Helical Spiral CT and multislics

Question 1

Explain the two types of slip rings?

2

Answer 1

high voltage which takes power from a high powered generator

low voltage power is provided to the slip ring. The rotating gantry has a high power transformer to deliver energy to the xray tube - used in modern helical scanners

Question 2

Define helical pitch

Answer 2

Question 3

Define three types of interpolation including a general definition of the term 4

Answer 3

1. Interpolation is the mathematical technique for a known function used to estimate a function from data on either side of the function. Unsed in helical data reconstruction
2. Linear - assumes a straight light between oppisite points on a helical acquision
3. 360 linear interpolation used 360 degrees appart from a two rotations
4. 180 degree interpolation used on single slice helical scanners

Question 4

Explain the advantages of helical scanning? 5

Answer 4

1. The table can move the patient continously through the xray beam without being limitatd by the gantry rotation (start stop motions).
2. Can have a pitches greater then one
3. Greater scan length can be acquired in one breath-hold with better temporal resolution and reduction of patient movement artefacts.
4. Various types of interpolation techniques can be applied to reconstruct data from nearby datapoints.

Question 5

Why would we not want to use axial scan appraoch to reconstruct the helical date 1

Answer 5

Introduces artefact when structures change along the z axis

Question 6

How can we fix stair step artefact? 2

Answer 6

This is due to insufficient data points by having overlaping helical reconstructions we can fix the stair step artefact.

Question 7

State the disadvantages of helical scanning 3

Answer 7

broadening of slice profile

helical interpolation artefacts apparent at high pitch

overscan - interpolation data is required on either side of the start and end slices so overscan needed to produce data points see image

Question 8

fill in the gaps

Answer 8

Question 9

How many fixed detector arrays would typically be used to generate 5mm slices

or

2.5 mm

Answer 9

4 detectors coupled at 1.25mm each adding up to 5mm

2 detectors at 1.25mm each

Question 10

Describe a adaptive detector array/ draw. state the problems 2

Answer 10

Wider detectors as the beam diverges towards the edges of the array.

cannot redice the reconstruction size of the outer detectors. Although, smaller detectors can be coupled together to represent larger data samples.

Question 11

Describe multislice detector switching 1

Answer 11

Data from like for like detectors can be acquired at one slice thickness but upscalled in thickness via reconstruction

Question 12

how can we achieve sub millimeter coverage with a 16 slice scanner 1

Answer 12

depends if it has a variable detector array but narrow collimation needs to be applied

Question 13

State two advantages of a 64 slice scanner 2

Answer 13

1. wide x axis collimation meaning wide volume coverage
2. sub millimeter acquisition at wide beam angle/ collimation

Question 14

explain the ways in which pitch is expresed in multiclice scanners

Answer 14

Question 15

Explain the difference in how interpolation is used between single slice and multislice scanners?

2

Answer 15

single slices used typically 2 data points wheresas multislice uses up to 8 in a filter width

Question 16

define cone beam artefacts? 2

Answer 16

Occurs as xray photons dirverge proportional to distence (in a cone beam shape) but the angle on the beam is also no longer parallel to the detector. Correction factor can be applied such as tilted or feldkemp which compensate to rectify images

Question 17

Define isotropic?

1

Answer 17

equal voxel values when reconstructed to eliminate stair artefact

Question 18

Describe what factors would effect temporal resolution?

4

Answer 18

1. Rotation time
2. number of slices and size of the detector array
3. Temporal resolution measured in milliseconds
4. scanning parameters such as the pitch
5. Dual energy - differentiation as dual source scanner would be capable of a higher tempral resolution but spectral detectors where the elements are subdevided will not effect the speed of the acquisition in the same way. Arguably, they could be capable of taking in more data sets which would influence the temporal resolution. A dua source could aquire the heartbeat in 1/4 of a rotation. A single source could potentially have a temporal resolution of 150 per 180 degree rotation.
6. The motion of the aatomy such as the resting heart rate to high pace heart beat would require different imaging parameters.

Question 19

Provide application of dual ENERGY ct?

Answer 19

Kidney stone analysis

Tissue characterising in oncology such as lunch tumours. After iodine, water,

Question 20

State four techniques that are currently used for dual energy imaging? 4

Answer 20

fast KV switching 80 140 - generates noisy images but scanner exposes at 80 kvp for a longer balance the CNR.

Slow KV switching works in a similar way but instead uses a slow pitch with xrays only being emmited at 180 degrees to reduce patient dose.

Dual source scanner tend to have two xray tubes at right angle to one another. One tube is uaully weaker and generates a lower KVp needed for the dual energy scan.

Dual layer detector one detector element may contain two different density scintillator material tope layer yytrium and bottom gadolinium oxysulphide. The top layer absorbes low enery photons while the botton absorbes the high energy. Each contains its own photodiode to convert the light into an electrical signal.

Question 21

Describe how a dual energy technology of your choice would be used in the clinical setting?

Answer 21

fast KV switching 80 140 - generates noisy images but scanner exposes at 80 kvp for a longer balance the CNR.

Slow KV switching works in a similar way but instead uses a slow pitch with xrays only being emmited at 180 degrees to reduce patient dose.

I will be discussing fast kvp switching. Thins in essence deeliveres two different spectrums of xrays typicalled the lower is best used to evaluate iodine contrast.

With the computing software being able to identify iodine from bone or other dense object it can use a dual phase scan to save patient dose by artificially rendering a virtual non contrast study.

DECT is less susceptable to beam hardening artefacts especially those associate with metal implants.

Currently lung tumours and nodules would be evaluated by measuring the HU by placing and ROI. Typically 20 to 60 HU would signify a malignant tumour. Studies have shown that dual energy quantative measurements could be used with a high level of sensitivity and specificity. IC measurements are made through an roi to illustrate the link between iodine intake of a tumour and vascularity. Alternatively wc measurements cal also be made to support the evaluation of lung nodules and distinguishing malignancy.

Artificial coloured images can also be rendered to illustrate the presence of iodine hence vascularity in lung nodures. Thereby, again showing malignancy.