Chapter 3- Physical Principles Of Ct

Front 1

CT has many differences from

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conventional radiography in order to overcome limitations

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Unlike conventional radiographic images, CT

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1. shows cross sectional (transaxial) views of the patient anatomy

2. shows 3D images that are computer generated with use of the transaxial data set

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In both RAD and TOM, xrays pass through the patient and are

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absorbed in different ways by the body tissues

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Because bone is denser, it absorbs

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more x-rays then less soft tissues

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Differential absorption is contained in the

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xray beam that passes through the patient and is recorded

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The major shortcoming of radiography is that the

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superimposition of all structures on the radiograph makes it difficult to discriminate to distinguish a particular part

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Other limitations - difficulty distinguising

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slight density changes of tissues

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Limitations prove to be a

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qualitative process rather than a quantitative process

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CT somewhat overcame the superimposition problem posed by conventional radiography however, these problems remained

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1. persistent image blurring

2. failure to demonstrate small differences in tissue contrast

3. degradation of image contrast

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The goal of CT is to overcome

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the limitations of radiography and tomography

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It does this by achieving the following:

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1. minimal superimposition

2. improved image contrast

3. the recording of very small differences in tissue contrast

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Physical principles

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1. data acquisition

2. data processing

3. image display, storage, communication

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Data acquisition refers to the

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systematic collection of information from the patient to produce the CT Image

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Two methods of data acquisition

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1. slice by slice data acquisition

2. volume data acquisition

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Slice by slice Acquisition

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the xray tube rotates around the patient and collects data for the first slice

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The tube stops, and the patient

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moves into position for the next slice

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The process is repeated until

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all slices are required

Front 18

Data collected through

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different beam geometries to scan the patient

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Volume Data Acquisition

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special beam geometry may be referred to as spiral or helical scanning

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Volume data acquisition is used to

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scan a volume of tissue rather than one slice at a time

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Two types of volume data acquisition scanners

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1. single slice CT

2. multi slice CT

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Spiral/helical CT

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x-ray tube rotates around the patient and traces a spiral/helical path to scan the entire volume of tissue while patient holds a single breath

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THis method generates a

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single slice per revolution of the xray tube - referred to as a single slice/spiral helical CT

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First step in data acquisition

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scanning

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Radiation attentuation

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the reduction of the intensity of a beam of radiation as it passes through an object - some photons absorbed, some scattered

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CT beam attenuation dependent on

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1. effective atomic density (atoms/volume)

2. atomic number of the absorber

3. photon energy

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Homogenous beam

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referred to as a monochromatic or monoenergetic beam

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The quality of the beam or beam energy

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does not change as it is attenuated

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If the starting beam energy is 88 kEv, the transmitted photons

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all have an energy of 88 kEv

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All photons have

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the same energy

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Heterogenous beam

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referred to as polychromatic beam

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Attenuation is not

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exponential but rather both the quality and quantity of the photons change

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As a result, the penetrating power of the photons

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increases and the beam becomes harder

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Heterogeneous beam- all photons have

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different energies

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Goal of CT

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to calculate the linear attenuation coefficient which indicated the amount of attenuation that has occured

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Xrays can be attenuated because of

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photoelectric effect

Front 37

Data acquisition geometries

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defined as the way that the x-ray tube and detectors are arranged to collect transmission measurements

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Two data acquisition geometries

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1. continuous rotation

2. stationary detectors

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Continuous rotation

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xray tube and detectors are coupled and rotated 360 degrees around the patient to collect transmission measurements by using a fan beam of radiaton

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Stationary detectors

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xray tube rotates 360 degrees around the patient and is positioned inside a stationary ring of detectors; radiation beam also describes a fan

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Data processing

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mathematical principles that basically occur in a two step process

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Data processing: 2 step process

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1. Raw data undergo some form of preprocessing

2. image reconstruction

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Raw data (data received from the detectors) undergo some form of preprocessing

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1. corrections are made

2. reformatting of data occurs

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Image reconstuction

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1. scan data converted into digital image characterized by CT #

2. reconstruction algorithms are applied

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Reconstruction algorithms are

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a mathematical procedure where conversion of the attenuation readings into a CT image is accomplised

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After data processing

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the reconstructed image is displayed for viewing and is sent for storage/communicated through PACS to remote sites for review by other physicians

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CT #

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each pixel in the reconstructed image is assigned a CT #

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# are related to the

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linear attenuation coefficients of the tissues in the slice

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Hounsfield scale

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based on the attenuation of water as a reference point

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Water

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0 HU

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Bone

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+1000 HU

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AIr

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-1000 HU

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What calculates CT #

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computer

-which can be printed as numerical image

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This image must be converted into a

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gray scale image because it is more useful to radiologist than a numerical printout

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The linear attenuation coefficient changes based on the

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energy level (keV) of the x-ray beam thus potentially affecting CT # because they can be calculated on the basis of the attenuation coefficients

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Attenuation coefficient changes with

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beam energy

Front 57

In CT, a high-kilovolt technique is generally used for the following reasons

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1. reduce dependence of attenuation

2. reduce contrast of bone relative to soft tissues

3. produce high radiation flux at the detector

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CT systems incorporate a # of correction schemes to

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maintain the precision of the CT # in order to reduce artifacts and avoid misdiagnosis

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Display device

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high performance monitors

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The display device is

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the gray scale image is displayed on a TV monitor (CRT) or liquid crystal display which is an essential component of the control/viewing console

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In the display/manipulation of grayscale images for diagnosis, important to

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optimize image fidelity

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Resolution is an important physical parameter of the gray scale monitor and is related to

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the size of the pixel, matrix, or matrix size

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Windowing

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CT image composed of a range of CT # that represent varying shades of gray

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Window width

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the range of CT #

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Window level

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window center

center of the range

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Both are located on the

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control console

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Both alter

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image contrast and brightness

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Windowing is the process of

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changing the CT image gray scale

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Window width controls

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image contrast

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Window level/center controls

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image brightness

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As the window level/center increases, the image

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goes from white (bright) to dark (less bright) and the image contrast changes for different values

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3 windows are shown

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1. bone window (optimized for imaging bone)

2. mediastinal window (optimized for imaging mediastinal structures)

3. lung window (optimized for imaging lungs)

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Format of the CT image

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1. scan FOV

2. display FOV

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FOV

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field of view, or reconstruction circle, which is a circular region from which the transmission measurements are recorded during scanning

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Scan FOV

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circular region rom which the transmission measurements are recorded during scanning

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Display FOV

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can be equal to or less than the scan FOV

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During image reconstruction and data collection, a matrix is

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placed over the scan FOV to cover the slice to be imaged

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The matrix and FOV are

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composed of pixels

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Because the slice has to be scanned and has the dimension of the depth, the pixel is

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transformed into a voxel, or volume element

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The radiation beam passes through

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each voxel and a CT # is then generated for each pixel in the displayed image

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Pixel size=

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FOV/Matrix

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Pixel size is generally ranging from

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1-10mm

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THe smaller the pixel size, the

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better the spatial resolution of the image

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The numerical value of the pixel represents the

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brightness of the image at that pixel position

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Pixel size can be computed from the

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FOV and the matrix size through the following relationship: pixel size, d= FOV/matrix size

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CT image can be characterized by the

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# of bits per pixel

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Image consists of a series of

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bit planes - referred to as bit depth

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Voxel size depends on the

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matrix and FOV (pixel size) plus slice thickness

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When the length, width, and height of a voxel are equal (a perfect cube) the voxel is referred to as

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isotropic

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The ultimate goal of a CT scanner is to produce

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high quality CT images with minimal radiation dose and physical discomfort to the patient

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How this goal is achieved is influenced by the

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design and major subsystems of the CT scanner

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Sequence of events

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10

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1. xray tube and detectors rotate around the patient who is positioned in the

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gantry aperture

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2. Radiation attenuated as it passes through the patient - transmitted photons

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measured by 2 sets of detectors - a reference detector (measures intensity of radiation from the xray tube) and another set that records xray transmission through the patient

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3. Transmitted beam and reference beam

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both converted into electrical current signals that are amplified by special circuits

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4. Before data sent to computer, it must

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be converted into digital form - this is done by ADC or digitizers

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5. Data processing

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beings

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6. Convolution performed on the data by the

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array processors

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7. Specific reconstruction algorithm reconstructs an

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image of the internal anatomic structures under exam

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8. Reconstructed image can then be

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displayed/stored on a magnetic/optical tape or discs

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9. Image processor allows the

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performance of various digital image postprocessing operations on the displayed image

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10. Control terminal usually an

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operators control console, which completely controls the CT system

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Advantages of CT

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1. excellent low contrast resolution

2. versatility in scanning options

3. 3D imaging

4. image modification

5. contrast scale can be varied by adjusting window width and window level

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Compared with conventional radiography and tomography, CT disadvantages include

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1. poorer spatial resolution

2. higher radiation dose

3. metallic objects produce streak artifacts

4. difficult to image soft tissue surrounded by bone due to the creation of artifacts