CT Physics and Instrumentation Outline

Question 1

what are the Physics of X-rays? (2)

Answer 1

X-rays are produced in the electron shell structure of the atom In x-ray tubes there are two different types of reactions that can produce x-rays:

Question 2

what are the details of Bremsstrahlung Radiation? (3)

Answer 2

German word meaning braking radiation 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

Question 3

what are the Characteristic x-rays? (4)

Answer 3

Are produced following the ionization of an atom which leaves an inner shell, classically the K shell, with an electron vacancy The outer-shell electron drops in to fill in the inner-shell vacancy and energy is lost Energy lost is released as electromagnetic radiation (x-ray) and emits a quantized photon “characteristic” of the element Not all outer-shell electrons produce X-rays

Question 4

Explain the CT X-ray Tube Design. (4)

Answer 4

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 also secondary collimation to improve image resolution

Question 5

What are the Energies required? (3)

Answer 5

For typical CT studies of the body energies ranges from 80-140kVp(peak kilovoltage) This 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

Question 6

Why Adjusting kVp for Scanning? (4)

Answer 6

Higher-energy x-ray photons are needed to penetrate denser, larger bones and thicker anatomy The kVp setting also defines the portion of photons that will successfully reach the detectors of the scanner Higher-energy photons are less attenuated by the body And aid more photons in reaching the detectors which result in lower quantum noise in the images, but the radiation exposure to the patient will increase slightly

Question 7

What are the Advantages of using a higher kVp? (3)

Answer 7

Greater penetration through the tissue of the body Decrease in quantum noise A reduction in beam hardening artifacts

Question 8

What are the disadvantages of using a higher kVp? (2)

Answer 8

Slightly increases patient dose Reduces differences in tissue densities

Question 9

Explain Current Variation. (3)

Answer 9

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

Question 10

Explain Current Variation in CT (4)

Answer 10

The range of currents that may be used in CT Scanner is commonly very wide, about 50-400mA It is continually variable and usually does not limit the operator to a few preset values The mA determines the number of x-ray photons produced by the CT x-ray tube. The mA and the time are the primary parameters that determine the radiation dose to the patient

Question 11

What are the Advantages of Current Variation (2)

Answer 11

Decrease in image noise Increase in contrast resolution

Question 12

What are the Disadvantages of Current Variation (1)

Answer 12

Higher mA increases dose to the patient

Question 13

Explain the X-ray Filter (4)

Answer 13

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 photons that would attenuate in the body and create noise; this would improve the image. The filter also decreases patient radiation exposure by as much as 50% *****Filtering helps in shaping the energy distribution of the beam. A filter also aids in preventing the edges of the beam from hardening and makes the distribution more uniform

Question 14

what does a CT scanner Filters do? (1)

Answer 14

Is a permanent installation for modifying the beam as mentioned previously and reduces unused low-energy radiation, which adds nothing to the image formation

Question 15

What are the Principles of Computed Tomography? (9)

Answer 15

Computed Tomography, CT for short, use X-ray technology and sophisticated computers to create images of cross-section “slices” through the body. CT exams allow a quick overview of pathologies and enable rapid analyses and treatment plans. 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. This x-ray based system created projection information of x-ray beams, which passed through the object from many points across the object and from many angles (projections). Early CT scanners were limited and only scanned the head. 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.

Question 16

Explain the CT Scanner Design (5)

Answer 16

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.

Question 17

What is the Composition of the Gantry? (10)

Answer 17

The gantry houses the key components of the scanner. Many components associated with the production, detection, and acquisition of the x-ray beam is 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. 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 patient’s body. Tissue attenuation is measured over a large region from one position of the x-ray tube. 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. 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.

Question 18

Explain X-ray Detectors (6)

Answer 18

Recent CT systems 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. 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. 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.

Question 19

Explain Collimation (6)

Answer 19

CT systems use collimators to protect the patient from unnecessary radiation and limit the beam to the size of the detector array that is active during data acquisition. It limits the width and shape of the beam to the area of interest for a single rotation of the tube and detectors around the patient. The slice thickness to be reconstructed can never be thinner than the collimator width. 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

Question 20

what are the Advantages of thinner collimation (4)

Answer 20

Less volume averaging Better resolution on reformatted sagittal and coronal images Increased spatial resolution Fewer streaking artifacts from high-density objects.

Question 21

what are the disadvantages of thinner collimation (3)

Answer 21

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

Question 22

Explain Rotation Speed (4)

Answer 22

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 the anatomy of interest Faster CT rotation speeds may be used to reduce the probability of patient motion artifacts.

Question 23

Explain Pitch (5)

Answer 23

Terminology in CT is associated with speed, and is actually the distance between turns. In spiral/ helical CT, the pitch is defined as the distance (mm) that the CT tables moves during one rotation of the x-ray tube. For single slice CT, Pitch is equal to the table movement per rotation divided by slice collimation. When the pitch is greater than 1.0 mm, there are interspaces created and some portion of the body is being missed. Formulas: For single-slice or single row detectors ***Pitch=table movement per rotation/slice collimation or slice thickness For spiral/helical or multi-row detectors ****Pitch= Movement of the Patient Table in one gantry rotation/Total Slice Volume ****Or, Pitch= Movement of the Patient Table in one gantry rotation/Thickness of a single slice

Question 24

Explain Increment (4)

Answer 24

The increment is the distance between the slices. 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.

Question 25

Explain Image Data Acquisition (3)

Answer 25

The high performance of CT scanners creates a volume of reconstructed data instead of a group of slices The raw data may be used to create several image sets to reveal different information from just the one set of raw data The beam width as set by the detector size and collimator determines the maximum resolution that may be obtained in the study

Question 26

Explain CT Image Reconstruction (3)

Answer 26

The reconstruction of a mult-islice 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

Question 27

What are the Values of CT Image Reconstruction (5)

Answer 27

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 ***For tissue and that of water the equation is: ***CTTissue = (µtissue - µwater)/µwater x 1000

Question 28

What are the Algorithms used in CT Image Reconstruction (4)

Answer 28

A variety of algorithms have been used to reconstruct CT images. Some includes: Back Projection (star artifact) Filtered back-projection (FBP) Iterative algorithms

Question 29

What are the Advantages and Disadvantages of CT image reconstruction (3)

Answer 29

Desired image detail is obtained Filters may sharpen or smooth reconstructed images Raw data may be reconstructed post-acquisition with a variety of filters

Question 30

What are the Disadvantages of CT image reconstruction(1)

Answer 30

Multiple reconstructions may be required if significant detail is required from areas of the study that contain bone and soft tissue

Question 31

Explain Post-processing Filtering (2)

Answer 31

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

Question 32

what are the CT Houndsfields Values? (11)

Answer 32

Tissue CT Number (HU) Dense Bone 1500-2000 Bone 1000 Calcification 150-2000 White Matter 50-65 Gray Matter 40-46 Blood 35-40 Cerebral Spinal Fluid 15 Water 0 Fat -50 to -100 Lungs -550 to 950 Air -1000

Question 33

Explain CT Display(1)

Answer 33

Digital image display of CT information creates unique challenges due to the wide variation in CT numbers in the image

Question 34

What are the Common Window Settings for Clinical Practice (9)

Answer 34

Window Setting Name Window Level Window Width Abdomen 40 HU 350 Brain 40 HU 100 Subdural 50 HU 150 HU Stroke 35 HU 35 HU Lung -600 HU 1300 HU Vocal Cord -300 HU 1000 HU Liver 50 150 Trabecular bone 250 2000 Cortical bone 350 3000

Question 35

Explain Image Quality (2)

Answer 35

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

Question 36

Explain Contrast Resolution (3)

Answer 36

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

Question 37

Explain High-Contrast Resolution (4)

Answer 37

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.

Question 38

Explain Low-Contrast Resolution (4)

Answer 38

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.

Question 39

Explain CT Artifacts (4)

Answer 39

Artifacts can degrade image quality and affect the perceptibility of detail. Includes Streaks ***patient motion, metal, noise, mechanical failure. Rings and bands ***Rings and Bands: Bad detector channels Shading ****Shading can occur due to incomplete projections.

Question 40

What are the Dose Measurement Formulas (2)

Answer 40

CTDI (vol) = CTDI(w)/pitch **Therefore, as you increase pitch, the dose per section, CTDI (vol), decreases DLP=CTDI (vol) x scan length