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Computed Tomography: Image
production in Computed Tomography
The general process of CT imaging may be divided into several steps which include:
- correct answer ✔✔1. Image Acquisition
- Data reconstruction
- Multidimensional image display
- Image Archival and Communication What is image acquisition? - correct answer ✔✔Measurement of the attenuation that occurs along each path through the patient from the x-ray tube to detector What is Data Reconstruction? - correct answer ✔✔The computerized processing of the transmission measurements into the CT image What is multidimensional image display? - correct answer ✔✔Display of the reconstructed grayscale image in two-dimensional (2-D) and/or three-dimensional (3-D) format; a representation of the attenuation that occurred across the scanned volume of tissue What is image archival and communication? - correct answer ✔✔Display as well as storage, both short-term and long-term (archival), of images on computer workstations What is scout image (localizer/scanogram) acquisition? - correct answer ✔✔Survey that can be obtained by keeping the x-ray tube and detector array in a fixed position while the patient bed is translated through the gantry What is Azimuth? - correct answer ✔✔Angle of the tube and detectors in relationship tp the patient position during scout acquisition
What does the CT gantry hold? - correct answer ✔✔Generator, x-ray tube, assorted collimators, data acquisition system (DAS), slip-rings, and detectors What is the slip-ring technology? - correct answer ✔✔Eliminates the need for cables by utilizing a system of contact brushes that supply electricity to power the system and enable the passage of transmission data to the computer system
- allows for continuous helical acquisition that has become the standard in modern CT imaging What are the technical elements of helical CT acquisition include: - correct answer ✔✔1. Continuous rotation of the gantry components (slip-ring technology)
- A powerful x-ray tube capable of long exposure output
- Continuous movement of the patient bed
- Mathematical reconstruction techniques The development of multidetector CT (MDCT) helped advance? - correct answer ✔✔the helical acquisition principle. Multidetector arrays enable CT sections of varying widths to be reconstructed at any point along the acquired volume What does the x-ray tube consists of? - correct answer ✔✔- Negative side (cathode)
- Positive side (anode) How many focal spots are in a CT tube? - correct answer ✔✔Dual filaments allow for a choice of two focal spot sizes ranging from .5 to 1.2mm in diameter. What focal spot helps improve spatial resolution? - correct answer ✔✔- Smaller focal spot size What is flying focal spot technology? - correct answer ✔✔Involves the electromagnetic steering of the electron beam emitted from the cathode. The beam of electrons is directed toward two separate locations on the rotating anode, resulting in two sources of x-radiation.
The mAs value for a given acquisition is directly proportional to what? - correct answer ✔✔- Patient radiation dose The mAs value for a given acquisition is inversely proportional to what? - correct answer ✔✔inversely proportional to image noise What is effective mAs? - correct answer ✔✔Describes the calculated mAs per acquired slice What is the main controlling factor of effective mAs? - correct answer ✔✔Table speed The pitch selected for a given acquisition determines? - correct answer ✔✔Speed at which the patient translates through the gantry for each tube revolution What is the effective mAs formula? - correct answer ✔✔effective mAs= mAs/pitch When using the effective mAs formula, as pitch increases, the mAs applied to each acquired slice? - correct answer ✔✔Decreases What does peak kilovoltage (kVp) controls? - correct answer ✔✔the quality of the x- ray beam with regard to its overall penetrating capabilities. Higher kVp settings produce x-ray beams with greater penetrating power The use of lower kVp does what to patient dose? - correct answer ✔✔Decreases patient radiation dose and improves contrast (especially for CTA procedures) When are higher kVp setting utilized during CT scans? - correct answer ✔✔Decrease the streak artifact that may occur as the beam passes through very dense parts such as those found in the posterior fossa during a head scan
Automatic tube voltage selection (ATVS) is what? - correct answer ✔✔advanced feature that may be available on some automatically modulates the tube potential based upon the changing patient attenuation along the scan acquisition range. What are dual-energy CT systems capable of doing? - correct answer ✔✔Applying multiple x-ray energies during a single CT acquisition.
- Transmission data are acquired from tissue interaction with both high- and low- kVp radiation. Differentiation and characterization of tissue composition are made possible by the difference in attenuation between the two x-ray spectra. Within a dual- source CT system how far apart do the tubes sit from each other? - correct answer ✔✔- 90 degrees from each other in the gantry
- The two x-ray tubes acquire data simultaneously at different kVp values. Dual energy ct has expanded clinical opportunities including: - correct answer ✔✔1. Improved resolution of soft tissue structures (ligaments, tendons) during musculoskeletal imaging
- The ability to visualize atherosclerotic plaque within contrast-enhanced vasculature during cardiac CT studies
- Contrast medium subtraction techniques for the demonstration of precontract images from a single scan acquired after the IV administration of iodinated contrast media
- Characterization of the biochemical composition of urinary tract calculi What is the Half-Value Layer (HVL)? - correct answer ✔✔The thickness of material that is capable of reducing the intensity of the x-ray beam to one-half of its original value. Half-value is measured by? - correct answer ✔✔Overall quality of the beam and is helpful in determining the amount of beam filtration necessary for a given CT system The polyenergetic (heterogeneous) x-ray beam undergoes filtration in an effort to do what? - correct answer ✔✔Remove low-energy x-ray photons. These are photons with insufficient energy to complete the path from source to patient to detector and would go unmeasured, adding solely to patient radiation dose.
- Detector (section) collimation What is the general purpose of beam collimation? - correct answer ✔✔Restrict the radiation exposure to the area of interest reducing patient dose and improving contrast Single Slice Computed Tomography (SSCT) incorporates what? - correct answer ✔✔Fan-Shaped beam with a transverse (x-y) dimension corresponding to the size of the detector array and the maximum SFOV The length of detector coverage in the transverse dimension is determined by what? - correct answer ✔✔Fan angle of the emitted beam. Wider fan angle extends coverage of the detector array along the x and y axes. Along the longitudinal or z axis, the beam is collimated to the anticipated slice thickness. Located just outside the tube housing the mechanical aperture responsible for beam restriction is termed? - correct answer ✔✔Pre-patient collimator Prepatient beam collimation in SSCT (Single Slice CT) directly controls what? - correct answer ✔✔The amount of tissue exposed to x-radiation as the tube rotates about the patient. Example pg 91 collimation number 9
- No adjustments to slice thickness are possible after the slice data have been acquired during a SSCT SSCT beam collimation consists of: - correct answer ✔✔- Prepatient collimation
- Postpatient collimation What is prepatient collimation? - correct answer ✔✔A mechanical device controls the aperture dimension at the x-ray tube, directly limiting the beam to the desired section width What is postpatient collimation? - correct answer ✔✔The radiation transmitted through the patient is controlled by the predetector collimators. Postpatient (predetector) collimators are used to improve image quality by limiting scatter radiation before it reaches the detectors
What are the two components of prepatient collimation? - correct answer ✔✔- The tube housing itself acts as an initial collimator
- The primary prepatient collimator is positioned just beyond the tube housing How does the tube housing act as a collimator? - correct answer ✔✔The tube aperature roughly shapes the primary beam into a fan or cone shape for SSCT or MDCT. The beam emerges from the tube housing at a specific fan angle, corresponding to the in-plane length of the detector array. How does the prepatient collimator located beyond the tube housing act as as a collimator? - correct answer ✔✔It restricts the beam along the z-axis, controlling slice thickness in the SSCT system. How does prepatient collimation work during MDCT? - correct answer ✔✔Beam is limited before reaching the patient, according to the desired section width and the number of detector rows to be utilized for image formation.
- MDCT utilizes a cone-shaped beam which is incident upon an expanded array of detectors.
- The detector array consists of multiple rows of individual detector elements. Where does Postpatient collimation lie within the CT tube and how is it performed? - correct answer ✔✔Occurs through high resolution comb placed over the detector array.
- Functions like a grid.
- Comb removes unwanted scatter radiation and off-axis photons that result from the more divergent nature of the MDCT beam. As the number of detector rows in an MDCT system increases the divergence of the cone beam does what? - correct answer ✔✔Increases How is MDCT beam collimated in CT? - correct answer ✔✔Collated to the total width of the slices acquired with each gantry location.
Definition of section interval? - correct answer ✔✔Describes the spacing between two adjacent CT images. It is measured as the distance between the center of one section and the center of the next adjacent section Definition of contiguous images? - correct answer ✔✔Those acquired with equal section thickness and interval. example: 5mm sections reconstructed every 5mm will completely cover a given volume of tissue, with no unmeasured tissue. Definition of noncontiguous images? - correct answer ✔✔Those acquired with interval greater than the section width. example: 1mm sections reconstructed every 10mm would not evaluate the intermittent 9mm of tissue. Definition of overlapping images? - correct answer ✔✔Reconstructed with a section interval that is less than the section width. A percentage value may be assigned to the extend of overlap. example: 1.0mm-thick images reconstructed every .5mm are said to possess 50% overlap Overlapping images are used to improve what? - correct answer ✔✔Quality of multiplanar reformation (MPR) and 3-D reconstructions. Definition of pitch? - correct answer ✔✔described the relationship between collimation and table movement during scanning. Important for both SSCT and MDCT
- The pitch of a helically acquired CT study characterizes the amount of longitudinal data acquired for every rotation of the gantry and the overall acquired speed. The original definition of pitch (detector pitch) for single-slice helical CT (SSCT) is? - correct answer ✔✔pitch= table feed per rotation/section width Increasing the detector pitch would result in what? - correct answer ✔✔Greater table translation with each gantry rotation
Higher detector pitch setting result in greater table coverage along the z-axis with each revolution of the gantry and does what to image quality? - correct answer ✔✔Reduces image quality In the helical MDCT system, section width is no longer primarily controlled by beam collimation at the x-ray tube. What now plays a role in reconstructed section width?
- correct answer ✔✔Detector configuration What is the equation for beam pitch? - correct answer ✔✔Beam pitch= table feed per rotation/total collimation Specific to MDCT, total collimation equals the combined thickness of all of the sections that are simultaneously acquired with each gantry rotation: - correct answer ✔✔total collimation= number of sections x section width A pitch setting less than 1 does what to table speed? Does what to collecting data? Image quality? - correct answer ✔✔- Slows down the table speed
- Increases acquired data
- Improves image quality Lower pitch settings does what to patient dose? - correct answer ✔✔Increases Increasing the pitch does what to the patient going through the gantry? - correct answer ✔✔- Moves the patient through the gantry faster
- Exposes tissue to the beam for short periods and reduces patient dose Definition of penumbra? How it may occur in CT? - correct answer ✔✔Unwanted/Scatter
- As the cone beam extends beyond the range of the linear detector array. Unwanted extension of the cone-shaped beam What is a way to help improve the accuracy of the cone beam in some CT scanners?
- correct answer ✔✔Flying focal spot configuration
What is the response time of a detector? - correct answer ✔✔Its ability to quickly measure x-rays and then recover before before the next measurement What does afterglow mean? - correct answer ✔✔Refers primarily to the tendency of scintillation crystal detectors to continue to glow in response to x-rays once the exposure source has been terminated. What does the high dynamic range mean? - correct answer ✔✔The CT detector must be sensitive enough to measure a broad range of x-rays transmission data and accurate enough to modulate signal in response to small changes in radiation transmission. CT scanners use to be composed of what? - correct answer ✔✔Either gas ionization chamber or scintillation crystals What is a gas ionization detectors? - correct answer ✔✔Convert x-ray energy directly into an electrical signal What is a solid-state scintillation crystal detectors? - correct answer ✔✔Converts x- ray energy first into light energy, which must then be concerted into electrical signal Gas ionization detectors consists of what? - correct answer ✔✔Small high-pressure chambers of xenon gas. Interaction with x-rays causes a proportional amount of ionization of the xenon gas, resulting in the discharge of a small electrical signal. An increase in absorbed x-ray energy with gas ionization detectors results in what? - correct answer ✔✔Increase in emitted signal What generation of CT scanners was the gas ionization detectors used in? - correct answer ✔✔Used primarily for conventional third-generation CT systems, before the advent of helical technology All modern MDCT systems utilize what type of detectors? - correct answer ✔✔Solid- state detectors, consisting primarily of a scintillating crystal material
The detector crystal emits a quantity of light energy proportional to the x-ray incident upon an individual detector (scintillation). The light is measured by what? - correct answer ✔✔Photodiode device and is concerted to an electronic signal to be sent for processing by the CT computer system. Crystal materials employed as CT detectors include: - correct answer ✔✔Cesium iodide, cadmium tungstate, ceramic gadolinium oxysulfide, and scintillating gemstone (garnet) minerals What types of detectors are preferred for MDCT? - correct answer ✔✔Solid-state scintillation-type (nonxenon) detectors are preferred for MDCT because of their ability to accurately record incident x-ray energy from any angle. - This flexibility is important because of the widened cone beam geometry inherent to MDCT systems What did the first generation of the CT scanner consist of: - correct answer ✔✔Consisted of 2 detectors (paired adjacently along the x-axis) exposed to a tightly collimated pencil beam of x-rays
- The tube and detectors would translate across the patient's head, rotate by 1 degree, and then translate back
- Method is known as the Rectilinear or translate- rotate scanning What did the second generation of the CT scanner consist of: - correct answer ✔✔- Utilizes a translate-rotate geometry
- increase in the number of detectors, allowing for a greater increments of rotation between translations -reduction in the overall scan time.
- First use of fan beam to expose a wider detector array. Expanded clinical capabilities beyond head imaging What did the third generation of the CT scanner consist of: - correct answer ✔✔- An expanded curvilinear array of detectors rotates with the x-ray tube around the patient.
- Rotate-rotate geometry eliminates the need for translation
- Larger fan beam is used to expose the expanded detector array
All MDCT systems utilize what generation geometry? - correct answer ✔✔3rd- generation All MDCT systems use what type of detector? - correct answer ✔✔Solid-state scintillation detectors Compared to a one dimensional detector array the MDCT is said to be 2-D. What is significant about using MDCT opposed to SSCT? - correct answer ✔✔MDCT= uses curvilinear detector array with multiple rows of individual detector elements.
- Results in larger coverage area over a given scan time
- Reconstruct images varying thicknesses at any given z-axis position within the volume data acquisition With thinner slices what happens to Spatial Resolution? With thicker slices? - correct answer ✔✔Increases= thinner Decreases= thicker Section Width in SSCT was controlled by what? - correct answer ✔✔Beam Collimators.
- Prepatient collimators restricted the primary beam at the venter of rotation equal to the desired section width MDCT the detector collimation determines the? - correct answer ✔✔the width of the reconstructed section
- By electronically adjusting the detector dimension, the operator can control the width of the x-ray beam contributing to a reconstructed section= beam collimation no longer directly controls section width. Overall, MDCT technology no longer indicates that the width of a CT is determined by prepatient collimator but instead controlled by what? - correct answer ✔✔Electronic separation or combination of signals from the individual detector rows What is detector configuration? - correct answer ✔✔Refers to the number, length, and organization of the individual detector elements in an MDCT system
What are the three general formats of MDCT detector configuration are as follows: - correct answer ✔✔- Uniform matric array
- Adaptive array
- Hybrid array What does uniform matric array mean? - correct answer ✔✔Utilizes multiple detectors in the longitudinal direction, each the same length. Ex: 4 slice MDCT employs 16 detector elements each 1.25mm in length What does adaptive array mean? - correct answer ✔✔Detectors are configured with the thinnest width at the center, surrounding by detectors of incrementally increasing widths along the z-axis. Ex: MDCT with an adaptive array of 16 rows may be composed of two middle detectors of 1.0 each (2 mm total), followed by pairs of 1.5 mm (3 mm total), 2. mm (5 mm total), and 5 mm (10 mm total), moving outward from the center of the array. Added together, the detectors total 20 mm along the z-axis What does hybrid array mean? - correct answer ✔✔Consists of two detector sizes. The narrower detectors are positioned midline, flanked by the wider detectors. Ex: The central rows of a 16 channel MDCT system are .625 mm, with the remaining peripheral rows at 1.25mm The quantity of data channels controls what? - correct answer ✔✔The number of sections the scanner can simultaneously acquire with each gantry rotation. Ex: What is commonly referred to as a 16-slice MDCT system has, in fact, 16 data channels at its disposal. The maximum number of sections that may be reconstructed from each gantry rotation is 16. The number of detectors in the array is equal to or greater than what? - correct answer ✔✔The number of data channels.
- The 16 slice MDCT system may utilize an adaptive array of 24 detectors With 16 possible data channels, the maximum number of sections this system can simultaneously acquire is what? - correct answer ✔✔ 16
