Methods of Data Acquisition-Part 1

Name the 3 methods of data acquisition

Preliminary (Localizer) Scanning

Axial Scanning

Helical (Spiral Scanning)

Are localizers cross-sectional?

They are NOT cross-sectional in nature

How do localizer scans compare to conventional radiography?

They are similar to conventional radiography

Localizer scans have slightly poorer/worse image quality compared to X-ray

Localizer scans deliver approximately equal radiation dose to the patient as X-ray

What tube positions acquire which localizer?

The X-ray tube does not rotate during their acquisition

Position of the X-ray tube determines orientation of the image

Example: if the tube is positioned above the patient. the resulting localizer scan will be an AP view whereas positioning the tube to the side of the patient will result in a lateral view

How many localizers are normally taken?

In all routine studies, at least 1 localizer is taken

Why are localizer scans important?

It includes all areas to be scanned and ensures that anatomy to be imaged has been placed within the range of the system

Correct positioning on the table

Mis-centering on the Z axis

The Z-axis is head to foot

Patient outside of the scan range at the bottom or top (clipped)

This would happen if the patient is positioned too close to the end of the scanning table

Mis-centering on the X-axis

Right to left

If the patient were to not be centered appropriately in the gantry

The patient would be lying more to one side of the table

This can result in out-of-field artifact

Mis-centering on the Y-axis

Anterior to Posterior

Occurs when the technologist sets the table too low or too high within the gantry

Can also result in out-of-field

What software uses the localizer, using proper centering imperative for its use

Automated Tube Current Modulation (ATCM)

What 2 things can be negatively affected if the localizer scan is not properly centered

Image Quality

Radiation dose to the patient

What could happen if directional instructions are not input correctly by the technologist

It will incorrectly annotate all subsequent images in the study and can result in a misdiagnosis and serious medical errors

How can a technologist check for location accuracy if a structure is hard to see on a localizer?

The technologist should take 1 slice through an area to check for accuracy

What 2 things are set using the localizer

DFOV and Image Center

What type of data do you need in order to change DFOV and Image Center retrospectively

Raw data

What is it called when the scout image is used to locate what level the slice is at

Reference Image

What table and tube movement happens during an axial scan

The table moves to the desired location and remains stationary while the x-ray tube rotates within the gantry, collecting data

What are axial images perpendicular to?

Produces images that are perpendicular to the z-axis AND parallel to every other slice

What are the advantages to Axial Scanning

High image quality due to the image being acquired perpendicular to the Z-axis (not slanted like helical scans)

Can be programmed 3 ways:

• Contiguous (gapless slices)

• Non-contiguous (gapped slices)

  • Decreases patient dose

  • You miss some information

  • Used for high resolution chest CTs

• Overlapped (Cine or Dynamic)

  • Done Rarely

  • Increase patient dose

What are the disadvantages of Axial Scanning

Brief pause between slice acquisition adds to the scan time

• Brief delay is significant when giving contrast (contrast does not stay in vessel long)

• Significant when it comes to motion from breathing (motion artifact)

  • Misregistration happens when you stop and start the exam in the middle so the patient can take a breath

    • Diaphragm movement may cause the anatomy to be at a different level

Limited reconstruction options

What are gapless slices called

Contiguous

What are gapped slices called

Non-contiguous

What is it called when slices are overlapped

Cine or Dynamic

What does SDCT stand for?

Single-Detector Row Systems

Used 1 row of detectors

What does MDCT stand for?

Multi-Detector Row Systems

Multiple rows of detectors

What does dual-source CT use

Uses 2 X-ray tubes and 2 detector assemblies

What factor is controlled by opening or closing the collimator in axial SDCT

Slice thickness

What problems come from opening the collimator beyond the detector element size

Opening the collimator more than 15 mm could not increase slice thickness since there is no detector element beyond that point, therefore the largest allowable slice thickness is set less than the detector width-usually 10 mm

If opening the collimation beyond the detector element size was allowed, it would

• Increase dose to the patient

• Increase the amount of scatter radiation

How do you calculate anatomy covered for contiguous SDCT exam

Number of slices X slice increment

How do you calculate anatomy covered for non-contiguous SDCT exam

Number of Slices + gap X slice increment

How does MDCT scanning work

A single rotation produces multiple slices

What are the 2 different types of arrays that can be used for MDCT axial scanning

Fixed array- parallel rows are equal size

Variable array- Rows not equal in size

What determines slice thickness in MDCT

Determined by a combination of the x-ray beam width (controlled by the collimators) and the detector configurations

What are the 3 advantages of MDCT axial scanning when compared to SDCT axial scanning

Reduces motion artifact

Enhances low contrast detectability

Optimizing iodinated contrast enhancement

Which of these diagrams is a fixed array

A and B

Which of these diagrams is a variable array

C and D

Why did the 4th generation CT design not work with MDCT

Because you would need as many as 4,800 detectors for a single row detector, so for a 64-slice scanner MDCT, you would need as many as 307,200 detectors

This design of scanner would need A LOT of maintenance

What is temporal resolution?

The time that it takes to acquire the data

What are the advantages of dual-source CT

Temporal resolution is halved in dual-source CT scanning

Does not increase patient dose

Good for cardiac studies-they may not have to give the patient beta blockers to slow the heart rate

Can use 2 different energy settings (kVp’s) can be used

• Known as Dual Energy CT or Spectral Imaging

What is the other name for dual-energy CT

Spectral Imaging

What is the difference between dual-source and dual-energy

Dual-Source imaging uses 2 set of X-ray tubes with 2 corresponding detector arrays

Dual-Energy imaging uses 2 different kVp settings simultaneously to acquire data (does not have 2 tubes and detectors)

Which image is Dual Source

A

Which image is Dual-Energy

A and B