MDCT PHYSICS

Dr Varun Bansal

Dept of Radio-Diagnosis

IGMC, Shimla

TOPICS TO BE COVERED

1. Basic Principles

CT concept

2. Generation of CT image

Scan/ data acquisition

Reconstruction

Display

3. Image Quality

Quantitative measurements

Image artifacts

Basic principle

CONCEPT:

Internal structure of an object can be reconstucted from multiple projections of the object.

CT image is a display of the anatomy of a thin slice of the body developed from multiple x-ray absorption measurements made around the body’s periphery.

Image in conventional tomography blurring out the information from unwanted regions

In CT constructed using data arising only from section of interest.

Routine CT generated axial image image is reformatted to coronal / sagittal.

Generation of CT imageScan / data acquisition:

Components:

Scan frame:early Ct scanners – rotating frames and recoiling system cable step and shoot methodsCurrent systems slip rings (transmitting electrical energy across rotating)

X ray Generators:operating frequencies 5 to 50 kilohertzpower 15 to 60 kW80 to 140 kV and 30 to 500 mA

Computer modulated generators:1. ability to obtain high x-ray flux when needed2. provision for tube heat management3. ability to change the current as needed to maintain high image quality within the context of ALARA.

X-ray Tubes:

rotating anodes, unique cooling methods

2 MHU with 300kHU/min 7 MHU with 1 MHU/min

enhance system’s ability to cover large areas of anatomy at

diagnostic levels of x-ray output.

Double tube designs:

increase x-ray source, faster acquisition time

spectral absorption differences of tissues

characterising tissues and abnormalities – fat content, stones.

Data acquisition system (heart of CT system):

1. Detector system

2. Analogue to digital conversion

3. Data processing

Detectors:

1. Scintillation crystals:

produce light when ionizing radiation reacts with them.

Scintillation detector

1st 2 gen used Thallium-activated NaI

Disadv- hygroscopic and very long afterglow.

Replaced by silicon photodiodes (a/c solid-state)

CeI, gadolinium oxysulfide and Cd tungstate. ( no afterglow)

2. Xenon Gas Ionization Chambers:

limited to use in rotate-rotate type scanners.

Should have- anode and cathode, inert gas, voltage, walls

separate, window

disadv- inefficiency

xenon (heaviest), compressing to 8 – 10 atm, long chamber size.

FIXED ARRAY DETECTORS ( equal width)ADAPTIVE ARRAY DETECTORS ( unequal width)

MultiDetector CT

MDCT sequential (axial) scanning Using sequential (axial) scanning, the scan volume is covered by

subsequent axial scans in a ‘‘step-and-shoot’’ technique.

In between the individual axial scans the table is moved to the

next z-position.

With the advent of MDCT, axial ‘‘step-and-shoot’’ scanning has

remained in use for only a few clinical applications, such as

- head scanning,

- high-resolution lung scanning,

- perfusion CT

- interventional applications

MDCT spiral (helical) scanning

Spiral/helical scanning is characterized by continuous gantry rotation and continuous data acquisition while the patient table is moving at constant speed.

Advantages:

1. Minimize motion artifacts

2. Decreased incidence of mis-registration between consecutive axial scans

3. Reduced patient dose

4. Improved spatial resolution in z- axis

5. Enhanced multi-planar or 3D rendering.

For general radiology applications, clinically useful pitch values range from 0.5 to 2. For the special case of ECG gated cardiac scanning, very low pitch values of 0.2–0.4 are applied to ensure gapless volume coverage of the heart during each phase of the cardiac cycle.

Reconstruction Process

A cross sectional layer of body is divided into many tiny blocks.

Then each block is assigned a number proportional to degree that the block attenuated the x-ray beam voxel

Their composition and thickness, along with the quality of the beam, determine the degree of attenuation.

Linear attenuation coefficient (µ) is used to quantitate attenuation.

Algorithms for Image Reconstruction 1. Back projection

2. Iterative methods

3. Analytical methods

BACK PROJECTION

oldest method. Not used now, but easiest to explain, prototype

Block is scanned from top and left side.

Heights of steps is proportional to the amount of radiation that passed through the block.

Rays from two projections are superimposed or back projected , they produce a crude

reproduction of the original object.

In practice, many more projections would be added to improve image quality, but the

principle is the same.

ITERATIVE METHODS:

• Starts with an assumption ( eg all points in the matrix have the same value) and compares this assumption with measured values, make corrections to bring the two into agreement, and then repeat the process over and over until the assumed and measured values are the same or within acceptable limits.

• Three variations:

1. Simultaneous reconstruction: all projections for the entire matrix are calculated at the beginning of the iteration, and all corrections are made simultaneously for each iteration.

2. Ray-by-Ray Correction: one ray sum is calculated and corrected, and these corrections are incorporated into future ray sums.

3. Point-by-Point correction: calculations and corrections are made for all rays passing through one point, and these corrections are used in ensuing calculations, again with the process being repeated for every point.

Analytic methods:

used in almost all CT today.

Differ from iterative methods in that exact formulas are utilized for

the analytical reconstructions.

Types:

1. Two-Dimensional Fourier Analysis:

basis any function of time or space can be represented by the

sum of various frequencies and amplitudes of sine and cosine waves.

Ray projections with squared edges, are the most difficult to

reproduce.

2. Filtered Back projection:

is similar to back-projection except that the image is filtered, or modified

to exactly counterbalance the effect of sudden density changes, which

caused blurring (the star pattern) in simple back-projection.

Those frequencies responsible for blurring are eliminated to enhance

more desirable frequencies.

Inside margins of dense areas are enhanced while the centres and

immediately adjacent areas are repressed.

Image reconstruction for Spiral / MDCT

Display:CT number (HU) After a CT scanner reconstructs an image, the relative pixel values represent

the relative linear attenuation coefficients.

A CT numbering system that relates a CT number to the linear attenuation coefficients of x-rays has been devised.

Where

K is magnification constant

µw attenuation coefficient of water

µ is the attenuation coefficient of the pixel in question.

Window level: the position describing the center of scale.

Window width: the range of CT numbers selected for gray-scale amplification.

ADVANCED DISPLAY

Multiplanar reformatting conventional CT study consists of several contiguous axial images

perpendicular to the long axis of the body,- coronal or sagittal images

are not possible except when the gantry is tilted or the body is

positioned to show the image in the desired plane.

This limitation of CT can be overcome by image manipulation

commonly referred to as multiplanar reformatting (MPR).

In this process, image data are taken from several axial slices and are

reformatted to form images.

In this viewing mode, the user defines the number of imaging planes and their position, orientation, thickness, and spacing, and the reformatted image is displayed in sagittal, coronal, or oblique planes.

3D Shaded Surface Reconstruction

For a surface reconstruction, the user selects a threshold range. This

allows the user to select only the tissue (e.g., bone) to be rendered.

The voxels with Hounsfield values within the threshold range are set to

the “on” state, whereas the rest of the voxels are set to the “off” state.

The second step is to project rays through the entire volume. As the

rays pass through the data, they stop when they identify the first “on”

voxel. For that particular ray, this first “on” voxel is part of the surface;

the other voxels are ignored. This is done for all the rays, and all of the

“on” voxels are used to create the surface.

A, Sample of eight-voxel data set with displayed CT number values. B, User-defined threshold of CT number values for tissue definition.

Three-Dimensional depth based shading With this method, those voxels that are closer to the viewer are

illuminated at a greater intensity than those that are farther back.

As with sagittal and coronal images, the quality of the image is

improved by interpolating the data between slices to form a smooth,

continuous image. When various three-dimensional views are

selected sequentially in time, the image can appear to rotate.

Volume Rendering technique that displays an entire volume set with control of the opacity or

translucency of selected tissue types. In this case, each voxel has an

associated intensity in addition to an associated opacity value.

advantage of VR over three-dimensional shaded-surface rendering is that it

provides volume information.

By using transparency, the operator can visualize information beyond the

surface. For example, VR is the preferred stent evaluation because stents

can be made transparent to view the lumen of vessels.

Other - ability to make plaque transparent for more accurate diagnosis of

vessel stenosis.

MIP (Maximum Intensity Projection) Unlike three-dimensional shaded-surface and VR displays, no preprocessing is required.

The rays are cast throughout the volume, and depending on whether it is maximum

intensity projection or minimum intensity projection, maximum or minimum values

along the rays are used in the final image display.

Using maximum intensity projection (MIP) for visualization permits easy viewing of

vascular structures or air-filled cavities.

MIP enables easy viewing of an entire vessel in one image.

This is because voxels representing the contrast-filled vessels are most likely to be the

ones with the highest values along the ray (assuming no bone along the ray).

Along the same lines, minimum intensity projection can be used to demonstrate air-

filled cavities.

Image QualityQuantitative Measurements

Spatial Resolution:

measured by the ability of a CT system to distinguish two small, high-contrast

objects located very close to each other under noise-free conditions.

required for evaluating high-contrast areas of anatomy,

such as the inner ear, orbits, sinuses, and bone in general, because of their

complicated shapes.

Spatial resolution can be specified by spatial frequencies, which indicate how

efficiently the CT scanner represents different frequencies. Modulation transfer

function (MTF) describes this property

Filter effects on resolution

the major role of the convolution filter is to remove the image

blurring created by the back-projection process.

Various filters control the amount of image blurring created by

accentuating high-frequency components found in the data.

For a crisp image, the high spatial frequencies are accentuated, and

this has the effect of sharpening the edges and improving resolution.

One pays for a crisp picture with a decrease in density resolution.

Similarly, by increasing density resolution, one pays by loss of some

spatial resolution and image crispness.

Opening size of Detector Aperture

The detector aperture MTF curve depends on the

magnification factor of the system and the physical size

of the detector.

If the object being viewed is smaller than the width of

the data ring, it will be difficult to resolve because it

occupies only a fraction of the space seen by the

detector.

Typical detector apertures of CT systems today range

from less than 1 mm to 1.5 mm, with center-to-center

detector spacing of approximately 1 mm.

Factors Affecting Spatial Resolution

Focal Spot Smaller focal spot, SR improves

Detector width Smaller Detector Width, SR improves

Number of Projections More projections, SR improves

Slice thickness Smaller ST, SR improves

Pitch Lower pitch, SR improves

Pixel Size Smaller Pixel Sixe, SR improves

FOV Decreasing FOV(everything else

constant), SR improves

Patient Motion Decreased Patient motion, SR improves

Pixel Size

the spatial resolution can be no greater than the size represented by

the pixel length.

In reality, pixel size should be 1.5 to 2 times smaller than the desired

resolution.

Unless a matrix element exactly coincides with an object, the object

representation will be averaged over two or more pixels and thus may

not be visualized.

It must be realized that the pixel size refers to the FOV (or body), not

the viewing screen or film.

Contrast Resolution

ability to differentiate the attenuation coefficients of adjacent areas of

tissue.

In the computation of any single pixel value, there is error in the form of

statistical variation; it is this variation that limits the ultimate contrast

resolution.

This variation (called image noise) is manifested as a grainy

background, or mottle.

The parameter used to evaluate this variation is the standard

deviation (SD).

NOISE can be reduced by Increasing tube voltage, tube current, scan time, FOV &Slice thickness Using reconstruction filters

Factors Affecting Contrast Resolution mAs More mAs, CR improves

Pixel Size FOV and pixel size increase, CR improves

Slice thickness ST increases, CR improves

Reconstruction filter Using Soft tissue improves CR

Patient Size For larger patients, at same technique,

more attenuation, detected photons

decreases, CR degrades.

Temporal resolution

refers to the ability of a CT scanner to capture objects that change

shape or position over time and depends primarily on the gantry

rotation speed and the reconstruction method used.

depends on:

1. gantry rotation speed

2. spiral interpolating algorithm used during reconstruction.

Ring Artifacts usually the result of difficulty with the detector.

each detector is associated with a data ring. A malfunction of any one

detector incorrectly back-projects along the data ring to produce the ring

artifact.

If a detector is not matched or is not intercalibrated accurately, the back-

projection for each data ring will be slightly different, causing multiple

rings.

Detectors in the center of the detector arc are most sensitive.

Metal and Bone Artifacts

The presence of objects having an exceptionally high or low

attenuation can create artifacts by forcing the detector to operate in a

nonlinear response region.

Because this incorrect response occurs at specific directions of the

beam through the object, incomplete cancellation of the back-

projected rays during reconstruction occurs and yields streaking

artifacts.

Beam-Hardening artifacts result from the preferential absorption of low-energy photons from the beam.

average beam photon energy is progressively increased.

toward the end of the x-ray path, the attenuation is less than at the beginning because

the attenuation coefficient is smaller with higher energy.

The reconstruction program, however, assumes a monochromatic beam and attributes

any change in beam intensity to a change in tissue composition rather than to the result

of a shift in average photon energy.

The assigned attenuation coefficients are thus in error, and the densities seen on the

image are in error.

The effect is most pronounced in regions of large attenuation, such as bone.

Stair-Stepping Artifact

occur when in one direction the pixel of the reformatted image has

the same length as the axial image

but in the other direction the pixel length is the same as the slice

thickness.

Because pixel length in most scans is considerably smaller than slice

thickness, the reformatted scan has an unusual appearance.

Uncommon in modern CT.

Thank YouReferences :- CT and MRI of the Whole Body: 5th edition; HAAGA Christensen’s Physics of Diagnostic Radiology. Recent Advances: AIIMS, PGI, MAMC Series. Volume CT: State-of-the-Art Reporting. AJR 2007;

189:528–534 Three-dimensional volume rendering of spiral CT data:

theory and methods. Radiographics 1999;745-764. Developments in CT. Imaging, 18 (2006), 45–61