CT Final Exam
3 steps involved in image reconstruction from a projection data
Data Acquisition, Image Reconstruction, and Image Display
Discuss ways to reduce dose to the patient in CT.
Dose parameters specific to the patient based on weight and the body part being imaged mAs: dose is directly proportional to mAs, if mAs is doubled, dose is also doubled Kvp: dose is proportional to the square of Kvp Pitch, collimation, penumbra
Calculate the effective dose to an organ based on exposure and tissue weighting factor.
Dose x weighing factor x tissue factor
Discuss terms associated with detector capabilities.
Efficiency- ability to capture, absorb, and convert x-ray photons into electrical signal • Stability- the steadiness of the detector response • Response time- the speed at which the detector can measure x-rays and recover to detect the next event •Dynamic range- ratio of the largest to the smallest signal that can be discriminated
Using TG66, discuss the necessary components of a CT scanner used as a simulator.
"CT scanner equipped with a flat tabletop and (preferably, but optional) external patient positioning lasers"
Discuss the advantages of large bore scanners when used in radiation therapy.
85 cm bore compared to 70 cm which allows for more flexibility in set up positions (fitting within the bore while in position). Also increased the SFOV 60 cm compared to 48 cm.
Make discussion on generations and characteristics of the generations of scanners.
1st Generation: based on parallel beam geometry and translate-rotate scanning motion (single pencil beam and single detector). 2nd Generation: based on fan beam geometry and translate-rotate motion; also known as rectilinear multiple pencil beam scanning. 3rd Generation: baes on fan beam geometry and complete rotation of tube and detectors. 4th Generation: based on fan beam geometry and complete/continuous rotation of the x-ray tube around a stationary ring of detectors (based on slip ring technology). 5th Generation: developed primarily for high speed CT scanning; also EBCT. 6th Generation: multiple x-ray tubes and detectors; intended specifically to image moving structures (heart); also known as DSCT. 7th Generation: use flat panel digital area detectors similar to the ones used in digital radiography. e of Teflon (low atomic # and high density). Features bilateral symmetry with a thickness that increases with the distance from the center of the beam. They compensate for differences in beam path length through the axial plane of the object so that a more uniform fluency is delivered to the detector.
Discuss the differences between analog and digital systems.
Analog System: Requires ADC to convert the image to digital Constant flow of information (infinite variable) When radiation passes through the patient it hits the detectors and emits an electronic analog signal absorbed by the detectors.
Discuss algorithms that can reduce the patient dose in CT.
Back-projection Iterative Algorithms Analytic Reconstruction Algorithms
Describe what is meant by beam geometry.
Beam Geometry is the size, shape, and motion of the beam and its direction. It plays a big role in spatial resolution.
Discuss the physical components of the scanner including the patient support system, gantry, tube, detectors, internal lasers, ADC, etc.
Better quality images/algorithm can provide 3D images (only axial slices are taken, sagittal/coronal slices are created from CT algorithm) More sophisticated tx plans possible Less tx dose possible to nearby critical structures.
Discuss bits and how they define the grayscale. Give examples.
Bits are also known as binary digits The number of bits per pixel is the bit depth For example: in a digital image with a bit depth of 2, each pixel will have 2^2 (4) gray levels (density) A bit depth of 8 implies that each pixel will have 2^8 (256) gray levels or shades of gray SO (256) = white And (0) = black
Discuss factors affecting contrast resolution.
Contrast resolution - the ability of the scanner to demonstrate small differences in tissue contrast; can be affected by the following: noise - fluctuation of CT numbers between points in the image for a scan of uniform material CT dose index Scattered radiation considerations
Discuss types of detectors for CT equipment. Describe their key attributes and discuss which ones are in use today.
Detectors convert x-ray photons (attenuated data) into electrical signals, which is then converted into a digital signal so computer can read the signal. Gas-Ionization Detectors: Consisted of Xenon gas were used primarily for conventional third-generation CT systems. Gas ionization chambers convert x-ray energy directly into an electrical energy. Delayed recovery time, before they can detect another signal. Obsolete. Scintillation Detectors: Scintillators are coupled to photodiodes. Consists of gadolinium oxysulfide and ceramic gemstone. Scintillating crystal materials emit light energy proportional to the x-ray it received. Fast recovery time. In use today.
Discuss the steps in the CT simulation process.
Explain the process of CT. B) Describe the examination to be performed. C) Emphasize the importance of keeping still. D) Giving appropriate breathing instructions. E) Have the patient empty the bladder before the examination so the patient is more comfortable. F) If a contrast medium is to be used, explain the reason for its use and question the patient about allergies. G) Reassure the patient that the technologist, although not in the room, will be able to see and talk to the patient
Discuss the physics concepts that result in image formation using CT.
Formation of CT images by CT scanner involves 3 steps: data acquisition; image reconstruction; image display/post processing, storage and communication Data acquisition - collection of x-ray transmission measurements from the patient X-rays fall onto detectors that measure transmission values, aka attenuation values Conversion of electrical signals to digital data **after enough transmission measurements have been collected, they are sent to computer for processing** Image reconstruction - math algorithms used to reconstruction CT image Image display/post processing & storage Reconstructed images can be displayed and recorded for subsequent viewing and stored for later analysis Post processing - used to make image more useful to observer for diagnostic interpretation Ex: smoothing, edge enhancement, gray scale manipulation, 3D image processing
Explain the different algorithms used in CT vs. MRI. How does each work?
Fourier Transform: Images can be acquired in the spatial frequency domain, such as those acquired in MRI. Frequency meaning the number of cycles per unit length, that is, the number of times a signal changes per unit length. Digital image processing can transform one image domain into another image domain. Example: and image in the spatial location domain can be transformed into a spatial frequency domain image. The Fourier Transform ™ does this. (page 31) Basically: used to reconstruct images of a patients anatomy in CT and also in MRI Back-Projection: The mathematical process primarily responsible for CT image reconstruction. The ray sum (measurement of the total x-ray absorption of a particular ray) is projected back onto a matrix. A summary of multiple projections to produce an image; can be characterized by a star pattern artifact Convolution: applied to reduce image unsharpness. It is to reduce blurry representation of the object caused by the gantry rotation. It is a algorithm or convolution kernel that acts as a mathematical filter. Step 1 = Convolution / Step 2: = Back Projection
Discuss the difference and roles of hardware and software components.
Hardware: High-voltage generator (x-ray production), x-ray tube (x-ray production), collimators (collimate to the area of interest), filters (reduce patient skin dose), air conditioners (keep gantry from over heating) Software: Data acquisition system (Converts attenuation information to digital, so the computer can read the information)
Discuss risk factors for contrast usage including those for contrast reaction and renal contraindications.
If a patient renal system is compromised then that is a contraindication for contrast. The patient will not be able to properly filter the contrast out of their system. Other contraindications are if a patient has seafood allergies, they will have an allergic reaction to the contrast.
Discuss image and data storage and technical formatting requirements.
Image data (reconstructed data) data that has been back-projected into the image matrix to create CT images displayed on a monitor. Digital filters are available to suppress noise and improve detail: Standard Algorithm: used when balance between image noise and image detail is mandatory Smoothing Algorithms: reduces image noise and show good soft tissue anatomy (Used in examinations where soft tissue discrimination is important to visualize very low contrast structures) Edge Enhancement Algorithms: This emphasizes the edges of structures and improve detail but create image noise. (Used in examinations in which final detail is important such as the inner ear, bone structures, thin slices, and pulmonary structures)
Define and discuss examples of the 3 subsystems of the CT scanner.
Imaging system: Gantry - Component of the CT system that surrounds the patient. It contains the slip rings (Which allow for the 360 degree rotation of the CT tube), x-ray tube, x-ray generator (To get enough power to create x-rays),collimators, detectors, and the DAS (Data Acquisition). Computer: Computer is responsible for: Image manipulation such as Windowing (Playing with contrast and image brightness); Image enhancement (Better Detail); Image enlargement (Magnify it); Image measurement (Check neoplasm size); Multi planar reformatting (Axial, Coronal, Sagittal). The computer has software that allows each hardware component to perform specific tasks such as pan and zoom, windowing, image rotation, zooming, and etc. PACS System: Purpose is see what you are scanning by looking on a flat panel monitor or cathode ray tube monitors (Old looking bulky monitors). It also provides a hard copy of the image on a recording medium. It also allows for the images to be stored and retrieved
Discuss CT gantry rotation in terms of imaging for radiation therapy.
In conventional CT the patient remains stationary; they are scanned slice by slice; and the procedure is time consuming. In SSCT, the x-ray tube rotates continuously around the patient while the patient moves through the gantry aperture during the scanning to cover an entire volume of tissue. This creates the path geometry that describes as spiral/helical CT. Other synonymous terms are spiral volume CT and helical volumetric CT. Pitch = movement of pt table in one gantry rotation/slice thickness. LOW PITCH- Greater Detail, More Dose. HIGH PITCH- Less Detail, Less dose, Faster scan.
Describe the meaning and purpose of machine definition of CT simulation or treatment planning software.
Includes verification of the virtual machines geometric resolution and accuracy. Correspondence between virtual treatment machines and real mechanical treatment machines.
Discuss input and output devices and name examples of each.
Input devices: devices such as the keyboard that can send information to the processor Output Devices: These are devices that display, view, or record completed processes of data; the final step of the IR loop that results in the final volumetric image.
Explain the differences between lossy and lossless compression.
Lossy: (irreversible) - when SOME information is lost in the process Lossless: (reversible) - when NO information is lost in the process
Describe multi planar reconstruction and its purpose.
MPR is a technique used in 2D imaging to create sagittal, coronal, and oblique views from the axial sections of the CT scan. (CT slices are only taken in axial plane
What is a slip ring? What is it's benefit?
Metal coil which allows tube to continually rotate
Discuss the physical components of the scanner including the patient support system, gantry, tube, detectors, internal lasers, ADC, etc.
Patient Support System: aka patient support assembly, patient couch. It is strong and rigid which is usually made of carbon fiber. Made up of the patient table and the pedestal Gantry: a mounted framework that surrounds the patient in a vertical plain. Contains a rotating scan frame which the x-ray generator, x-ray tube, and other components are mounted. Also houses the slip rings, x-ray tubes, high-voltage generator, collimators, detectors, and the DAS. Gantry Aperture: usually 70 cm (large bore being 85cm) Tube: Responsible for the x-ray production Detectors: Receive x-ray radiation that has passed the body through the process of attenuation Internal lasers: grants the ability to align the patient mimicking external lasers and have the ability of accurate reproducibility on the treatment machine. ADC: Analog to digital converter - converts the analog signal to digital DAS: Data Acquisition System - converts the information to digital, so the computer can read the information
Describe the relationship between pitch and patient dose.
Pitch and patient dose have an inverse relationship When the pitch is increased, patient dose decreases
Explain the types of filtration used in CT and their purpose.
Radiation from x-ray tubes is polychromatic, but the beam needs to appear monochromatic to satisfy the requirements of the reconstruction process. Filtration is the addition of sheets of metal to a beam of x-rays to alter the energy spectrum, penetrating ability, and imaging characteristics of the radiation. Inherent filtration= 3mm Al equibalent, added filtration= copper sheets of 0.1- 0.4mm thick (between tube and patient) Filtration serves two purposes: First, it removes long wavelength x-rays to "harden" the beam because they are not useful in imaging; Second, it shapes the energy distribution across the beam to produce a beam of uniform intensity. Bowtie filter is most commonly used for CT.
Discuss types of software, specifically reconstruction, pre-processing and post-processing.
Reconstruction: The reconstruction process is based on the use of an algorithm that uses the attenuation data measured by the detectors to build up the image for viewing and interpretation. Older CT scanners collected data over 180 degrees, while new CT scanners collect more attenuation data over 360 degrees to generate better quality images. (Fourier Transform and Filtered Back Projection are reconstruction algorithms) Pre-processing: Processes the signal data collected from the detectors. Post-processing: Manipulates the image data for review and interpretation. It includes window leveling, and MPR (Multiplanner reformation).
Explain the purposes of pre patient collimation.
Reduce the dose and increase image quality.
Discuss the benefits of using a CT scanner for simulation purposes. CT now used over conventional sim
Shorter scan time improves localization "virtual" sims can be done without patient physically being there Better quality images/algorithm can provide 3D images (only axial slices are taken, sagittal/coronal slices are created from CT algorithm) More sophisticated tx plans possible Less tx dose possible to nearby critical structures.
Name and describe the different rooms required of a CT scanning unit.
The imaging system is located in the scanner room The computer system is located in the computer room The display, recording, and storage system is located in the operator's room (Where you sit and input all the demographics
Compare and contrast spatial resolution and contrast resolution.
Spatial: ability to distinguish between two very small objects placed closely together (high contrast resolution). Contrast: ability to distinguish relatively large objects which differ only slightly in density from background (low contrast resolution).
Describe the different types of detector arrays.
Three types used in MSCT: FIXED/uniform/matrix/linear detectors ADAPTIVE/nonuniform/variable detectors (reduction of dead space) Hybrid
Discuss and compare acquisition times of traditional, single slice and multislice scanners. s
Traditional CT acquisition times is approx. 25 minutes Single slice can scan rapidly with scan times shorter than 1 second Multi-slice can scan rapidly with scan times as low as 0.5 second
Define a MIP and discuss how and when it is used.
Volume-rendering 3D technique in which the algorithm ensures only the tissues with the greatest attenuation will be displayed for viewing by observer does not require sophisticated computer hardware MIP computer program renders a 2D image on a computer screen from a 3D dataset Images can be displayed in rapid sequence to allow observer to view an image that can be rotated continuously back and forth to enhance 3D visualization of complex structures with postprocessing technique
Explain the purpose of a topogram.
What are the synonyms used to describe this scan? (Scout, pilot image) For accurate scan volume and scan location.
Define a range of the most common slice thicknesses used in radiation therapy.
Why is this range appropriate for the purpose of planning? 2.5-3mm is used for better soft tissue resolution -3-5mm is used in lumbar region (lower back)
Discuss the best preset windows and the theory behind them for imaging brain, lung, bone and soft tissue structures.
Wide windows (400-2000 HU) should be used to encompass tissues of greatly differing attenuation within the image Ex: body scans: fat, fluid & muscle: 350-600 HU Lung/bone: 1000-2000 HU to include air space & vessels (lung) cortex & marrow (bone) Narrow windows (50-350 HU) should be used to display soft tissues within structures that contain different tisssues of similar densities Ex: brain 80-150 HU to show differences between gray and white matter Liver: 100-250 HU to highlight liver mets.
Discuss ways that the window width and window level change the image. Show examples of ways that brightness and darkness can be manipulated by changing the central hounsfield value.
Window width- wider window width, wider range of numbers, more soft tissue structures. Window level- if level is decreased, image will be brighter Changing HU- 2000 HU= wider window width. Narrow width= <1000 HU.
Define window width and window level.
Window width: the range of CT numbers that compose a CT image's gray scale, controls the CT image contrast Window level: the center or midpoint of the CT number range that composes a CT image's gray scale, controls the CT brightness
Define temporal resolution. temporal resolution-
an indication of a CT system's ability to freeze motions of the scanned object; performance parameter that deals in time or speed of data acquisition; important parameter when wanting to minimize motion ex: "shutter" speed of camera when taking photo at sports events **cardiac imaging is most challenging because heart motion is continuous**
Discuss mAs and kVp in terms of patient dose.
mAs is the quantity and can increase dose, kVp is the quality and controls contrast you can increase kVp so that mAs can be decreased and decrease dose to patient.
Discuss the purpose of filtered back projection.
•A mathematical technique used in magnetic resonance imaging and CT to create images from a set of multiple projection profiles.
Discuss the locations and roles of both the ADC and the DAC.
•ADC- computer program that takes analog input and turns it into digital. DAC- computer program that takes digital input and converts it into analog output.
Define interpolation and discuss how it is used.
•Interpolation- estimate of data that lies between 2 known values. Allows for reconstruction of a thin, motion free image from a volumetric data set acquired from a moving patient.
Discuss how changes to the SFOV change the image quality.
•Smaller SFOV- better image quality
