* CT Image Processing & Display *

What is attenuation

The reduction of an x-ray beam as it passes through an object due to absorption

What factors affect attenuation

Density, thickness, and atomic number of the material

What is beam attenuation measured in

Hounsfield Units (HU)

What does 1 HU represent

0.1% difference between the linear attenuation coefficient of water

What is an algorithm in CT

A finite set of steps performed to solve a problem

What is image reconstruction

The process of using raw data to create an image by assigning CT numbers to pixels

What is interpolation

A mathematical method of estimating an unknown value using known values on either side

What is raw data

Data collected from detectors that has not yet been assigned Hounsfield Units

What is image data

Processed raw data in which each pixel has been assigned a Hounsfield Unit

What is convolved data

Data that has been filtered by a kernel during filtered back projection

What is the difference between raw data and image data

Image data has been processed and assigned HU values

What is reconstruction time

The time between the end of scanning and when the image is displayed

What is prospective reconstruction

An image produced during scanning

What is retrospective reconstruction

The process of creating a new image from raw data after scanning has finished

What is a ray in CT imaging

The path an x-ray beam takes from the tube to the detector

What is a ray sum

The measurement of how much beam attenuation occurs for each ray

What is a view

A complete set of ray sums

What is an attenuation profile

The account of attenuation properties for each ray sum correlated to its position

What is back projection

A summation technique that converts attenuation profiles into a matrix to form an image

What is filtered back projection

A method that applies a correction filter to improve image sharpness and edge accuracy

What is convolution

Applying a filter function to an attenuation profile

What are common convolution filters

Standard, smoothing, edge enhancement, bone, and detail filters

What is adaptive statistical iterative reconstruction (ASIR)

A reconstruction technique that uses noise profiles to enhance image clarity and suppress noise

How much can ASIR reduce patient dose

Up to 50%

What is the benefit of iterative reconstruction compared to filtered back projection

Provides similar image quality at lower patient dose

What is interpolation used for in spiral CT

To estimate unknown values between known data points before filtered back projection

What is the scan field of view (SFOV)

The area within the gantry from which raw data is collected

What does SFOV determine

The number of detectors collecting data for a scan

What is the display field of view (DFOV)

The portion of the collected data used to create the image; must be equal to or smaller than the SFOV

What happens if anatomy is outside the SFOV

It cannot be reconstructed

What does a smaller DFOV do

Acts like zoom, displaying a smaller area in greater detail

What is post-processing

Manipulating raw and image data after scanning to enhance visualization

What data is stored in PACS

Image data

What does image reformation or rendering use

Image data to produce images in different planes or 3D views

What can be changed with retrospective reconstruction

DFOV, image center, reconstruction algorithm, image incrementation, or slice thickness

What is the advantage of changing image incrementation

Reduces stair-step effect in multiplanar and 3D images

Why can't slice thickness be smaller than detector acquisition

Because the CT system cannot reconstruct data it didn't collect

Why isn't raw data stored

Too large and complex to maintain

What is multiplanar reformation (MPR)

A technique that produces new slices in different orientations such as sagittal, coronal, or oblique

What is curved planar reformation (CPR)

A reformation following the center of a tubular structure like vessels or colon

What data is required for MPR and 3D reformations

Image data with Hounsfield Units

Why must DFOV and image center match for MPR

Ensures identical alignment across planes

What is 3D reformation

A technique representing the full scan volume in one image

What is surface rendering (shaded surface display)

3D display using only the surface information within a defined HU threshold

What type of values does surface rendering use

Pixel values based on HU thresholds

What is volume rendering

Uses all voxel data, assigning opacity values to each voxel to create a semi-transparent 3D image

What is the difference between surface and volume rendering

Surface uses pixel surface data; volume uses all voxel data

What is maximum intensity projection (MIP)

A 2D image reconstructed from 3D data showing the highest attenuation values; used for angiography

What is minimum intensity projection (MinIP)

A display showing the lowest HU values, highlighting air-filled structures

What are quantitative measurements

Software-based calculations of volume, distance, and density within CT images

What is QCT used for

Quantitative CT bone densitometry; measures beam attenuation of bones to assess density

What is a CT number

The measurement of linear attenuation coefficient for a tissue, expressed in HU

What are CT numbers also known as

Hounsfield numbers

What are the standard Hounsfield Unit values

Water = 0, Air = -1000, Dense bone = +1000

What does pixel brightness correspond to

Hounsfield Unit value

Can attenuation of tissue be changed

Yes, by using contrast media

Why are Hounsfield Units clinically useful

They help identify and measure different tissues and pathologies

What is windowing

Adjusting displayed image contrast by manipulating window width (WW) and window level (WL)

What does window width (WW) represent

The range of CT numbers displayed

What does reducing window width do

Increases image contrast

What does a wide window width do

Creates a long gray scale with low contrast (used for bone imaging)

What does a narrow window width do

Creates a short gray scale with high contrast (used for soft tissue imaging)

What does window level (WL) represent

The center CT number of the window width; determines tissue brightness

Where should WL be set for bone imaging

Between +300 and +1000

Where should WL be set for fluid structures

Around 0

Where should WL be set for lung imaging

Around -500

How does increasing window level affect the image

Shifts the gray scale toward higher HU (brighter tissues)

How does decreasing window level affect the image

Makes the image appear darker

What happens if window width increases

Longer gray scale and lower contrast

What happens if window width decreases

Shorter gray scale and higher contrast

What does window level tell you

The tissue type of interest

What is region of interest (ROI)

A selected area on the image used for measuring HU values and standard deviation

What does standard deviation indicate in ROI analysis

Variation in HU within the selected region; closer to zero means more uniformity

Why is ROI size important

It should match the size of the structure being measured

What is distance measurement used for in CT

To measure size or distance between structures (e.g., aortic diameter)

What is image annotation

Adding text or markers on an image to convey additional information

What is a reference image

A scout or overview image showing slice locations relative to anatomy

What is image magnification

Temporarily enlarging a portion of an image for detailed viewing

What are two methods of image magnification

Changing DFOV (permanent change) and using magnifying glass (temporary zoom)

What is cine imaging

Continuous scanning without table movement; used to monitor contrast enhancement in vessels

When is cine imaging most commonly used

During arterial phase imaging to capture timing of contrast peaks