Lecture 5.2.pdf

CT: IMAGE QUALITY AND ARTEFACTS Dr. Anastasia Hadjiconstanti Acknowledgements: Dr. Constantinos Zervides LECTURE LOB’S 37. DESCRIBE THE ENGINEERING ASPECTS OF CT SCANNERS. 38. DESCRIBE HOW IMAGE QUALITY IS AFFECTED. 39. DISTINGUISH BETWEEN PATIENT AND MACHINE CAUSED ARTEFACTS. CT SCANNER GENERATIONS...

CT: IMAGE QUALITY AND ARTEFACTS Dr. Anastasia Hadjiconstanti Acknowledgements: Dr. Constantinos Zervides LECTURE LOB’S 37. DESCRIBE THE ENGINEERING ASPECTS OF CT SCANNERS. 38. DESCRIBE HOW IMAGE QUALITY IS AFFECTED. 39. DISTINGUISH BETWEEN PATIENT AND MACHINE CAUSED ARTEFACTS. CT SCANNER GENERATIONS First generation detectors: one type of beam: pencil-like x-ray beam tube-detector movements: translate-rotate duration of scan (average): 2530 mins Third generation detectors: multiple, originally 288; newer ones use over 700 arranged in an arc type of beam: fan-shaped x-ray beam tube-detector movements: rotate-rotate duration of scan (average): approximately 5 sec Second generation detectors: multiple (up to 30) type of beam: fan-shaped x-ray beam tube-detector movements: translate-rotate duration of scan (average): less than 90 sec Fourth generation detectors: multiple (more than 2000) arranged in an outer ring which is fixed type of beam: fan-shaped x-ray beam tube-detector movements: rotatefixed duration of scan (average): few seconds TYPES OF THE BEAM SEQUENTIAL AND SPIRAL CT I Originally, all CT scanners sequentially acquired axial slices according to the stop-and-shoot principle. In sequential scanning, during acquisition of a slice, the table remains stationary. After completion of the acquisition the table moves to a new position to perform the next scan. The disadvantage of this method is the relatively long scan time. SEQUENTIAL AND SPIRAL CT II Spiral CT scanners allow for continuous tube-detector rotation and fast acquisition of data. This was made possible using slip-ring technology. SEQUENTIAL AND SPIRAL CT III Sequential CT scanners rely on a physical connection in the form of cables between the rotating elements (X-Ray tube, detectors, stationary base). This necessitates unwinding of the wires after each acquisition. Spiral CT systems transmit energy and data via electrically conductive brushes and rotating rings. This enables spiral CT scanners to rotate continuously. During acquisition, the table moves at a constant speed through the gantry. SEQUENTIAL AND SPIRAL CT IV The path of the acquisition relative to the subject resembles that of a helix or spiral, hence the names. Continuous acquisition of data allows coverage of larger sections in the same amount of time. Because the X-ray tube generates energy for an extended period more heat storage capacity is required. Also, the large amount of data that is being produced in a very short period requires expanded storage and processing capacity. SEQUENTIAL AND SPIRAL CT V ELECTRON BEAM CT I In mechanical CT, including multi-slice spiral CT, the X-Ray tube and detector array physically rotate around the table. The forces created during rotation restrict rotation speed, and temporal resolution of mechanical CT. The electron beam CT (EBCT) was developed to image the heart. Instead of a physically rotating the tube-detector unit, EBCT generates and directs electrons along a stationary tungsten ring. ELECTRON BEAM CT II ELECTRON BEAM CT III Emitted X-rays from the target ring are collimated, and after passing through the patient collected by the stationary detectors on the opposite side. In the absence of rotating parts, the temporal resolution of EBCT is 100 ms. This was an enormous improvement of the resolution of mechanical CT at the time of development (1980’s). Although EBCT can be used for non-invasive coronary angiography, it is now most often used for quantification of coronary calcium. MULTI-SLICE CT I By increasing the number of detector rows, multiple channels of data can be acquired simultaneously. The advantage of multidetector or multi-slice CT acquisition is the increased longitudinal coverage that can be achieved per rotation. MULTI-SLICE CT II This is particularly useful for cardiac acquisitions, which deal with an inherently longer scan time compared with imaging of non-moving organs. Current state-of-the-art CT technology simultaneously acquires 320 slices ($2.5 million). 320-slice scanners are physically equipped with 320 detector rows. MULTI-SLICE CT III CT manufacturers also offer 16,32,64, 128 and 256 detector systems. Some systems allow for twice their slice acquisition by double sampling in the longitudinal direction. By rapid, longitudinal alternation of the focal spot during acquisition, two partially overlapping sets of projections from slightly differing positions are acquired. Compared with systems with single z-sampling, double z-sampling results in an improved longitudinal resolution at the penalty of a longer total scan time. MULTI-SLICE CT IV With the expansion of the number of detectors, the individual detector width decreased. From 4 to 64 slice CT the detector width decreased from 1.0 / 1.25 to 0.4 / 0.5 mm. Combined with overlapping reconstruction of slices, this has improved the spatial resolution in the longitudinal direction. MULTI-SLICE CT V Coverage of the Multi-Slice CT scanners increased when more active detector rows became available. DUAL SOURCE CT Dual-source CT scanners are equipped with two X-Ray tubes rotating at a 90o angle to each other. The advantage of dual source CT is the improvement of the temporal resolution. An interesting application of dual source CT is dual-energy CT. During scanning both tube detector systems operate using a different tube voltage (kV), which improves tissue differentiation. PERFORMANCE CHARACTERISTICS OF CT Rot. Time per 360o Data per 360o scan Image matrix Power Slice thickness Spatial resolution 1972 1980s 1990s 2000s 2010s 300 s 5 - 10 s 1-2s 0.33 - 0.5 s 0.27 - 0.35 s 57.6 kB 0.2 – 1 MB 1 – 2 MB 5 – 20 MB 0.1 – 1 GB 80 x 80 256 x 256 512 x 512 512 x 512 512 x 512 2 kW 10 kW 40 kW 60 – 100 kW 80 – 120 kW 13 mm 2 – 10 mm 1 – 10 mm 0.5 – 1 mm 0.4 – 0.6 mm 3 Lp/cm 8 – 12 Lp/cm 10 – 15 Lp/cm 12 – 16 Lp/cm 12 – 25 Lp/cm IMAGE QUALITY AND ARTEFACTS I Image quality is primarily determined by the detector size and the number of angular projections. Typical resolution of clinical CT scanners is 0.2–1 mm with slice thicknesses of 0.4–5 mm. Often, a higher slice thickness is chosen to reduce radiation dose and improve SNR at the expense of axial resolution. The focal spot of the X-ray tube is another key determinant of image quality. A larger focal spot blurs the image. This is a major aspect in high-resolution CT, where special tubes with 3–5µm focal spot size are employed. IMAGE QUALITY AND ARTEFACTS II Also, CT units use collimators. The interaction of X-rays with tissue creates randomly scattered photons which create image noise and cloud the image. Collimators in front of the detector act as anti-scatter grids. They eliminate X-ray photons that deviate from a straight source-detector path. Additionally, collimators in front of the X ray tube reduce the beam size and limit beam thickness. This creates a small apparent focal spot. IMAGE QUALITY AND ARTEFACTS III IMAGE QUALITY AND ARTEFACTS IV Artefacts are any discrepancy between the CT numbers represented in the image and the expected CT numbers (HU units). Common artefacts:      Beam Hardening Partial Volume Effect (PVE) Bad Detector Metal Patient motion IMAGE QUALITY AND ARTEFACTS V BEAM HARDENING EFFECT In extreme cases, a detector element may fail giving a constant output signal. Detectors and their associated signal amplifiers are also the primary source of noise. Electronic noise-suppression filters and software noise suppression reduce the amount of image noise. Noise can also be reduced by the operator, provided that the loss of detail is acceptable. Since X-ray sources cannot provide monochromatic beams, an artifact related to beam hardening is common. Beam hardening occurs when the X-rays pass through strongly absorbing materials. IMAGE QUALITY AND ARTEFACTS VI Lower-energy X-rays are absorbed and the energy peak shifts toward higher energies. Thus, absorption values are underestimated. Beam hardening can be reduced by pre-hardening the beam, by placing thin metal plate (molybdenum or tungsten) in front of the tube. Algorithms have also been proposed and are used to compensate for beam hardening in the projections. IMAGE QUALITY AND ARTEFACTS VII IMAGE QUALITY AND ARTEFACTS IX PARTIAL VOLUME EFFECT Partial volume effects occur whenever a pixel represents more than one kind of tissue. This is particularly relevant when a tissue boundary lies within a CT slice. Partial volume effects blur the intensity distinction between adjacent tissues. Higher resolution or sometimes repositioning the patient may reduce partial volume effects. IMAGE QUALITY AND ARTEFACTS X RING ARTEFACTS The detectors are a critical element of the unit. Detectors need to be centered with respect to the source; otherwise, the image will be blurred. In addition, detectors need to be calibrated so that the output signal intensity is identical for all detectors with the same incident X-ray intensity. Uncalibrated detectors create RING ARTIFACTS in the reconstruction. IMAGE QUALITY AND ARTEFACTS XI IMAGE QUALITY AND ARTEFACTS XII METAL ARTEFACTS Metal materials can cause the streaking artefacts due to block parts of projection data. For example, dental fillings, prosthetic devices, surgical clip, etc. Remove the metal material as possible to reduce the artifact. IMAGE QUALITY AND ARTEFACTS XIII CT images of a patient with metal spine implants IMAGE QUALITY AND ARTEFACTS XIV PATIENT MOTION Motion blur occurs when the patient moves (e.g., breathes) during the scan of one slice. Motion blur cannot be corrected, but the risk of motion blur can be reduced with shorter acquisition times and with the use of the helical scanning principle. Often patients are asked to hold their breath during the scan. Detailed imaging of the heart continues to pose problems. IMAGE QUALITY AND ARTEFACTS XV CT image of a shoulder phantom shows streaking artifacts caused by photon starvation. IMAGE QUALITY AND ARTEFACTS XVI CT image of the body obtained with the patient’s arms down but outside the scanning field shows streaking artifacts. IMAGE QUALITY AND ARTEFACTS XVII EFFECT OF REDUCING PROJECTIONS IMAGE QUALITY AND ARTEFACTS XVIII EFFECT OF REDUCING RAYS EXERCISE FOR HOME - SBA Originally, all CT scanners sequentially acquired axial slices according to the stop and shoot principle. The sequential acquisition method was eventually replaced by the spiral acquisition method. Which development allowed for the above to occur? A. B. C. D. E. Use of electron gun. Use of fan-shaped collimation. Use of multiple imaging detectors. Use of slip ring technology. Use of two X-ray tubes. EXERCISE FOR HOME – SBA SOLUTION Originally, all CT scanners sequentially acquired axial slices according to the stop and shoot principle. The sequential acquisition method was eventually replaced by the spiral acquisition method. Which development allowed for the above to occur? A. B. C. D. E. Use of electron gun. Use of fan-shaped collimation. Use of multiple imaging detectors. Use of slip ring technology. Use of two X-ray tubes. QUIZ – SBA 1 A patient undergoes a CT examination of the abdomen. During the post-processing phase, the CT radiographer detects an artefact, as seen in Figure 1. What is responsible for the appearance of the image in Figure 1? A. B. C. D. Beam hardening. Metallic material. Patient motion Uncalibrated detectors. QUIZ – SBA 1 SOLUTION A patient undergoes a CT examination of the abdomen. During the post-processing phase, the CT radiographer detects an artefact, as seen in Figure 1. What is responsible for the appearance of the image in Figure 1? A. B. C. D. Beam hardening. Metallic material. Patient motion Uncalibrated detectors. QUIZ – SBA 2 A patient is to undergo a CT examination of the head. During the scouting phase, the CT radiographer indicates that the image has an acceptable axial resolution but an unacceptable Signal to Noise Ratio (SNR). What can the radiographer correct instantaneously, in order to improve SNR? A. B. C. D. Decrease CT detector size. Decrease slice thickness. Increase focal spot size. Increase slice thickness. QUIZ – SBA 2 SOLUTION A patient is to undergo a CT examination of the head. During the scouting phase, the CT radiographer indicates that the image has an acceptable axial resolution but an unacceptable Signal to Noise Ratio (SNR). What can the radiographer correct instantaneously, in order to improve SNR? A. B. C. D. Decrease CT detector size. Decrease slice thickness. Increase focal spot size. Increase slice thickness. SUMMARY I Originally, all CT scanners sequentially acquired axial slices according to the stop-and-shoot principle. Spiral CT scanners allow for continuous tube-detector rotation and fast acquisition of data. The electron beam CT (EBCT) was developed to image the heart. Instead of a physically rotating the tube-detector unit, EBCT generates and directs electrons along a stationary tungsten ring. Increasing the number of detector rows, multiple channels of data can be acquired simultaneously. SUMMARY II Dual-source CT scanners are equipped with two X-Ray tubes rotating at a 90o angle to each other. Image quality is primarily determined by the detector size and the number of angular projections. Higher slice thickness reduces radiation dose and improves SNR at the expense of axial resolution. The focal spot of the X-ray tube is another key determinant of image quality. CT artifacts are common and can occur for various reasons. SUMMARY III Knowledge of these artifacts is important because they can mimic pathology (e.g. partial volume artefact) or can degrade image quality to non-diagnostic levels. CT artifacts can be classified according to the underlying cause of the artifact: Patient based artifacts. Motion artifact. Transient interruption of contrast. Physics based artifacts. Beam hardening. Partial volume averaging. Noise. Photon starvation. Aliasing in CT. Hardware based artifacts. Ring artifacts. Out of field artifacts. Tube arcing. Helical and multichannel artifacts. Windmill artifacts. Cone beam effect. Zebra artifact. Stair step artifact. REFERENCES Authors Title Edition Publisher Year ISBN E. Seeram Computed Tomography: Physical Principles, Clinical Applications, and Quality Control 4th Edition Saunders 2015 9780323312882 Medical Imaging and Radiation C. Martin, P. Dendy and Protection for Medical Students and R. Corbertt Clinical Staff 2nd Edition The British Institute 2014 of Radiology 9780905749549 M.A. Haidekker Medical Imaging Technology 1st Edition Springer 2013 9781461470724 A.B. Wolbarst, P. Capasso and A.R. Wyant Medical Imaging: Essentials for Physicians 1st Edition Wiley-Blackwell 2013 9780470505700 L.E. Romans Computed Tomography for Technologists: A Comprehensive Text Wollters Kluwer Health / Lippincott 2010 Williams & Wilkins 9780781777513 st 1 Edition Artifacts in CT: Recognition and Avoidance. Julia F. Barrett and Nicholas Keat RadioGraphics 2004 24:6, 1679-1691