CT Physics and Instrumentation :CT Physics and Instrumentation Group # 3 – Daniel Ferro, Mathew Variamattom, Jason Chok


Objectives :Objectives Describe the physics processes involved in the production of x-rays Describe the role of each component in the x-ray tube Discuss the role of proper adjustment of x-ray tube voltage and current in CT Name the principle parts of a CT scanner Discuss the function of each CT scanner component Discuss how CT image data are acquired and processed Describe the calculation process of Hounsfield units


Objectives cont’d :Objectives cont’d Describe CT number values assigned to various tissues and how these values are assigned into meaningful display windowing Discuss CT image quality issues List the origin of CT image artifacts and describe their prevention Discuss appropriate parameters for the acquisition of low-dose CT for PET attenuation correction Describe the parameters and image characteristics required for a diagnostic-quality CT scan Discuss the importance of CT quality control


Introduction :Introduction This will provide an overview of the principles and operation of a computed tomography (CT) scanner as well as look at Image data acquisition, CT image construction , display, image quality protocols, artifacts, radiation safety and quality control


Physics of X-rays :Physics of X-rays X-rays are produced in the electron shell structure of the atom Gamma rays are created by the nuclear forces at the center of the atom In x-ray tubes there are two different types of reactions that can produce x-rays: Bremsstrahlung Radiation Characteristic x-rays


Bremsstrahlung Radiation :Bremsstrahlung Radiation German word meaning braking radiation It occurs when energetic electrons pass very near the nucleus of an atom The closer the distance between the electron and the nucleus, the greater the deceleration of the electron and the higher the x-ray energy emitted


Characteristic X-rays :Characteristic X-rays These are produced following the ionization of an atom that leaves an inner shell, typically the K shell, with an electron vacancy The outer-shell electron that drop in to fill in the inner-shell vacancy must lose energy The energy lost is released as electromagnetic radiation (x-ray) with a specific or characteristic energy determined by the energy difference between the two shells


X-ray Tube design :X-ray Tube design The x-ray tube is a glass envelope that contains a high vacuum so that accelerated electrons from the internal electrodes may move with ease Within the x-ray tube is a cathode that has a very small filament, which is several millimeters in length X-ray tubes for CT scanners have an anode that rotates thousands of revolutions per minute in order to prevent the beam of electrons from the cathode from burning the anode and to remove heat from the anode X-ray tubes in CT scanners may use standard, high-resolution, and sometimes ultrahigh-resolution focal spot sizes and secondary collimation to improve image resolution


Voltage Variation :Voltage Variation An increase in the high voltage applied to the x-ray tube will result in a corresponding increase in the maximum energy of the x-rays that are produced


Energies required :Energies required For typical PET/CT studies of the body that range will be 80-140kVp(peak kilovoltage) This also depends on the size of the patient and the detail that is required The energies that can be selected on a CT scanner are usually defined by a limited set of energies such as 80, 100, 120, and 140kVp A general rule is that an increase of approximately 15% in kVp is roughly equivalent to doubling the milliampereseconds (mAs)


Tweaking kVp for Scanning :Tweaking kVp for Scanning Higher-energy x-ray photons are needed to penetrate large bones of the shoulder, hip and vertebrae The kVp setting also defines the fraction of photons that will successfully reach the detectors of the scanner Higher-energy photons are less attenuated by the body More photons reaching the detectors will result in lower quantum noise in the images, but the radiation exposure to the patient will increase


Beam Hardening :Beam Hardening The spectrum of x-rays passes though the body, the lower-energy (softer) x-rays are most easily absorbed As the beam passes though there is a shift in energy distribution so that a larger fraction of the remaining photons have a higher energy; this spectral shift is called beam hardening Beam hardening will create artifacts seen as dark streaks that radiate from the outside toward the inner part of the body


Advantages and disadvantages of using a higher kVp :Advantages and disadvantages of using a higher kVp Advantages Greater penetration through the body Decrease in quantum noise A reduction in beam hardening artifacts Disadvantages Higher dose to patient Reduces differences in tissue densities


Current Variation :Current Variation The applied current is another factor that can be adjusted in the x-ray tube There is a proportional increase in the number of x-rays, at all energies, when the current in milliampere (mA) is increased Both the Bremsstrahlung and characteristic x-rays that are produced increase proportionally with the current


Current Variation in CT :Current Variation in CT The range of currents that may be used in CT Scanner is commonly very wide, about 50-400mA It is continually variable and typically does not limit the operator to a few preset values The mA is a direct defining parameter of the number of photons generated by the x-ray tube The mA and the time are the dominant parameters that determine the radiation dose to the patient


Advantages and Disadvantages of Current Variation :Advantages and Disadvantages of Current Variation Advantages Decrease in image noise Increase in contrast resolution Disadvantages Higher mA increases dose to the patient


X-ray Filter :X-ray Filter Both planar x-ray and CT imaging system position an absorbing x-ray filter into the x-ray beam The beam-modifying filter has tow purposes: Filtering absorbs low energy x-rays that would attenuate in the body and create noise; this would improve the image. The filter also reduces patient radiation exposure by as much as 50% Filtering also helps in shaping the energy distribution of the beam. A filter also aids in preventing the edges of the beam from hardening and this makes the distribution more uniform


Filters on a CT scanner :Filters on a CT scanner The filters on a CT scanner is not interchangeable with other filters It is a permanent installation for modifying the beam as mentioned and reduces the unused low-energy radiation, which adds nothing to the image formation


Principles of Computed Tomography :Principles of Computed Tomography Computed Tomography, CT for short (also referred to as CAT, for Computed Axial Tomography), utilizes X-ray technology and sophisticated computers to create images of cross-section “slices” through the body. CT exams and CAT scanning provide a quick overview of pathologies and enable rapid analyses and treatment plans.


Principles of Computed Tomography :Principles of Computed Tomography Tomography is a term that refers to the ability to view an anatomic section or slice through the body. Anatomic cross sections are most commonly refers to transverse axial tomography. CT scanner was developed by Godfrey Hounsfield in the very late 1960s.


Contd… :Contd… This x-ray based system created projection information of x-ray beams passed through the object from many points across the object and from many angles (projections). Early CT scanners were limited to use in the brain. In 1974 Robert Ledley introduced techniques that led to the development of the first CT scanner that could perform whole-body imaging of patients. CT produces cross-sectional images and also has the ability to differentiate tissue densities, which create an improvement in contrast resolution.


CT Scanner Design :CT Scanner Design The basic technology employed in the CT scanner is designed to provide a source of x-rays to be transmitted through the patient. The CT system consists of: a computer work station for operation of the scanner, image processing computers, electronic cabinets, the gantry and the patient imaging table.


Composition of the Gantry :Composition of the Gantry The gantry houses the key components of the scanner. Many components associated with the production of the x-ray beam and detection and acquisition of the beam must be located within the rotating portion of the gantry. The fan-beam x-ray tube sits opposite the detector array within the rotating gantry. The three phase power generator is also within the gantry module. Heat from the generator, x-ray tube, and other components must be removed efficiently.


Contd… :Contd… The x-ray tube in a CT scanner is designed to produce a fan beam of x-rays that is approximately as wide as the body. Tissue attenuation is measured over a large region from one position of the x-ray tube. The x-ray tube on a CT scanner is a much more heavy duty unit than the tubes used for standard x-ray imaging. On the opposite side of the patient is the detector array that measures the strength of the x-ray beam at various points laterally across the body.


X-ray Detectors :X-ray Detectors Modern CT scanners use solid state detectors that have very high efficiency at the low energy of x-rays produced by CT scanners. Solid state detectors are made of a variety of materials that create a semiconductor junction similar to a transistor. Ultrafast ceramic detectors use rare earth elements such as silicon, germanium, cadmium, yttrium or gadolinium, which create a semiconducting p-n junction. Ceramic solid-detectors are very fast, can be extremely stable, and are produced to form an array of very small, efficient detectors that can cover a large area.


contd… :contd… Detector systems in a multiple-slice CT scanners use detectors that are in a multiple transverse line across the patient in a two-dimensional (2D) array. The array size for a 16 slice CT may have more than 800 detector elements laterally across the gantry. The size of the individual detector elements is less than 1 mm on each side.


Collimation :Collimation CT scanners use collimators to protect the patient and limit the beam to the size of the detector array that is active during data acquisition. The configuration of CT collimation is adjacent to the x-ray tube. It restricts the width and shape the beam to the area of interest for a single rotation of the tube and detectors around the patient.


Contd… :Contd… The slice thickness that may be reconstructed can never be thinner than the collimator width. Selecting an appropriate collimation thickness will prevent small lesions from being missed. Thinner collimation produces less streaking of high density objects. Collimation should be sufficiently thinner than the lesions to be detected. Slice thickness is important because it defines the volumetric dimension of the image voxels.


Advantages and disadvantages of thinner collimation :Advantages and disadvantages of thinner collimation Advantages are: Less volume averaging Better resolution on reformatted sagittal and coronal images Increased spatial resolution Fewer streaking artifacts from high-density objects.


Contd… :Contd… Disadvantages are: Increased quantum mottle Increased scanning time May result in an increased number of slices as defined by the reconstruction slice thickness.


Rotation Speed :Rotation Speed The rotation speed is the time required for the gantry to make one complete revolution. In helical (spiral) CT the gantry rotates continuously in one direction on the slip-ring system, While the patient table moves at a constant speed through the gantry to quickly perform the scan of a relatively large body area. Faster CT rotation speeds may be desirable for reducing the likelihood of patient motion artifacts.


Pitch :Pitch In helical CT, pitch is the ratio of the patient’s movement through the gantry during one 360 degree rotation relative to the beam collimation. For single slice CT, Pitch is equal to the table movement per rotation divided by slice collimation. On most CT and PET/CT systems the pitch is entered as a parameter that is set in defining the procedure for the acquisition of patient data. When the pitch is greater than 1.0 mm, there are interspaces created and some portion of the body is being missed.


Increment :Increment The increment is the distance between the slices. In CT data are acquired as projection information along the helical path. Slices may be reconstructed at various positions along the helix. One important factor in defining the slices and their positional relationship to one another is the distance between the reconstructed axial slices in the ‘z’ direction.


Image Data Acquisition :Image Data Acquisition The high performance of CT scanners creates a volume of reconstructed data instead of a group of slices The data acquired from the helical motion of the gantry are obtained as projection information, as in both SPECT and PET The raw data may be used to create several image sets to reveal different information from just the one set of raw data


Image Data Acquisition cont’d :Image Data Acquisition cont’d An example is the raw data may be used to generate one set of data to be used for attenuation correction of SPECT or PET Another reconstruction set of images may be high-resolution thin slices, but another may be smoother, thicker slices The beam width as set by the detector size and collimator determines the maximum resolution that may be obtained in the study A typical CT acquisition for combined SPECT/CT or PET/CT may take less than 1 minute


CT Image Reconstruction :CT Image Reconstruction The reconstruction of a multislice CT images will be computed from the helical projection data Since the x-ray beam is either fan-shaped or cone-shaped in a helical pattern, special reconstruction algorithms are used to account for this unique geometry Image reconstruction may use different reconstruction algorithms that will calculate the attenuation at each pixel in the slice


Values of CT Image Reconstruction :Values of CT Image Reconstruction CT numbers (Hounsfield units) are the pixel values assigned in the image They are computed by calculating the relative difference between the linear attenuation coefficient of tissue and that of water


Hounsfield Units (HU) or CT numbers Calculation :Hounsfield Units (HU) or CT numbers Calculation For tissue and that of water the equation is: CTTissue = (µtissue - µwater)/µwater x 1000 The CT number for water has a value of 0 because: (µtissue - µwater)/µwater = 0


CT numbers for various Tissues in Hounsfield units (HU) :CT numbers for various Tissues in Hounsfield units (HU)


Algorithms used in CT Image Reconstruction :Algorithms used in CT Image Reconstruction A variety of algorithms have been used to reconstruct CT images: Filtered back-projection (FBP) Iterative algorithms


Reconstruction Kernels :Reconstruction Kernels This is another important parameter that is set prior to CT reconstruction The kernel is an algorithm that, in the case of CT reconstruction, defines the clinical application and amount of smoothing that will be applied in the reconstruction process Each vendor’s CT scanner will have a variety of kernels that optimize the image quality to a particular body area or tissue type Bone images will require an algorithm that maintains sharpened edges or bone margins


Example of Kernels in Reconstruction :Example of Kernels in Reconstruction An example is a Siemens system the kernels are defined by a letter (or letters) followed by a number The letter identifies the body area or clinical application (e.g H=head, B=body, U=ultrahigh resolution, C=Child head) The typical numbers ranges from 10-90 Most clinically useful filters use numbers in the range of 30 to 40 The higher the kernel number, the shaper the image On the other hand the lower the number the smoother the image


Advantages and Disadvantages of CT image reconstruction :Advantages and Disadvantages of CT image reconstruction Advantages Desired image detail is obtained Filters may sharpen or smooth reconstructed images Raw data may be reconstructed post-acquisition with a variety of filters Disadvantages Multiple reconstructions may be required if significant detail is required from areas of the study that contain bone and soft tissue


Post-processing Filtering :Post-processing Filtering A post-processing filter such as low-contrast enhancement (LCE) filter will improve low-contrast resolution while it decreases image noise A high-contrast enhancement (HCE) filter will produce sharpening of high-contrast differences as in the head imaging and brain imaging


CT Display :CT Display Digital image display of CT information creates unique challenges due to the wide variation in CT numbers in the image An example is the chest region contains lung tissue (CT numbers in the range of –850 to –200), fat (-250 to –30), soft tissue (10 to 80), and bone (150 to 500 or more) This range of CT numbers in the image is from –850 to 500 numbers


Limitations of CT display :Limitations of CT display The human eye is thought to be able to discern only 30 to 100 different shades of gray; because of this only a limited range of CT values are displayed in the image Digital imaging is often limited to 8 bits of data and therefore only 256 intensity values are assigned over the entire gray scale from black to white


Examples of CT window selection :Examples of CT window selection A bone window may be set with CT values selected from 400 to 1000 to view details of the dense bone in the vertebrae Windowing from CT numbers 0 to –1200 allows the lung tissue to be viewed While windowing from –85 to 165 creates a scale to better evaluate soft tissue


Display of Volumetric Image Data :Display of Volumetric Image Data Some of the most dramatic display options are those that allow the CT data to be depicted as though the true anatomy of the patient were being visualized Although these images are attention-grabbing they typically have little use to the radiologist in interpreting the examination But it can have useful application in surgical planning and marketing radiology studies Computed tomography images are also the standard display used in radiation therapy treatment planning


MIP :MIP Maximum intensity projection (MIP) images are generated by taking a stack of image slices and projecting back the maximum intensity of the brightest pixel along that path This technique uses the same principles as MIP imaging for SPECT and PET


SSDs :SSDs The creation of shaded surface displays (SSDs) images is typically done in three steps: Reconstruct the CT data from the 2D slices using thin overlapping slices Segment out unwanted data using thresholding of the CT numbers or cropping or segmenting out overlying structures Render or shade the projected images to provide depth perception and illumination of the patient


Image Quality :Image Quality Although CT images are relatively simple displays, the generation of the images is based on settings and parameters, as well as some some others Each parameter setting that is selected prior to imaging will have some effect on the final image


Contrast Resolution :Contrast Resolution Ability to differentiate between different tissue densities in the image High-Contrast Low-Contrast


High-Contrast Resolution :High-Contrast Resolution Ability to see small objects and details that have high density difference compared with background. very high density difference from one another Ability to see a small, dense lesion in lung tissue See objects where bone and soft tissue are adjacent.


High Contrast Image :High Contrast Image


Low-Contrast Resolution :Low-Contrast Resolution Ability to visualize objects that have very little difference in density from one another. Better when there is very low noise Visualizing soft-tissue lesion within the liver. Differentiate gray matter from white matter in the brain.


Low Contrast image :Low Contrast image


Image Noise :Image Noise Noise level: % of contrast Number of detected photons Matrix size Slice thickness Algorithm Electronic noise Scattered radiation Object size.


CT Protocols :CT Protocols All CT scans begin by performing a scout scan. used in planning the exam and to establish where the target organs are located. Not critical for examination Operator should limit radiation exposure to paient from scout scan.


Diagnostic CT :Diagnostic CT Diagnostic CT may not clearly define the exact procedure being preformed. Can mean the image quality is excellent with low noise by using optimal slice thickness along with sufficient kVp and mAs settings to appropriately detect disease. Contrast media have been administered to the patient and that the CT scan is preformed following appropriate procedures.


CT Artifacts :CT Artifacts Artifacts can degrade image quality and affect the perceptibility of detail. Includes Streaks Rings and bands Shading


Streaks :Streaks Streaks: patient motion, metal, noise, mechanical failure.


Rings and Bands :Rings and Bands Rings and Bands: Bad detector channels.


Shading :Shading Shading can occur due to incomplete projections.


Quality Control :Quality Control The definition of QC consists of 2 parts. Quality assurance requires a measurement of the CT scanner’s performance to ensure the scanner is operating at some acceptable level. Quality control carries the concept of quality assurance one step further – if the quality is inadequate, then steps are taken to correct the problem.


Why do we need QC? :Why do we need QC? Maintain high performance standards for our patiant's Regulatory agencies demand QC on equipment that can potentially harm patients Examples: Mechanical parts wear slowly. Electronic parts can change characteristics and drift out of optimal adjustment.


References :References Waterstram-Rich, Christian (2007). Nuclear Medicine and PET/CT. St. Louis, Missouri: Mosby Elsevier. Seeram, Euclid (2001). Computed Tomography. USA: W.B. Saunders .


Question 1 :Question 1 QC consist of 2 parts, Which one is NOT? Quality Control Quality Assurance Scanners are always good


Answer 1 :Answer 1 C) Scanners are always good


Question 2 :Question 2 Why is QC needed? Maintain high performance standards Mechanical parts wear slowly. Both A & B None of the above


Answer 2 :Answer 2 C) Both A & B


Question 3 :Question 3 Which of the following images has the most resolution? A or B


Answer 3 :Answer 3 B) Because it has less detail.


Question 4 :Question 4 T / F: Streaks can be caused my metal obstructing the scanners ROI


Answer 4 :Answer 4 TRUE


Question 5 :Question 5 T / F: Rings and Bands can be caused by patient movement


Answer 5 :Answer 5 False


Question 6 :Question 6 In 1974 Robert Ledley introduced techniques that led to the development of the first CT scanner that could perform whole-body imaging of patients. True or False?


Answer 6 :Answer 6 True.


Question 7 :Question 7 What is the use of collimators in CT scanners?


Answer 7 :Answer 7 CT scanners use collimators to protect the patient and limit the beam to the size of the detector array that is active during data acquisition.


Question 8 :Question 8 How is a Bremsstrahlung radiation created?


Answer 8 :Answer 8 The Bremsstranhlung radiation is created by an electron passing very close by the nucleus of an atom and it loses energy that lost energy is then emitted as an x-ray


Question 9 :Question 9 T/F are filters on a CT scanner interchangeable?


Answer 9 :Answer 9 False


Question 10 :Question 10 Name 2 types of reconstruction algorithms?


Answer 10 :Answer 10 2 types are filtered back-projection (FBP) and the iterative algorithms