1. Spiral CT
Bushong Chapter 5

2. Spiral CT
Continuous source rotation with the patient translation through x-ray beam
Patient couch moves as x-ray tube rotates

3. Spiral CT High voltage supplied by slip rings or on board generator
Slip ring replaces cables
No interscan delay
Interscan delay is a small delay between slices or volumes that is needed during standard axial scanning for the x-ray tube to stop and reverse direction. Interscan delay can also be used to allow extra time during a scan for tube cooling
Volumetric imaging within one breath-hold, at least 25s

4. Spiral CT Hyperventilation may help patients extend breath-hold
Tell the patient to breath in and out several times before final breath hold
Contrast enhanced examination requires less contrast media
Contrast amounts between ml are still common
Some multislice/multidetector scanners are so fast that technologists need to be careful about not scanning faster than the contrast bolus moves through the vascular system
Detector data transferred to computer by slip rings

5. Spiral CT Examination time is greatly reduced
Patient comfort is much improved
Because the patient is moved through the gantry while the x-ray tube rotates, a spiral pattern results

6. Spiral CT Z-axis resolution is slightly reduced with spiral CT
Effective slice thickness increases with pitch

7. Pitch
Pitch is the patient couch movement per rotation divided by slice thickness
Contiguous spiral
Pitch = 1, that is 10mm/10mm
Extended spiral
Pitch = 2, that is 20mm/10mm
Overlapping spiral
½, that is 5mm/10mm
Page 78 Bushong shows diagram

8. Pitch Low pitch results in better z-axis resolution
Narrow collimation/high pitch results in better z-axis resolution than wide collimation/low pitch
Narrow collimation/high pitch is recommended for high contrast, thin slice examination, for example, lung nodules

9. Pitch Data is collected continuously but not from a transverse plane
Instead a helical volume is created
As spiral CT pitch increases, patient dose is reduced
Patient dose is approximately proportional to 1/pitch
Patient dose is proportional to slice thickness divided by couch movement

10. Pitch
Pitch in excess of 2 is not recommended for any clinical examination
Too much anatomy would be skipped
Couch incrementation is usually set to equal collimation, pitch = 1
Couch speed (mm/s) should not exceed slice thickness (mm) in order to obtain best compromise between image quality and image volume

11. Pitch Couch speed will not normally exceed 10mm/s
When pitch exceeds 1, 180 interpolation must be used to limit loss of z-axis resolution
Longitudinal (z-axis) image coverage is the product of couch velocity (mm/s) and image time (s)
If couch moves 10mm/s and the imaging time is 5s then 50mm (5cm) of axial coverage will result
The larger the pitch, the more anatomy is covered per examination

12. Interpolation
Reconstruction of spiral CT images is the same as that for conventional CT except for interpolation
A transverse planar image can be reconstructed at any position along the axis of the patient (z-axis)
The transverse image is reconstructed from spiral data first by interpolation, then by filtered back projection

13. Interpolation
Either 360 degrees or 180 degrees interpolation may be employed
Usually 180 degree interpolation is preferred
Contiguous reconstruction can result in partial volume effect when object is contained in adjacent slices

14. Interpolation
Overlapping reconstruction may be necessary to ensure that object is fully contained within a slice
Data acquisition is continuous along the z-axis; therefore by interpolation, image reconstruction is at any z-axis position
Regardless of z-axis position, slice thickness is determined by collimation

15. Interpolation Volume averaging increases with increasing pitch
By increasing the pitch the helix becomes more spread out and small structures can be missed
This is similar to stretching a slinky toy

16. Interpolation
Image noise is higher with spiral CT versus conventional CT regardless of pitch
Interpolation is the computation of an unknown value using known values on either side
Z-axis resolution is improved with 180 degree interpolation compared to 360 degree interpolation

17. Interpolation
Extrapolation is the computation of an unknown value using known values on one side
180 degree interpolation results in a thinner slice than 360 degree interpolation
180 degree interpolation results in a noisier image than 360 degree interpolation

18. Interpolation
180 degree interpolation results in approximately 20% higher noise than conventional CT
360 degree interpolation results in approximately 20% less noise than conventional CT
180 degree interpolation results in better z-axis resolution on reformatted longitudinal images than 360 interpolation

19. Interpolation
180 degree interpolation allows scanning at a higher pitch than 360 degree
360 interpolation broadens sensitivity profile more than 180 degree interpolation
In general, image noise is less for 360 degree interpolation, spiral CT than for conventional CT
In general, image noise is much higher for 180 degree interpolation, spiral CT than for conventional CT

20. Interpolation
Whether 180 degree or 360 degree interpolation, there are linear and higher order reconstruction algorithms
Two characteristic spiral CT artifacts have been identified as breakup and stair step
Both the breakup artifact and the stair step artifact occur as a consequence of reformatting interpolation transverse images to the longitudinal plane – coronal or sagittal

21. Sensitivity Profile
Generally when covering a given length of anatomy, thinner collimation and higher pitch are preferred because the result is better spatial resolution
Pitch greater than 2:1 is not clinically useful because of a broadened sensitivity profile and reduced z-axis resolution
Generally, higher pitch results in thinner slice thickness and less partial volume artifact

22. Sensitivity Profile
During spiral CT with pitch >1, the sensitivity profile (z-axis resolution) is wider than that of conventional CT
Spiral CT sensitivity is described by the full width at tenth maximum (FWTM) rather than the conventional full width at half maximum (FWHM)
The higher the pitch, the wider will be the sensitivity profile

23. Design Features – Slip Ring Technology
Slip ring technology made spiral CT possible
Normal spiral CT gantry rotation is 1 revolution per second
Although .5s revolution is possible, the engineering required by the stress of centrifugal force is formidable

24. Slip Ring Technology
There may be multiple slip rings, both high voltage and low voltage
The design of spiral CT imagers is based on both third and fourth generation with no clear advantage to either
The slip ring contacts or brushes wear and are designed to be replaced during preventive maintenance

25. X-Ray Tube
Spiral CT requires less than 1s 360 degree rotation time and at least 5 MHU x-ray tubes
For very long scan times, mA must be reduced so that x-ray tube loading will not be exceeded
Regardless of heat capacity (MHU) and anode cooling (kHU/min), spiral CT is usually limited by the heat capacity of the focal track
High anode heat capacity (6-8 MHU) and rapid cooling (1 MHU/min) are required

26. X-Ray Tube
In spite of the high heat load, tube life is comparable to conventional CT at about 50,000 exposures

27. Technique Selection Reconstruction
Index is the interval at which images are reconstructed
Index is reconstruction distance divided by collimation
An index of less than one indicates image overlap
An index of greater than one indicates a gap

28. Reconstruction
An index of less than one should be employed to visualize suspected lung nodules
Spiral CT significantly improves coronal and sagittal slice reconstruction
High quality two-dimensional and three-dimensional image reformation are made from overlapping transverse images

29. Reconstruction
Spiral CT operation requires the following unique technique selections
Scan time
Beam collimation
Couch feed velocity
Z-axis spacing for image reconstruction
Spiral images cannot be reconstructed as rapidly as they are acquired. Hence computer memory must be excessive

30. Reconstruction
Scan time cannot exceed the patient’s breath-hold capacity, usually about 25s
Collimation and couch velocity can be selected as pitch

31. Z-Axis Resolution
Z-axis resolution is compromised in spiral CT but not significantly
The ability to reconstruct images at any z-axis location improves small lesion detection by reducing partial volume effects

32. Advantages and Limitations
Image noise is usually less with spiral CT
More data is acquired in spiral CT; therefore, image reconstruction takes a little longer
Spiral CT replaces single scan techniques with volume acquisition techniques
Spiral CT misses no anatomy in the scanned volume

33. Advantages and Limitations
Spiral CT images can be reconstructed at any z-axis position
Multiple overlapping transverse images are possible in a single breath-hold with no additional patient dose
Overall scan time is less with spiral CT resulting in improved patient throughput
Spiral CT takes a bit longer for image processing because of the required interpolation before planar image reconstruction

34. Advantages Faster image acquisition Contrast can be followed quicker
Reduced patient dose at pitch > 1
Physiologic imaging
Improved 3d imaging
Angiographic imaging
Fewer partial volume artifacts
Freeze breathing
Fewer motion artifacts
No misregistration
Increased throughput
Improved patient comfort
Unlimited z-axis resolution
Real time CT biopsy