CT Scanner Technology

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Questions and Answers

What is the main advantage of helical CT scanners?

They have slower scan times

They can acquire data while the table is moving (correct)

They use more contrast agent

They can produce higher-resolution images

What technology allows the gantry to rotate continually in third- and fourth-generation scanners?

Slip ring technology (correct)

Helical pattern scanning

X-ray tube design

Multiple detector arrays

What is the main benefit of using multiple detector arrays in seventh-generation scanners?

They have slower scan times

They use less contrast agent

They produce higher-resolution images

They make better use of the x-rays produced by the x-ray tube (correct)

What is the difference between helical CT and spiral CT?

Spiral CT is an inaccurate term for helical CT Signup and view all the answers

What is the main advantage of sixth-generation scanners?

They can acquire data in 50-msec scan times Signup and view all the answers

What is the purpose of interpolating the raw data from helical CT scanners?

To approximate the acquisition of planar reconstruction data Signup and view all the answers

What is the main limitation of third- and fourth-generation scanners?

The gantry had to be stopped after each slice was acquired Signup and view all the answers

What is the main advantage of X-ray tubes designed for CT?

They have impressive heat storage and cooling capabilities Signup and view all the answers

What is the estimated number of contiguous 7-mm CT scans equivalent to 100mm?

14 Signup and view all the answers

Where is a single CT image acquired in the pencil chamber?

At the center of the pencil chamber Signup and view all the answers

What is the conversion factor for air kerma to dose in mGy/mGy?

0.893 Signup and view all the answers

What is the purpose of drilling holes in standard phantoms?

To allow the placement of the pencil chamber Signup and view all the answers

What is the relationship between the radiation dose in CT and the mAs used per slice?

The radiation dose is directly proportional to the mAs Signup and view all the answers

What is the limitation of using the 100-mm pencil ion chamber?

It cannot be used to measure the CTDIFDA for slice thicknesses other than 7 mm Signup and view all the answers

What is the advantage of using the CTDI100 over the CTDIFDA?

It provides a better estimate of the MSAD for small slice thicknesses Signup and view all the answers

What is the difference between the dose to soft tissue and the dose to Lucite?

The dose to soft tissue is 20% larger than the dose to Lucite Signup and view all the answers

What percentage of the signal in typical fan beam scanners is accounted for by scattered radiation?

5% Signup and view all the answers

What is the main advantage of third-generation CT scanners?

Reduced scan time Signup and view all the answers

How do the detectors in a fan beam geometry CT scanner detect scattered radiation?

Only scattered radiation in the same plane as the detector is detected Signup and view all the answers

What is the primary difference between fan beam geometry and open beam geometry?

Detection of scattered radiation Signup and view all the answers

How many detectors are typically used in third-generation CT scanners?

More than 800 detectors Signup and view all the answers

What is the motion of the x-ray tube and detector array in third-generation CT scanners?

Rotate-rotate Signup and view all the answers

Why are third-generation CT scanners more expensive?

Due to the high number of detectors used Signup and view all the answers

What is the typical scan time of newer third-generation CT scanners?

One half second Signup and view all the answers

What should be used for dose calculations in CT scanners with multiple detector arrays?

Collimator pitch Signup and view all the answers

How is the dose in helical CT calculated?

Using the CTDI, with a correction factor when the pitch is not 1.0 Signup and view all the answers

What is the relationship between the helical dose and the axial dose?

The helical dose is 133% that of the axial dose Signup and view all the answers

Why do helical scans often use less mAs per 360-degree gantry rotation than axial scans?

Not specified in the content Signup and view all the answers

What is the purpose of current modulation in CT scanners?

To adjust the mA during the scan based on tissue thickness Signup and view all the answers

Why is the SNR in the final image related to the number of x-rays that pass through the patient and are detected?

Because the SNR is directly proportional to the number of x-rays detected Signup and view all the answers

What is the advantage of reducing the mA during acquisition through the thinner tissue projections?

Reduced patient dose with little loss in image quality Signup and view all the answers

What happens to the high SNR detected in the thinner angular projections?

It is wasted because the noise levels from the thicker angular projections dominate in the final image Signup and view all the answers

What is the effect of using too few angular views in CT image reconstruction?

View aliasing Signup and view all the answers

What is corrected during preprocessing of raw data in CT scanners?

Electronic gain of detectors Signup and view all the answers

What is the purpose of calibration scans in fourth-generation CT scanners?

To measure geometric efficiencies Signup and view all the answers

What is the result of breaking down the total attenuation coefficient µt?

Linear attenuation coefficient Signup and view all the answers

What is the purpose of interpolation in helical CT scanning?

To convert helical data into planar image data Signup and view all the answers

What is the benefit of helical scanning in CT?

Production of additional overlapping images with no additional dose Signup and view all the answers

What is the result of log computation in CT data processing?

Reconstructed value in each pixel Signup and view all the answers

What type of CT scanners have the x-ray source rotating in an arc around each of the detectors?

Fourth-generation scanners Signup and view all the answers

Study Notes

X-ray Beam Geometry

In fan beam geometry, scattered radiation accounts for approximately 5% of the signal, which is significantly lower compared to other imaging modalities. This is due to the focused beam shape, which reduces the amount of radiation that is scattered in various directions.
In open beam geometry (conventional projection radiography), the highest detection of scatter occurs, with a scatter-to-primary ratio (s/p) of 4. This is attributed to the broader beam shape, which increases the likelihood of radiation interacting with the surrounding tissues, resulting in scattered radiation.

Characterized by rotate/rotate motion, wide fan beams, and more than 800 detectors, third-generation CT scanners feature a dramatically improved ability to interrogate the entire patient with the x-ray beam, thanks to an increased angle of the fan beam. The detector array forms an arc, allowing for more efficient data collection and improved image reconstruction.

Rotate/rotate, wide fan beam, and more than 800 detectors.
The angle of the fan beam was increased to allow the x-ray beam to interrogate the entire patient.
The detector array forms an arc, eliminating the need for translational motion.
The multiple detectors in the array capture the same number of ray measurements in one instant as was required by a complete translation in earlier scanner systems.
Scan time is reduced substantially, with early third-generation scanners delivering scan times shorter than 5 seconds and newer systems delivering scan times of one half second.

Sixth-Generation CT Scanners (Helical)

The gantry rotates continually, untethered by wires, using slip-ring technology.
Helical CT scanners acquire data while the table is moving, and the x-ray source moves in a helical pattern around the patient.
Helical scanning allows the use of less contrast agent.

Reconstruction of Planar Sections

The raw data from the helical data set are interpolated to approximate the acquisition of planar reconstruction data.
The interpolated data are used to produce reconstructions of planar sections of the patient.

Seventh-Generation CT Scanners (Multiple Detector Array)

X-ray tubes designed for CT have impressive heat storage and cooling capabilities, making better use of the x-rays produced.
When multiple detector arrays are used, the collimator spacing is wider, and more x-rays are used.
The effect of using too few angular views (view aliasing) is reduced with more views.

Processing of the Data

The raw data acquired by a CT scanner is preprocessed before reconstruction.
Correction data are used to adjust the electronic gain of each detector in the array.
Variations in geometric efficiencies caused by imperfect detector alignments are also corrected.

Interpolation (Helical)

Before the actual CT reconstruction, the helical data set is interpolated into a series of planar image data sets.
Helical scanning allows the production of additional overlapping images with no additional dose to the patient.

Dose Considerations

The CTDI (Computed Tomography Dose Index) is used to estimate the dose delivered to the patient.
The CTDI100 provides a better estimate of the MSAD (Mean Signal Above Dose) for thin slices.
The dose in helical CT is calculated in exactly the same manner as it is with axial CT, using the CTDI, but a correction factor is needed when the pitch is not 1.0.

Current Modulation in Computed Tomography

Scanners capable of modulating the mA during the scan can reduce patient dose with little loss in image quality.
The mA is reduced during acquisition through the thinner tissue projections, resulting in a reduction in patient dose.

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Description

Understand the basics of X-ray beam geometry and CT scanner technology, including fan beam and open beam geometry, scatter-to-primary ratio, and detector array formation.