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Ct Physics Module 2 > Powerpoints > Flashcards

Powerpoints Flashcards

1
Q

Digital imaging process(5)

A

Scanning

Sampling

Quantization

Analog to digital conversion

Digital to analog conversion

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2
Q

Process(3)

A

X-ray beam scans across the patient in synchrony with a detectors on the opposite side of the tube

Sufficient number of transmission measurements are taken at different orientations of X-ray source and detector , then the distribution of attenuation coefficients within the layer may be determined

By assigning different levels to different attenuation coefficients an image image can be reconstructed with aid from a computer that represents various structures with different attenuation properties

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3
Q

Attenuation coefficient

A

Reflects the degrees by which the X-ray beam intensity is reduced by the tissue it passes through

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4
Q

Reconstruction

A

Process in which attenuation data is Summed up from thousands of angles used

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5
Q

Sampling

A

When detectors measure the beam intensity on it

Intensity varies depending on attenuation of beam and the projection .

Aliasing, a streaking artifact appear if not enough samples are obtained

Measurements taken in hounsfield units (hu) , calibrated universally , with air at -1000 and water at 0 and bone at 1000(hu)

There is a new sampling technique( pg 131) the z- flying focal spot technique. Where two overlapping slices for each detector row are obtained at same time per 360 degree rotation

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6
Q

Aliasing

A

Streak artifact appears if not enough samples obtained

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7
Q

Quantization

A

Digital processing quantization is process of mapping a large set of input values to a countable a smaller set

Difference between an input value and it quantized value such as round off error is referred to as quantization error . A device or algorithmic function that performs quantization called quantizer

Analog digital converter is example of quantizer

Also forms core essentially lossy compression algorithms

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8
Q

Quantization

A

Process of mapping a large set of input values to a smaller set

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9
Q

Quantization error

A

Difference between input value and it’s quantized value such as round off error

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10
Q

Quantizer

A

Device or algorithmic function that performs quantization

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11
Q

Analog digital conversion

A

Radiation is attenuated as it passes through patient

Transmitted photons are measured by the detectors

Transmitted beam is converted into electrical current signals that are amplified by special circuits. These signals are sent to das

Then followed by logarithmic amplification in which the transmission readings are changed into attenuation and thickness data . Then this data is sent to analog digital converter or digitizer . Adc dynamic range must be large dynamic range of X-ray image. The outputs of adcs are routedto an image signal processor

Data processing begins

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12
Q

Analog to digital conversion 3 components

A

Components of the adc are the sampler, quantizer, coder

Coder assigns a binary bit sequence to each discrete output from the quantizer. Ex 01001 is digital signal output

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13
Q

Pacs

A

Needs an interfacing between Ct scanner and pacs

Related to work lists , image distribution, his/ris and dicom specifications

Typically is addressed by radiologists, physicians, technologists staff

Soft copy display of images can be viewed on monitors

Image distribution is an important issue in Ct pacs. It is the eligibility of system to send images to caregivers

Integration of all system is quite critical

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14
Q

Dicom

A

Digital imaging and communication in medicine

Dicom is imaging protocol pacs

Image management and archival needs be compliant with dicom

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15
Q

Digital to analog conversion

A

The resulting X-ray image data set is converted to an image by image processor

Image processor is typically very high speed computer which performs massive calculations required for tomographic image construction

Further image processing is necessary to map large dynamic range most effectively into limited visible display range

Then output digital data are subsequently converted into an analog signal needed to operate analog display devices such as computer screen

This conversion is done by dac ( solid state electronics)

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16
Q

Data acquisition methods

A

Slice by slice- smct

Volumetric- msct

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17
Q

Beam geometry

A

Size, shape, motion of beam and its path. For ex pencil, fan shaped

Components - physical devices that shape and define beam. Ex: detector position and number of detectors plus pre- collimation

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18
Q

Scanning / raw data/ image data(4)

A

Rays
Views
Profiles
Sampling

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19
Q

Rays

A

Part beam that falls on the detector

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20
Q

Views

A

Collection of rays for one translation

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21
Q

Profiles

A

Electrical signal coming from the detector . Signal represents a profile( pixels, matrices, voxels)

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22
Q

Sampling

A

Each transmission measurement is a data sample

23
Q

Attenuation

A

Reduction of beam

24
Q

Lambert beer law

A

Attenuation based on a homogeneous beam

25
Linear attenuation coefficient
Indicates the amount of attenuation that has occurred in tissue
26
Ct/ housnfield numbers
Scale is a linear transformation of the original linear attenuation coefficient measurement into one which the radiodensity of distilled wate at standard pressure and temperature (stp)is defined as zero hounsfield unit while the radiodensity of air at stp is defined as -1000 hu
27
Profiles
Pixel Matrix Voxel
28
Pixel
Picture element ( individual box) each pixel represent a wide range of different shades gray ( from white to black)
29
Gray scale or bit depth ranges
Number grey shades 8bits- to 12 bits
30
Matrix
Series of boxes in rows and columns. Each box displays a numerical value ( visual brightness or density level). The greater the matrix for the same fov the better the special resolution
31
Voxel
Volume element
32
Selectable scan factors
Scan field of view Matrix size Slice thickness
33
Scan field of view
Region from which transmission signals are recorded during scanning. Chosen technologist and it should be appropriate to exam
34
Matrix size
Chosen by tech before procedure and depends on anatomy of interest
35
Slice thickness ssct vs msct
In single slice volume Ct the slice thickness is determined by the pitch and the width of the pre collimator ( beam width or bw ) . In msct the slice thickness is determined by the bw, the pitch and other factors such as the shape and width of the reconstruction filter in z-axis . Should be adapted to area of interest
36
Window width
Range of Ct numbers used for particular protocol, controls image contrast
37
Window level
Center of the range , controls image brightness
38
Kvp and mas
Preset by manufactured but should be manipulated by technologist. Increasing these improves signal to noise ratio but increase patient dose. Automatic tube current( atcm) is commonplace in Ct
39
Algorithm
Reconstruction algorithmns such as brain, standard, bone
40
Scan time and rotational arc
Increasing rotational time will increase resolution because image is generated from more projections
41
Roi
Region of interest measurement can be taken or can be used to track contrast concentration in predetermined area. Part of basic to aid in visualization. Roi can be transferred from slice to slice
42
Magnification
Part of basic tool
43
Focal spot size and beam geometry
Selectable scan factors
44
Pitch
Ratio of the distance the table travels per rotation( 360 degrees) to the total collimated X-ray beam width. Increasing helical pitch results in shorter scan time but reduces overall image quality because of interpolation in scan data
45
Slice sensitivity profile
Describes how thick a section imaged and to what extent details within the section contribute to signal
46
Image reconstruction Type computer used? Conventional Ct? Single detector row? Multidetector row?
Needs a computer - an array processor Uses different reconstruction algorithmns depending on scanner used Conventional Ct uses filtered back projection Single detector row spiral: linear interpolation Multidetector row spiral: longitudinal interpolation with z-axis filtering, interlaced sampling, convolution algorithmn
47
Image reconstruction single slice conventional Ct | Filtered back projections( aka convolution method) steps
All projection profiles are obtained Logarithm of the data obtained Logarithmic values are multiplied by the digital filter or convoluting filter, to generate a set filtered profiles Filtered profiles are the back projected Filtered projections are summed up producing an image free of blurring
48
Convolution
Digital image processing technique to modify images through a filter function
49
Image reconstruction single slice Ct
Same as filtered back projection algorithm used with additional considerations Because patient moves continuously through the gantry for a 360 rotation, reconstructed images will be blurred and therefore interpolation is necessary before filtered back projection is used Interpolation algorithm uses 360 degree linear interpolation. Algorithmn used two know values to reconstruct data Then 180 interpolation algorithmn used. Algorithmn uses calculated complimentary data, then the distance between two points of data set are used to interpolate. The distance between two points is known as z gap
50
Z gap affects image quality- smaller gap???
Better image quality
51
Image reconstruction multi slice spiral Ct
The z gap is determined by pitch and by detector row spacing Interpolation still occurs using z sampling Logituidinal interpolation by z filtering can be used as reconstruction algorithm. Interpolation of the spiral helical data is still needed. But then a filter is used to improve image quality Another reconstruction method called interlaced sampling used smaller z gaps by adjusting or selecting pitch to separate the complimentary data from 180 data Fan beam reconstruction can be used if the number of detector rows is small. This method uses multiple fan beams to approximate the cone beam geometry of msct
52
Isotropic scanning
Dimensions of the size of the voxel( x,y,z) are all equal Happens when slice thickness equals pixel size Data set acquired is isotropic Major goal of isotropic imaging in Ct is achieve excellent special resolution in all imaging planes, specially in mpr and 3D May require narrow collimation which increases scan time This gives more dose to patient
53
3D
Needs powerful computer workstation with advanced image processing capabilities and increased memory ( use of optical disk storage allows for increased image storage versus optical disk storage) 3d allows uses to render various 3d images from axial data set 3d falls under three catagoies( ssd( surface rendering), vr( volume rendendering), and mip and minIP Cta's are application of 3d imaging. Tools include 4d, vessel tracking skull removal and multiple target volume( 4 d is opacity instead of time)
54
Coder
Coder assigns a binary bit sequence to each discrete output from the quantizer. Ex 01001 is digital signal output

Ct Physics Module 2 (1 decks)

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