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104 Cards in this Set
- Front
- Back
CT has many differences from |
conventional radiography in order to overcome limitations |
|
Unlike conventional radiographic images, CT |
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 |
absorbed in different ways by the body tissues |
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Because bone is denser, it absorbs |
more x-rays then less soft tissues |
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Differential absorption is contained in the |
xray beam that passes through the patient and is recorded |
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The major shortcoming of radiography is that the |
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 |
slight density changes of tissues |
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Limitations prove to be a |
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 |
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 |
the limitations of radiography and tomography |
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It does this by achieving the following: |
1. minimal superimposition 2. improved image contrast 3. the recording of very small differences in tissue contrast |
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Physical principles |
1. data acquisition 2. data processing 3. image display, storage, communication |
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Data acquisition refers to the |
systematic collection of information from the patient to produce the CT Image |
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Two methods of data acquisition |
1. slice by slice data acquisition 2. volume data acquisition |
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Slice by slice Acquisition |
the xray tube rotates around the patient and collects data for the first slice |
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The tube stops, and the patient |
moves into position for the next slice |
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The process is repeated until |
all slices are required |
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Data collected through |
different beam geometries to scan the patient |
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Volume Data Acquisition |
special beam geometry may be referred to as spiral or helical scanning |
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Volume data acquisition is used to |
scan a volume of tissue rather than one slice at a time |
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Two types of volume data acquisition scanners |
1. single slice CT 2. multi slice CT |
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Spiral/helical CT |
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 |
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 |
scanning |
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Radiation attentuation |
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 |
1. effective atomic density (atoms/volume) 2. atomic number of the absorber 3. photon energy |
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Homogenous beam |
referred to as a monochromatic or monoenergetic beam |
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The quality of the beam or beam energy |
does not change as it is attenuated |
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If the starting beam energy is 88 kEv, the transmitted photons |
all have an energy of 88 kEv |
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All photons have |
the same energy |
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Heterogenous beam |
referred to as polychromatic beam |
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Attenuation is not |
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 |
increases and the beam becomes harder |
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Heterogeneous beam- all photons have |
different energies |
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Goal of CT |
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 |
photoelectric effect |
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Data acquisition geometries |
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 |
1. continuous rotation 2. stationary detectors |
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Continuous rotation |
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 |
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 |
mathematical principles that basically occur in a two step process |
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Data processing: 2 step process |
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 |
1. corrections are made 2. reformatting of data occurs |
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Image reconstuction |
1. scan data converted into digital image characterized by CT # 2. reconstruction algorithms are applied |
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Reconstruction algorithms are |
a mathematical procedure where conversion of the attenuation readings into a CT image is accomplised |
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After data processing |
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 # |
each pixel in the reconstructed image is assigned a CT # |
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# are related to the |
linear attenuation coefficients of the tissues in the slice |
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Hounsfield scale |
based on the attenuation of water as a reference point |
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Water |
0 HU |
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Bone |
+1000 HU |
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AIr |
-1000 HU |
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What calculates CT # |
computer -which can be printed as numerical image |
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This image must be converted into a |
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 |
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 |
beam energy |
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In CT, a high-kilovolt technique is generally used for the following reasons |
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 |
maintain the precision of the CT # in order to reduce artifacts and avoid misdiagnosis |
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Display device |
high performance monitors |
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The display device is |
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 |
optimize image fidelity |
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Resolution is an important physical parameter of the gray scale monitor and is related to |
the size of the pixel, matrix, or matrix size |
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Windowing |
CT image composed of a range of CT # that represent varying shades of gray |
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Window width |
the range of CT # |
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Window level |
window center center of the range |
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Both are located on the |
control console |
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Both alter |
image contrast and brightness |
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Windowing is the process of |
changing the CT image gray scale |
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Window width controls |
image contrast |
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Window level/center controls |
image brightness |
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As the window level/center increases, the image |
goes from white (bright) to dark (less bright) and the image contrast changes for different values |
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3 windows are shown |
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 |
1. scan FOV 2. display FOV |
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FOV |
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 |
circular region rom which the transmission measurements are recorded during scanning |
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Display FOV |
can be equal to or less than the scan FOV |
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During image reconstruction and data collection, a matrix is |
placed over the scan FOV to cover the slice to be imaged |
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The matrix and FOV are |
composed of pixels |
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Because the slice has to be scanned and has the dimension of the depth, the pixel is |
transformed into a voxel, or volume element |
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The radiation beam passes through |
each voxel and a CT # is then generated for each pixel in the displayed image |
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Pixel size= |
FOV/Matrix |
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Pixel size is generally ranging from |
1-10mm |
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THe smaller the pixel size, the |
better the spatial resolution of the image |
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The numerical value of the pixel represents the |
brightness of the image at that pixel position |
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Pixel size can be computed from the |
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 |
# of bits per pixel |
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Image consists of a series of |
bit planes - referred to as bit depth |
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Voxel size depends on the |
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 |
isotropic |
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The ultimate goal of a CT scanner is to produce |
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 |
design and major subsystems of the CT scanner |
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Sequence of events |
10 |
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1. xray tube and detectors rotate around the patient who is positioned in the |
gantry aperture
|
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2. Radiation attenuated as it passes through the patient - transmitted photons |
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 |
both converted into electrical current signals that are amplified by special circuits |
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4. Before data sent to computer, it must |
be converted into digital form - this is done by ADC or digitizers |
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5. Data processing |
beings |
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6. Convolution performed on the data by the |
array processors |
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7. Specific reconstruction algorithm reconstructs an |
image of the internal anatomic structures under exam |
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8. Reconstructed image can then be |
displayed/stored on a magnetic/optical tape or discs |
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9. Image processor allows the |
performance of various digital image postprocessing operations on the displayed image |
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10. Control terminal usually an |
operators control console, which completely controls the CT system |
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Advantages of CT |
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 |
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 |