Methods of Data Acquisition Part 2 (Helical)

What are the AKA’s for helical scanning?

Spiral

Volumetric

Continuous Acquisition Scanning

What was eliminated with helical scanning compared to axial scanning?

Eliminated the interscan delay

How does helical scanning affect the scan time compared to axial scanning?

Increased scanning speed

What are the 3 defining characteristics of a helical scan

Continuously rotating x-ray tube

Constant x-ray output

Continuous table movement (uninterrupted)

What happens to image resolution when you increase the scan speed

Improved Image Resolution

What 3 things are improved when increasing the scan speed (that improves resolution)

Images with improved iodinated contrast concentration

Decreased respiratory and cardiac motion artifact

Superior multiplanar and 3D reformation capabilities

Why are using helical scanning multi row detector systems good when scanning the heart

Because a 320 row detector scanner can cover 160mm in one gantry rotation and this is enough to image the entire heart

Slip Ring Improvement

Gantries with a slip ring design eliminated the need for electrical cables

Tube Cooling Improvement

More efficient tube cooling

Helps dissipate heat

Helical scanning needs a constant x-ray output which creates a lot of heat

CT rooms should be kept cool to help dissipate heat

Higher x-ray output Improvement

Higher x-ray output (increased mA capabilities)

If you lower your time, you increase your mA to keep the optimal mAs
High frequency generators are now used instead of 3-phase to accommodate increased mA

Smoother Table Movement Improvement

Smoother table movement (not jerky)

Otherwise we would see motion artifact

Software that adjusts for table motion Improvement

Software that reduces artifact of the predictable table movement (we know the table speed)

Unfortunately, this software does not work for unpredictable motion (breathing, peristalsis, etc)

Improved raw data management Improvement

The scanner is acquiring a large data set which must be stored until the technologist has time to extract and create all image sets needed by the radiologist. The amount of time will depend on workload of the technologist and the radiologists

If the radiologist doesn’t read the exam for 3 days and then decided that he wants the tech to create a thinner slice, the raw data would still need to be stored in order to make that reconstruction

More efficient detectors Improvement

Manufacturers have also started using more efficient detectors because you can use less mAs when you increase the detector efficiency

What is interpolation

Complex statistical methods to take the slant and blur out of the helical image and create images that closely resemble those acquired in a traditional axial mode

What are the 2 disadvantages of interpolation

Loss of image resolution (minimal)

Negative impact on effective slice thickness

What needs to happen to interpolation as the slice becomes steeper

As the angle of the slice becomes steeper, the interpolation required to adjust the data increases

What factor controls the slice angle getting steeper

Increasing pitch increases the slice angle (steeper)

Effective Slice Thickness Definition

Thickness of the slice that is actually represented on the CT image, as opposed to the size selected by the collimator opening

Effective Slice Thickness AKA

Slice-sensitivity profile

How does helical scanning affect the effective slice thickness

In helical scanning, because of the interpolation process, the effective slice thickness may be wider than the selected slice thickness

What is blooming and degradation

When the effective slice thickness is wider than the selected slice thickness

What is pitch

Relation of table speed to slice thickness

The travel distance of the CT scan table per 360 degree rotation of the x-ray tube, divided by the x-ray beam collimation width

How is pitch calculated

Travel distance of table per 360 rotation/X-ray beam collimation width

What will pitch be when the table feed and beam collimation are equal/identical

1

What will the pitch be when the table feed is less than the beam collimation

Less than 1

When the table feed is less than the beam collimation, what will happen to the slices

Overlapping slices

When using helical scanning, what is table speed directly related to

Table speed and slice thickness are directly related

What happens when you increase pitch (when beam collimation/slice thickness remains the same)

Increase anatomic coverage

Decreased radiation dose

Improved tube heat limit (better)

What happens to the slant of the acquired data when you increase pitch

Increases

What happens to interpolation when you increase pitch

Increases

What happens to the negative effects of interpolation when you increase pitch

Increases

What are the 3 disadvantages of interpolation

Image Unsharpness

Effective slice thickness (blooming)

Image noise: fewer data being acquired (under-sampling)

What pitch would this represent

1

Requires less interpolation

Table moves the same distance as beam width

What pitch would this represent

1.5

Table moves 1.5 times the beam width

What pitch would this represent

2

Requires more interpolation

Table moves twice the beam width

What amount/setting should pitch not exceed so that the negative effects are minimized

1.5

What does it mean to change slice incrementation retrospectively

Changing where the slice lies over the anatomy (slice location)

When changing slice incrementation retrospectively, what data do you need

Raw data

What does changing slice incrementation retrospectively allow for the creation of

Allows for the creation of overlapping slices retrospectively

When changing slice incrementation retrospectively how is the patients radiation dose affected

When acquiring overlapping images in axial scanning, the patient is getting a higher dose because they are getting extra radiation where the overlap is

When changing slice incrementation retrospectively how is partial volume averaging is affected

May help reduce partial volume averaging

How does reconstructing slice thickness work in CT

MDCT Helical scanning allows for you to build a new set of images at a different slice thickness using retrospective reconstructions, without having to rescan the patient

What data do you need to reconstruct slice thickness in CT

Raw data

What will using thinner slices do

When reconstructing slice thickness into thinner slices it will reduce the amount of tissue in 1 voxel resulting in a Hounsfield unit with a more accurate representation on the image

What type of slice interval is this

Contiguous 5/5 mm

What type of slice interval is this

Overlapped 5/2.5 mm

What type of slice interval is this

Non-Contiguous 5/7.5 mm