CT Image - Hounsfield Units and Linear Attenuation Quiz

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Questions and Answers

What is the primary difference between Axial Scanning and Helical Scanning in CT imaging?

Scan efficiency

Exposure time

Table movement (correct)

Slice thickness

In Computed Tomography (CT) scanning, what type of projection is used for Scouts?

PA projection

Supine projection

Lateral projection

AP projection (correct)

How is the 'Pitch' defined in the context of CT scanning?

Overlap during scanning (correct)

Slice thickness

Distance between slices

Table speed

Which factor affects the patient dose in CT scanning when increased?

mAs (current and scan time) Signup and view all the answers

What is the main advantage of Computed Tomography (CT) over traditional x-rays?

3-D information on structure locations Signup and view all the answers

How does Computed Tomography (CT) handle the issue of superimposition of structures?

By reconstructing slices to show 3-D information Signup and view all the answers

Why is it important to have a 3rd dimension in medical imaging, as discussed in the text?

To better understand the exact locations of structures within the body Signup and view all the answers

What makes Computed Tomography (CT) able to distinguish between adjacent like objects with very little difference in contrast?

Reconstruction of data from transmission measurements at multiple angles Signup and view all the answers

What is used to represent a tiny block of tissue in a CT image?

Voxel Signup and view all the answers

Which tissues typically have a low Hounsfield Unit (HU) number?

Air Signup and view all the answers

How is the level of grey in a CT image determined?

By the Hounsfield Unit Signup and view all the answers

What happens to the dose in a CT image if the mAs is reduced to maintain a constant noise level?

The dose decreases Signup and view all the answers

What is the purpose of acquiring a scout in CT image production?

To plan the sequence of slices using the scout Signup and view all the answers

How does the Scan Field of View (SFOV) impact CT image production?

Decides the portion of the patient's anatomy that will be scanned Signup and view all the answers

What is the role of computer processing in CT image production?

Building images from the obtained data Signup and view all the answers

Why is it important for Radiation Therapists to consider the Scan Field of View (SFOV) during CT Simulation?

To capture contours just beyond the patient's anatomy Signup and view all the answers

Study Notes

Differences in CT Scanning

Axial scanning captures images in discrete slices, while helical scanning acquires continuous data as the patient moves through the scanner, allowing for faster imaging.
Helical scanning results in overlapping images, enhancing the reconstruction process and creating more detailed three-dimensional datasets.

Projection Used for Scouts

Scouts utilize a standard radiographic projection, typically a two-dimensional, panoramic view of the area being examined, serving as a preliminary check before full scans.

Definition of Pitch

'Pitch' in CT is defined as the ratio of table feed per rotation to the total width of the collimated x-ray beam, influencing image quality and exposure time.

Factors Affecting Patient Dose

Increasing mAs (milliampere-seconds) directly raises the patient dose in CT scanning, as it governs the amount of radiation used for image acquisition.

Advantage of CT Over Traditional X-rays

CT provides cross-sectional images, allowing for detailed visualization of complex structures and differentiation between adjacent tissues, which standard x-rays cannot achieve.

Handling Superimposition of Structures

CT imaging utilizes computer algorithms to reconstruct images in multiple planes, effectively removing superimposition and revealing hidden anatomical details.

Importance of 3D in Medical Imaging

Having a third dimension in imaging enables precise localization and relationship understanding among anatomical structures, enhancing diagnostic accuracy.

Distinguishing Adjacent Objects

CT employs advanced spatial resolution and contrast distinction, supported by high-quality detectors and algorithms, allowing differentiation of similar-density tissues.

Representation of Tissue in CT Images

A voxel is used to represent a tiny block of tissue in a CT image, indicating volume in three dimensions for accurate assessment.

Tissues with Low Hounsfield Unit (HU)

Fat and air typically exhibit low HU numbers, indicating lower density compared to other tissues, useful for identifying abnormalities and characterizing lesions.

Determination of Grey Level in CT Images

The grey level in a CT image corresponds to the Hounsfield Units, providing visual representation of tissue density based on specific attenuation values.

Effect of Reducing mAs

Reducing mAs to maintain constant noise level will lead to an increase in the dose needed to ensure adequate image quality, potentially compromising patient safety.

Purpose of Acquiring a Scout

Scouts are crucial for localization and planning, offering a quick reference to the anatomy for correct positioning and optimizing the subsequent scanning process.

Impact of Scan Field of View (SFOV)

SFOV affects the area imaged and the detail captured; proper selection ensures relevant anatomy is included, enhancing the diagnostic capability of scans.

Role of Computer Processing in CT Images

Computer processing is vital for reconstructing raw data into usable images, applying algorithms that enhance contrast and detail for better clinical interpretation.

Considerations for Radiation Therapists Regarding SFOV

Radiation Therapists must consider SFOV during CT simulations to ensure accurate treatment planning and to minimize unnecessary radiation exposure to surrounding tissues.

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Description

Test your knowledge on CT images, Hounsfield Units (HU or CT Numbers), Linear Attenuation Coefficient (LAC), and how they affect the quality of images. Understand the relationship between mAs, dose, and image noise in Computed Tomography.