The formation of a CT image is a distinct three phase process.

The scanning phase produces data, but not an image.

The reconstruction phase processes the acquired data and forms a digital image.

The visible and displayed analog image (shades of gray) is produced by the digital-to analog conversion phase.

There are adjustable factors associated with each of these phases that can have an effect on the characteristics and quality of the image.

During the scanning phase a fan-shaped x-ray beam is scanned around the body.

The amount of x-radiation that penetrates the body along each individual ray (pathway) through the body is measured by the detectors that intercept the x-ray beam after it passes through the body.

Like all medical images, CT images have the five specific image quality characteristics.  They are:

• Contrast Sensitivity (very high for CT)

• Blurring and visibility of Detail

• Visual Noise

• Artifacts

• Spatial (Tomographic slice or volume views)

Each of these characteristics are affected by the selection of protocol factor values that control the imaging process.

The projection of the fan-shaped x-ray beam from one specific x-ray tube focal spot position produces one view.

Many views projected from around the patient's body are required in order to acquire the necessary data to reconstruct an image.

As the x-ray beam is scanned around the body, forming many views, the data recorded by the detectors are stored in computer memory for later image reconstruction.

Illustration from Scientific American

A ray is the pathway of a portion of the x-ray beam from one specific focal-spot position to a specific detector position.

As the ray passes through the body, it measures the total x-ray attenuation (or penetration) along it's path.  This is the data recorded by the detector.

A view, as seen previously, is made-up of many individual rays.

Illustration from Scientific American 

A complete scan is formed by rotating the x-ray tube completely around the body and projecting many views.

Each view produces one "profile" or line of data as shown here.

The complete scan produces a complete data set that contains sufficient information for the reconstruction of an image.

In principle, one scan produces data for one slice image.  However, with spiral/helical scanning, there is not always a one-to-one relationship between the number of scans around the body and the number of slice images produced.

The principle objective of CT imaging is to produce a digital image (a matrix of pixels) for a specific slice of tissue.

During the image reconstruction process, the slice of tissue is divided into a matrix of voxels (volume elements).

As we will see later, a CT number is calculated and displayed in each pixel of the image.  The value of the CT number is calculated from the x-ray attenuation properties of the corresponding tissue voxel.

There are two distinct motions of the x-ray beam relative to the patient's body during CT imaging.

One motion is the scanning of the beam around the body as we have just seen.

The other motion is the movement of the beam along the length of the body.  Actually, this is achieved by moving the body through the beam as it is rotating around.

Scan and step is one mode of scanning. It was the first scanning mode developed and is still used today for some procedures.

It works like this, one complete scan around the body is made while the body is not moving.  Then the body is moved to the next slice position.

The principle characteristic (and limitation) of this mode is that the data set is fixed to a specific slice of tissue.  This means that the slice thickness, position, and orientation is "locked in" during the scanning phase.

We are about to see that spiral/helical scanning offers an alternative.

Spiral or helical scanning (both names are correct) is a more recently developed mode and is used for many procedures.

The patient's body is moved continuously as the x-ray beam is scanned around the body.

This motion is controlled by the operator selected value of the pitch factor.

As illustrated, the pitch value is the distance the body is moved during one beam rotation, expressed as multiples of the x-ray beam width or thickness.

If the body is moved 10 mm during one rotation, and the beam width is 5 mm, the pitch will have a value of 2.

During a spiral scan, the body is actually moved through the x-ray beam.

However, it is helpful to think of it from this perspective like we were moving along with the patient.

What we see is the beam moving along the body, distributing radiation and collecting data continuously.

As we see here, when the pitch is increased, the x-ray beam appears to move faster along the patient's body.

During the same time (as illustrated), the x-ray beam will be spread over more of the body when the pitch is increased.  This has three major effects.

1. Scan time will be less to cover a specific body volume.

2. The radiation is less concentrated so dose is reduced.

3. There will not be as much "detail" in the data and image quality might be reduced.

The radiation detectors are very small elements (individual detectors) that are arranged in rows that span and intercept one view.

A specific CT machine can be designed to have either a single row of detectors or multiple rows as shown.

There are advantages in having multiple rows.

A body section can generally be scanned faster with a multiple row detector system because there are multiple fan beams scanning simultaneously.

This is especially significant when covering a relatively large body section with thin beams for the purpose of producing thin, high-detail slice images or 3-D volume images.

A major advantage of spiral/helical scanning it that it produces a continuous data set extending over some volume of the patient's body.

The data set is not broken up into slices as with the scan/step slice acquisition method.

As we will soon see, the volume data set can be sliced many ways later during the image reconstruction phase.

The scan and step slice acquisition method produces data sets that are fixed to specific slices of tissue.

A major advantage of spiral scanning is that the thickness, position, and orientation of image slices can be adjusted during the reconstruction phase.

Images of overlapping slices can be created.

The reconstruction can be repeated to produce images with different spatial characteristics.

A volume data set can be used to reconstruct 3-D images.

A general requirement for good-quality 3-D images is that the data set have "good detail" in the long patient axis direction.  This is achieved by scanning with thin beams and relatively low pitch values.

Image reconstruction is the phase in which the scan data set is processed to produce an image.  The image is digital and consist of a matrix of pixels.

Filtered back projection is the reconstruction method used in CT.

"Filtered" refers to the use of the digital image processing algorithms that are used to improve image quality or change certain image quality characteristics, such as detail and noise.

"Back projection" is the actual process used to produce or "reconstruct" the image.  It will be illustrated later.

 This is a very simple example illustrating the principle of image reconstruction by back projection.

We start with one scan view through a body section (like a head) that contains two objects.  As we know, the data produced is not a complete image, but a profile of the x-ray attenuation by the objects.

Let's now take this profile and attempt to draw an image by "back projecting" the profile onto our image surface.

As we see, there is only enough information in the profile to allow us to draw in streaks, kind of like shadows across the image area.

We have now rotated the x-ray beam around the body by 90⁰ and obtained another view.

If we now back project this profile onto our image area we see the beginnings of an image showing the two object.

Two views does not give us a high-quality image.

  Several hundred views are used to produce clinical CT images.

As we have already discovered, the CT image is a digital in the form of a matrix of pixels.

A part of the reconstruction process is the calculation of CT number values for each image pixel.

Here we see the formula used in the reconstruction process to calculate the CT numbers.

The CT numbers are calculated from the x-ray linear attenuation coefficient values for each individual tissue voxel.  It is the attenuation coefficient that is first calculated by the reconstruction process and then used to calculate the CT number values.

Note that water is the reference material for CT numbers and has an assigned value of zero.

Tissues or materials with attenuation (density) greater than water will have positive CT numbers.  Those that are less dense will have negative CT numbers.

X-ray attenuation depends on both the density and atomic number (Z) of materials and the energy of the x-ray photons.  For CT imaging a high KV (like 120-140) and heavy beam filtration is used.   This minimizes the photoelectric interactions that are influenced by the Z of a material.  Therefore, CT numbers are determined by the density of the tissues or materials.

CT numbers are in Hounsfield Units.

The CT image is, for all practical purposes, an image of the densities of the tissue.

We have now observed two of the three phases of the CT imaging process.

The scan phase produces a data set, but not an image.

The image reconstruction phase produces a digital image from the scan data set by the process of filtered back projection.

We now move to the final phase in which the digital image is converted into a visible analog image.

In this phase the digital image, consisting of a matrix of pixels with each pixel having a CT number, is converted into a visible image represented by different shades of gray or brightness levels.

There are  several adjustable factors that control this process.

The windowing has level and width controls

The area of the digital image that is actually displayed is controlled by the zoom control.

We will now look at both of these.

The window is the range of CT numbers that will be displayed with the different shades of gray, ranging from black to white.

Tissues within the window will have different shades of gray (brightness) and will have visible contrast.

All tissues and materials that have CT numbers above the window will be all white and no contrast within this range.  All that have CT numbers below the window will be all black and without contrast.

The level control adjust the center of the window.

The width control adjust the range of CT numbers that will be displayed with contrast.  The width controls the contrast in the displayed image.  Reducing window width increases the displayed image contrast among the tissues.

The ability to window is what gives CT a very high contrast sensitivity.  This is because a window can be set to display and make visible very small differences in tissue densities.

Zooming is the process of selecting some smaller area within the total digital image to cover the full display.

It is in principle a magnification mode in that it enlarges a small area.

Zooming is very different from changing the field of view during image reconstruction.

As we will see in the next module (CT Image Quality and Dose Management) reducing the field of view has the effect of increasing image detail.  Zooming does not have the same effect.

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