Computed Tomography For Technologist Flashcards
Main advantages of CT over conventional radiography are
☑️elimination of superimposed structures
☑️ability to differentiate small differences of anatomic structures and abnormalities
☑️superior quality of the images
____________ describes the ability of a system to define small objects distinctly.
Spatial Resolution
_________ refers to the ability of a system to differentiate, on the image, objects with similar densities.
Low-contrast resolution
Refers to the speed that the data acquired.this speed is particularly important to reduce or eliminate artifacts that result from object motion, such as those commonly seen when imaging the heart.
Temporal Resolution
The thickness of the cross sectional slice is referred to as its ______
Z Axis
Is the grid formed from the rows and columns of pixels.
Matrix
The number of photons that interact depends on the:
☑️thickness
☑️density
☑️atomic number of the object
Defined as the mass of a substance per unit volume and a degree which matter is crowded together, or concentrated
Density
The relative number of photons interact _________ with the increased density, thickness and atomic number of the object.
Increases
The degree to which an xray beam is reduced by an object is referred to as
Attenuation
The amount of the xray beam that is scattered or absorbed per unit thickness of the absorber is expressed by the ________
Linear Attenuation Coefficient
Quantify the degree that a structure attenuates the xray beam.
Hounsfield Units
Hounsfield Number for air
-1000
Hounsfield number for bone
+1000
Factors that contribute to an inaccurate Hounsfield measurement include
☑️poor equipment calibration
☑️image artifacts
☑️volume averaging
All xray beam sources for CT and conventional radiography produce xray energy that is _________which xray beam comprises photons with varying energies
Polychromatic
Appear as dark streaks or vague areas of decreased density sometimes called cupping artifacts.
Beam-hardening artifacts
The process in CT by which different tissue attenuation values are averaged to produce one less accurate pixel reading is called
Volume Averaging or Partial Volume Effect
The reconstruction that is automatically produced during scanning is often called
prospective reconstruction.
Brain of CT scanner
CPU
The CT process can be broken down into three main segments:
Data Acquisition → Get Data Image Reconstruction → Use Data Image Display → Display Data
The Hounsfield unit value is ________ related to linear attenuation coefficient. _____ HU equals a 0.1percent difference bet the LAC of the tissue as compared with the LAC of water.
Directly
1
Use a brush-like apparatus to provide continuous electrical power and electronic communication across a rotating surface.
Permits the gantry frame to rotate continuously, eliminating the need to straighten twisted system cables.
Slip Rings
________ is used to increase the intensity raeof the beam, increasing its penetrating ability and thereby reducing patient dose. Help also to reduce the heat load on the xray tube by allowing a lower mA setting
High KV
Is often used for the anode target material because it produces higher-intensity xray beam. Has an atomic number of _____.
Tungsten, 74
Are used to shape the xray beam, helps to reduce the radiation dose to the patient and improves image quality.
Compensating filters
A body scanning filters, used to reduce the beam intensity at the periphery of the beam corresponding to thinner areas of a patient’s anatomy
Bow tie filters
Restrict the xray beam to a specific area thereby reducing scatter radiation. Control the thickness by narrowing or widening the xray beam.
Collimators
Located below the patient and above the detector array, shapes the beam after it has passed through the patient, primary function are to ensure the beam is in the proper width as it enters the detector and to prevent scatter radiation from reaching the detector.
Predetector collimators
Is used to describe the entire collection of detectors included in a CT system, comprise detector elements situated in an arc or ring, each of which measures the intensity of transmitted xray radiation along a beam projected from the xray source to that particular detector element.
Detector Array
Refers to the ability with which the detector obtains photons that have passed through the patient
Capture Efficiency
Refers to the number of photons absorbed by the detector and is dependent on the physical properties of the detector face (thickness material)
Absorption Efficiency
Is the time required for the signal to the detector to return to zero after stimulation of the detector by x-ray radiation so that it is ready to detect another x-ray event.
Response time
Is the ratio of the maximum signal measured to the minimum signal the detectors can measure.
Dynamic Range
Are also called scintillation detectors because they use a crystal that fluoresces when struck by xray photon.
Solid-state detectors
The size of detector opening is called the
Aperture
Characteristics of Solid-state crystal detector
☑️high photon absorption ☑️sensitive to temperature,moisture ☑️solid Material ☑️Can exhibit afterglow ☑️No front window loss
Characteristics of Pressurized Xenon Gas detector
☑️Moderate photon absorption ☑️Highly Stable ☑️Low density material (gas) ☑️no afterglow ☑️Losses attributable to front window and the spaces taken up by the plates
The configuration of the xray tube to the detector determines _____________.
Scanner generation
A _________scanner design is one in which the xray tube is placed opposite the detector array. Both the tube and the detector move in a circle within the gantry. Also called rotate-rotate scanner.
Third Generation
A system that allow xrays to be focused directly on the detector bank, which reduces the amount of scatter that reaches the detector.
Third generation
A ________scanner design uses a detector array that is fixed in a 369 degrees circle within the gantry. The tube rotate within the gantry. Also called Rotate Only scanner.
Fourth generation
Measures the number of photons that strikes the detector, converts the information to a digital signal and sends the signal to the computer
Data-Acquisition System (DAS)
Why is an ADC a necessary part of ADS?
Because one of the task of ADS is to convert the analog signal to a digital format through ADC (analog to digital converter)
Why it is important for all CT staff to set landmarks in the same way?
Helpful if the scan will be repeated at a later date, exclusively through the area of interest determined on the earlier scan. Accurate table referencing helps to maintain consistency between examinations
Explain why artifacts are common in CT systems with a third generation design?
Because the same bank of detectors is used repeatedly, even a very small alignment of a single detector will result in visible ring artifact.
What is the advantage and disadvantage of small focal spot?
☑️Advantage: reduced penumbra, produce sharper images (netter spatial resolution)
☑️Disadvantage: they concentrate heat onto a smaller portion of the anode, cannot tolerate as much heat
Is a precise set of steps to be performed in a specific order to solve a problem. The basis for most computer programming.
Algorithm
Mathematical method of estimating the value of an unknown function using the known value on either side of the function. A mathematical method of creating missing data.
Interpolation
The principal components in a computer are
- an input device
- an output device
- a central processing unit (CPU)
- memory.
Components of computer that are ancillary pieces of computer hardware designed to feed data into the computer or accept processed data from the computer.
input and output device
- Examples of input device are keyboard, mouse, touch-sensitive plasma screen, and CT detector mechanisms.
- Output devices include monitor, laser camera, printer, and archiving equipment such as optical disks or magnetic tape.
interprets computer program instructions and sequences tasks. It has been referred to as the “brain” of the CT system.
central processing unit, or CPU
Explain how computer memory works. In what situation could scan data be lost?
*The computer is turned on.
• T he computer loads data from ROM and performs tests to make sure all the major components are functioning properly. The computer loads the basic input/output system from ROM, providing information about things such as storage devices, start-up sequences, security, and ancillary device recognition.
• T he computer loads the operating system (OS) from the hard disk into the system’s RAM. The critical parts of the OS are maintained in RAM as long as the computer is on, allowing the CPU to have immediate access to the OS.
• W hen an application is opened, it is loaded in RAM. This temporary storage area allows the information to be more readily accessed by the CPU. Hence, the CPU gets the data it needs from RAM, processes it, and writes new data back to the RAM in a continuous cycle.
• When you save a fi le and close the application, the fi le is written to the specific storage device, and then it and the application are purged from RAM. If the fi les are not saved to a permanent storage device before being purged, they are lost.
• I n CT systems, software instructs the computer to automatically save reconstructed images to a permanent storage device to prevent unintentional loss.
is imprinted at the factory and is used to store frequently used instructions such as those required for starting the system.
ROM
includes instructions that are frequently changed, such as the data used to reconstruct images. Is so named because all parts of it can be reached easily at random. Is very fast, but is also volatile, losing the stored data in the case of a power loss.
RAM
Refers to computer storage devices that can be written to once, but read from many times.
WORM
Includes all measurements obtained from the detector array. Storage requires much more computer storage space than that of image data.
Raw Data
All of the thousands of bits of data acquired by the system with each scan are called
Raw Data
- The terms scan data and raw data are used interchangeably to refer to the data sitting in the computer waiting to be made into an image.
The process of using raw data to create an image is called
Image Reconstruction
The reconstruction that is automatically produced during scanning is often called
Prospective Reconstruction
Are those which result once the computer has processed the raw data. One Hounsfi eld unit value is assigned to each pixel.
Image Data
The path that the x-ray beam takes from the tube to the detector is referred to as a
Ray
The DAS reads each arriving ray and measures how much of the beam is attenuated. This measurement is called a
Ray Sum
A complete set of ray sums is known as a
View
The system accounts for the attenuation properties of each ray sum and correlates it to the position of the ray.
Attenuation Profile
Compiles the information from all of the attenuation profi les to create an image.
Back Projection
There is a significant drawback to back projecting data onto a matrix: it produces streak artifacts in a star pattern on the image. To minimize these artifacts, a process called ________ is applied to the scan data before back projection occurs. The process of filtering is done through complicated mathematic steps. The process of applying a filter function to an attenuation profile is called ________.
Filtering
Convolution
Use Fourier theory to reduce statistical noise and create an image that is pleasing to the eye.
Filtered Back-Projection Algorithm
is also called calibration field of view. Selecting the this determines the area, within the gantry, from which the raw data are acquired.
SFOV (Scan Field of View)
Selecting this determines how much of the collected raw data is used to create an image. Also called zoom or target.
Display Feld of View (DFOV)
The section of data selected for display on the image.Selecting this determines how much of the collected raw data is used to create an image. Also called zoom or target.
Display Feld of View (DFOV)
Change the digital signal from the computer memory back to an analog format so that the image can be displayed on the monitor.
DAC (Digital-to-analog converters)
Is an output device that transfers the image from the monitor to the film
Camera
By pass the image on the display monitor and transfer data directly from the computer, bypassing the video system entirely, thereby signifi cantly improving image quality.
Laser Cameras
The film used in CT consists of a _________ that is sensitive to either the light-emission spectrum of the video screen phosphor (for the multiformat camera) or to the laser beam light.
Single Emulsion
The number of Hounsfield units assigned to each level of gray is determined by the
Window Width
Is used to display CT images. This system assigns a certain number of Hounsfield units to each shade of gray
Gray Scale
________ assigns the quantity of pixel values to the gray scale. ________ determines the center pixel value in the gray scale.
Window Width
Window Level
Selects which Hounsfield values are displayed as shades of gray.
Window Level
The width stays unchanged at 300, but the center is moved to 200. The new range of Hounsfield numbers to be included in the gray scale is from 50 to 350. Determining the range of Hounsfield values requires only simple arithmetic. How?
First, divide the window width in half. Next, subtract the quotient from the window level to determine the lower limit of the range, and add the quotient to the window width to determine the upper limit.
300/2 = 150
200-150 = 50
300+50 = 350
CT number = from 50 to 350
All values higher than those in the selected range will appear ______ on the image. All values lower than those in the selected range will appear _____ on the image.
white
black
The manipulation of window width and window level to optimize image contrast is referred to as ________.
windowing
The ________ should be set at a point that is roughly the same value as the average attenuation number of the tissue of interest.
window level
An area on the image defined by the operator. Is the first step in a number of image display and measurement functions.
Region of Interest
Typical window setting for Head Posterior fossa
window width - 150
window level - 40
Typical window setting for Brain
window width - 100
window level - 30
Typical window setting for Temporal bone
window width - 2800
window level - 600
Typical window setting for Neck
window width - 250
window level - 30
Typical window setting for Chest Mediastinum
window width - 350
window level - 50
Typical window setting for Lung
window width - 1500
window level - -600
Typical window setting for Abdomen Soft tissue
window width - 350
window level - 50
Typical window setting for Liver (high contrast)
window width - 150
window level - 30
Typical window setting for Pelvis Soft tissue
window width - 400
window level - 50
Typical window setting for Pelvis Bone
window width - 1800
window level - 400
Typical window setting for Spine Soft tissue
window width - 250
window level - 50
Typical window setting for Spine Bone
window width - 1800
window level - 400
The amount of CT number variance within the region of interest is indicated by the
Standard Deviation
Image magnification is NOT the same as
decreasing the display field of view.
Is a graphical display showing how frequently a range of CT number occurs within an ROI
Histogram
When is it helpful to magnify a CT image?
Magnification allows relevant clinical detail to be more easily seen and more accurately measured. However, magnification has inherent limitations and should not be used as an alternative to correct display field selection.
True or False
All routine CT studies require at least one localizer scan. Proper centering of the localizer scans is particularly important when automatic exposure control options are used.
True
True or False
Incorrectly inputting directional instructions into the CT scanner can result in images that have been mislabeled and can result in misdiagnosis and serious medical errors.
True
The older, traditional method of scanning is referred to as ________________. Key to this method is that the CT table moves to the desired location and remains stationary while the x-ray tube rotates within the gantry, collecting data.
axial scanning, conventional scanning, serial scanning, sequence scanning, or step-and-shoot scanning.
What are the advantages and disadvantages of axial scan methods?
Advantages:
- highest image quality, superior to that of helical methods.
- Axial slices can be programmed so that the data acquired are contiguous, gapped, or overlapping.
Disadvantages:
*cumulative effect of the pauses between each data acquisition adds to the total examination time. Although the cumulative delay may be less than 30 seconds, even this brief delay is often signifi cant.
In _______ scanners a single gantry rotation can produce multiple slices. Therefore, it provides longer and faster z axis coverage per gantry rotation.
MDCT
Explain how the detectors in MDCT systems can be grouped to produce slices of various thicknesses.
The slice thickness of an MDCT scanner is not determined solely by the degree of physical collimation, but also by the width of the detectors in the slice thickness (z axis) dimension. Combining different numbers of individual detector elements together changes the width of the detectors.
What are the three basic ingredients that define a helical scan process?
- a continually rotating x-ray tube
- constant x-ray output, and
- uninterrupted table movement.
To take helical scanning from theory to practice, many obstacles associated with traditional axial CT had to be overcome. The major improvements leading to its development were
1) x-ray gantries with a slip ring design,
2) more-effi cient tube cooling,
3) higher x-ray output (i.e., increased mA capability),
4) smoother table movement,
5) software that adjusts for table motion,
6) improved raw data management, and
7) more-effi cient detectors.
__________ is often referred to as volumetric scanning. This refers to the fact that the end result of such a scanning method is a block of data, not separate slices, as occurs in traditional axial scanning.
Helical Scanning
Why are special interpolation methods required for helical data?
The specific interpolation technique used depends on the manufacturer and the detector configuration. They improved the quality of the helical image, in most cases making them indistinguishable from axial images.
What trade-offs are necessary when pitch is increased?
Increasing the pitch will result in a scan covering more anatomy lengthwise for a given total acquisition time. It will also reduce the radiation dose to the patient (if other scan parameters are held constant). A decrease in pitch slows down the table speed. A pitch of less than 1 will result in overlapping slices. Therefore, decreasing the pitch will decrease the amount of anatomy covered per unit time and increase the radiation dose to the patient.
Is a parameter that is commonly used to describe the CT table movement. It is most commonly defi ned as the travel distance of the CT scan table per 360° rotation of the x-ray tube, divided by the x-ray beam collimation width.
Pitch
True or False
A pitch of less than 1 is not commonly used in SDCT. In fact, on some SDCT systems, a pitch of less than 1 is not an available option.
True
The most prevalent application for using an extended pitch is _________. Because these studies require scanning to take place while vascular structures contain high amounts of an iodinated contrast agent, scan speed is paramount.
CT Angiography
In an ______ helical scanner the thickness of the x-ray beam equals the thickness of the final slice.
SDCT
In an ______ spiral scan the thickness of the x-ray beam is divided over parallel rows of detectors, in this illustration producing four slices with each gantry rotation.
MDCT
________ can be defined as table movement per rotation divided by beam width.
Beam Pitch
The beam width can be determined by multiplying the number of slices by slice thickness. For example, with a 4-slice MDCT at 4 × 1.25-mm slice thickness and a tablefeed of 6 mm per rotation, the pitch is
1.2
6/(4x1.25) = 6/5 = 1.2
The pitch for an MDCT scanner is defined by using the __________, rather than the x-ray beam width.
detector aperture
It is defined as table movement per rotation time divided by the selected slice thickness of the detector.
Detector pitch
In general, in both SDCT and MDCT systems a pitch between ____ and ____ is most common.
1 to 1.5
- Extended pitch values may result in an increase in streak artifact.
What is the formula for determining how many images are created and how much anatomy (lengthwise)that is covered in a helical scan acquisition builds on the formula used to calculate distance covered in an axial scan sequence.
SDCT Scan Coverage Number of Images Created in a Helical Scan Sequence
Pitch x total acquisition time x 1/rotation time = number of images
Example 1: When it takes 1 second for each tube rotation, a 30-second helical scan acquisition will produce 30 images when the pitch is set at 1. That is, 1 × 30 × 1/1 = 30.
Distance Covered in an SDCT Helical Scan Sequence
Pitch x total acquisition time x 1/rotation time x slice thickness = amount of anatomy covered
Example 2a: A helical scan with a 30-second total acquisition time, a 1-second rotation time, and a 5-mm slice thickness, with a pitch of 1.
1 x 30 s x 1/1 s x 5 mm = 150 mm of anatomy covered
Example 2b (change in slice thickness): When slice thickness is 10 mm (other factors unchanged from the proceeding example).
x 30 s x 1/1 s x 10 mm = 300 mm of anatomy covered Example 2c (change in gantry speed rotation): When the gantry takes just 0.5 seconds to make a complete rotation (other factors unchanged from example 2a). 1 x 30 s x 1/0.5 s x 5 mm = 300 mm of anatomy covered
________ allow the slice incrementation to be changed, retrospectively. This allows the creation of overlapping slices, without increasing the radiation dose. In some situations, changing the slice incrementation can reduce the partial volume effect.
Helical data
Explain why it might be beneficial to retrospectively change the slice incrementation of helical data.
Helical Data allow the slice incrementation to be changed, retrospectively. This allows the creation of overlapping slices, without increasing the radiation dose. In some situations, changing the slice incrementation can reduce the partial volume effect.
The thinnest image that can be reconstructed for a helical data set is often predetermined by the __________ used for the data acquisition.
slice thickness
For _______ acquisitions the image thickness may be greater than the slice thickness, but in many cases the image thickness cannot be less than the slice thickness.
MDCT
Among the factors that the operator can control are
☑️milliampere (mA) level ☑️scan time ☑️slice thickness ☑️field of view ☑️reconstruction algorithm ☑️kilovolt-peak (kVp).
Higher ____ settings allow shorter scan times to be used. A short scan time is critical in avoiding image degradation as a result of patient motion.
MA
the thicker and denser the part being examined, the more ____ that is required to produce and adequate image.
mAs
Reducing the mAs while holding the kVp constant and kVp is reduced while the mAs is held constant will
decrease patient dose
Using digital technology, the image quality is not directly linked to the dose, so even when an mA or kVp setting that is too high is used, a good image results.
Uncoupling effect
Define partial scan and overscan.
☑️Partial Scan - which is acquired from 180° plus the degree of arc of the fan angle. Although this scan is slightly more than a half circle, these scans are often referred to as half-scans. Only half of the otherwise available data are available to reconstruct the image with partial scans therefore, they are inferior to standard 360° scans. However these scans do have a limited application in studies that require short scan times.
☑️ Overscan - is the 400° scan, commonly used in fourth-generation scanner designs. By allowing some overlap of data from the fi rst and last tube positions, overscans reduced motion artifacts.
_________ relates to how well the image represents the object scanned. However, the true test of the quality of a specifi c image is whether it serves the purpose for which it was acquired.
Image quality
The two main features used to measure image quality are:
Spatial Resolution
Contrast Resolution
The ability to resolve (as separate objects) small, high-contrast objects.
Spatial Resolution (Detail Resolution)
The ability to differentiate between objects with very similar densities as their background.
Contrast Resolution
The number of line pairs visible per unit length is also called
spatial frequency
☑️ How frequently an object will fit into a given space
The _________ is the most
commonly used method of describing spatial resolution ability, not only in CT, but also in conventional radiography. It is often used to graphically represent a system’s capability of passing information to the observer.
MTF ( modulation transfer function )
The spatial resolution of a CT image can be described in what two dimensions.
☑️in-plane resolution - resolution in the xy direction
☑️longitudinal resolution - resolution in the z direction
Matrix size and DFOV selection determine
pixel size
_______ refers to how many pixels are present in the grid.
Matrix Size
☑️Because the perimeter of the square matrix is held constant, the greater the total pixels present in the image, the smaller each individual pixel. Therefore, matrix size is one factor that controls pixel size. In practice, matrix size seldom varies in CT.
Determines how much raw data will be used to reconstruct the image
DFOV
☑️ Changing the DFOV will also alter the size of the image on the screen.
☑️ Increasing the DFOV increases the size of each pixel in the image.
☑️ pixel size = DFOV/matrix size
TRUE OR FALSE
Thinner slices produce sharper images because to create an image the system must flatten the scan thickness (a volume) into two dimensions (a fl at image). The thicker the slice, the more flattening is necessary.
TRUE
_________ plays an important role in volume averaging, thereby affecting spatial resolution in the image.
Slice thickness
New CT scanners allow for very thin slice thickness; often the goal is to produce __________.
isotropic voxels
In general, ________ the pitch reduces resolution in the image.
increasing
The ability to distinguish an object that is nearly the same density as its background is referred to as
low contrast detectability.
____ is superior to all other clinical modalities in its contrast resolution. For comparison, in screen-film radiography, the object must have at least a __ difference in contrast from its background to be discernible on the image. On CT images, objects with a ____ contrast variation can be distinguished.
CT
5%
0.5%
______ is the undesirable fluctuation of pixel values in an image of a homogeneous material. Grainy appearance or “salt-and-pepper” look on an underexposed image.
Noise
TRUE OR FALSE
The presence of noise on an image degrades its quality, particularly its contrast resolution.
TRUE
How is the CT image affected by too low an mAs setting? How is it affected if the mAs setting is too high?
☑️Increasing mAs, will improve contrast resolution, but at the cost of a higher radiation dose to the patient.
☑️ Examples of indications in which a lower mAs might be acceptable are solid nodule detection in the lung, coronary artery calcium detection, or the identifi cation of emphysema in the lung.
Doubling the mAs of the study increases the SNR by ____.
40%
☑️Therefore, if the initial image was degraded by quantum noise then doubling the mAs will improve the contrast resolution of repeat scans.
Keeping all other scan parameters the same, as pixel size decreases, the number of detected x-ray photons per pixel will
decrease
TRUE OR FALSE
Bone algorithms produce lower contrast resolution (but better spatial resolution), whereas soft tissue algorithms improve contrast resolution at the expense of spatial resolution.
TRUE
TRUE OR FALSE
For the same x-ray technique, larger patients attenuate more x-rays photons, leaving fewer to reach the detectors. This reduces SNR, increases noise, and results in lower contrast resolution.
TRUE
__________ refers to how rapidly data are acquired. It is controlled by gantry rotation speed, the number of detector channels in the system, and the speed with which the system can record changing signals.
Temporal Resolution
Give examples of examinations in which images may need to be reconstructed twice, using different algorithms.
Certain studies require the data to be reconstructed with two separate algorithms. One optimizes low-contrast detectability in the soft tissue. The second provides optimal high spatial resolution and is preferable for bone.
Example: images of internal auditory canal
Factors that affect spatial resolution.
List the factors that affect contrast resolution.
Contrast Resolution
☑️mAs/Dose, Slice Thickness, Reconstruction Algorithm, patient size
Spatial Resolution
☑️Matrix Size, Display Field of View, Pixel Size
Slice Thickness, Sampling Theorem, Reconstruction Algorithm, Focal Spot Size, Pitch
Because an object may not lie entirely within a pixel, the pixel dimension should be half the size of the object to increase the likelihood of that object being resolved.
Sampling Theorem
