detector.

Explain advantages and disadvantages of binning

• Advantages - less raw data to reconstruct reduces time required, increases signal-to-noise ratio

• Disadvantages - decreases spatial resolution

Explain slice acquisition rate (SAR)

The number of slices acquired in one 360 degree rotation/acquisition time

State the Z-axis coverage formula in multi-slice helical CT imaging

Z = (no. of slices x slice width x pitch x total imaging time) / gantry rotation time.

Differentiate between the types of x-ray beam used in single-slice and multi-slice CT scanners

• Single-slice = fan beam (widened lateral collimation, tight axial collimation)

• Multi-slice = cone beam (widened lateral and axial collimation

State the advantages and disadvantages of multi-slice CT scanners

Advantages

• High speed

• Multiple slices acquired simultaneously

• Broader axial collimation reduces demands on x-ray tubes

Disadvantages

• High cost due to no.of detector elements

• High demands on computer system