HSMI 3314 Introduction to Computed Tomography PDF

1 LEARNI NG OUTCOME (W1) Expe ctat io n 1 Historical perspective towards the development of the CT scanner. 4 The advantages of advanced CT vs. Previous Generation. 2 Limitations of conventional rad...

1 LEARNI NG OUTCOME (W1) Expe ctat io n 1 Historical perspective towards the development of the CT scanner. 4 The advantages of advanced CT vs. Previous Generation. 2 Limitations of conventional radiography and linear/ conventional tomography. 5 The advantages and disadvantages of CT over other imaging modalities 3 The characteristics of various CT technologies/generation: 1. 1ST - 5TH Generation 2. Spiral/Helical CT (MSCT/MDCT) 3. Dual Source CT 2  HISTORICAL PERSPECT IVE 1967 1. If x-ray beam were passed through an object in all direction and measurement were made of all the x-ray transmission, information about the internal structure of the body could be made 2. Construct and experiment apparatus to investigate the clinical feasibility (Americium Gamma Source) 3. Collaboration with radiologist - evaluate the usefulness of the machine by evaluating the human brain specimen able to differentiate the tumor tissue with the gray and white matter able to show detail such as ventricle and pineal gland in controlled experiment using fresh brain from bullocks 19711 st clinical prototype CT brain scanner was installed at Atkinson Morley’s Hospital processing time for picture reduced to 20 minutes introduction of minicomputers - processing time reduced to 4.5 minutes 1972 1 st patient was scan using clinical prototype CT - woman with suspected cerebral lesion image show a clear dark circular cyst in the brain able to distinguish between normal and diseased tissue 1973 1. 1 st significant technical development - 1st whole body CT scanner 2. Introduction of 3rd generation of CT scanner 1974 1. Introduction of 4 th generation of CT scanner 1975 Introduction of Dynamic Spatial Reconstructor (DSR) Allow dynamic volume scanning to accommodate imaging of the dynamic organ and functional aspect of the cardiovascular and pulmonary system with high temporal resolution 3  HISTORICAL PERSPECT IVE 1980 Electron Beam Computed Tomography (EBCT) was introduced Invented to image the cardiovascular system without motion artifact Capable to acquired multi-slice image in 50ms and 100ms 1988 Research and development of MSCT 1990 Introduction of single-slice Spiral/Helical CT scanners Allow volume scanning by moving the patient continuously through the gantry of the scanner while the x-ray tube and detector rotate continuously for several time 1992 Introduction dual Spiral/Helical CT scanners 1998 Introduction Multislice Spiral/Helical CT scanners 4  TERMI NOLOGIES COR ON AL AX IAL S A G I T TA L H Y P ERD EN SE I S O DAN SE HY POR DENSE 5  LIM ITATI ON OF CONVENTI ONAL RADIOGRAPHY Major short coming: superimposition of all structure on the film 1 Impossible to distinguish particular detail especially structures differ slightly in density. 2 Difficult to differentiate between 3 homogenous tissue of difference thickness and a heterogenous tissue of uniform thickness 4 A qualitative examination rather than quantitative. 6 A q u al i ta t i ve e xa m i n at i o n r a t h e r t ha n q u an t i ta t i v e. 1 2 3 4 7 LIN EAR TOM OG RAPHY 8 LIM ITATI ON OF LINEAR TOMOGRAPHY Limited to single-plane imaging with a persistence image blurring that cannot be removed 1 Degradation of image contrast due to the presence of scattered radiation created by the open geometry of the 2 x-ray beam 3 Limited diagnostic accuracy due to poor image 4 resolution and contrast differentiation. Linear tomography might expose patients to more radiation because it may require multiple exposures to capture images at different angles or planes. 9 LIM ITATI ON OF CONVENTI ONAL RADIOGRAPHY & LIN EAR TOM OG RAPHY 1 Both fail to adequately demonstrate a slight difference in subject contrast characterized by soft tissue: LINEAR ATTENUATION COEFICIENT Fat 0.194 Water 0.222 HUMAN CSF 0.227 Plasma 0.227 Red cells 0.246 2 Contrast cannot be manipulated after it has been recorded on the film – conventional 10  GOALS OF OCOM PUTED TOMOGRAPHY V E R C O M E L I M I T AT I O N O F B O T H Overcome the limitation by: These were archived by: 1. Minimal Superimposition A beam is transmitted through a specific cross-section of the patient This step removed the problem of superimposed position of structure above and below the specific cross-section or slice of tissue 2. Improved image contrast & detail A high collimated x-ray into a thin beam - only passed through the cross-section of tissue to be imaged This step intended to minimize the scatter production thus improved the image contrast 3. Capable to record a very small Special detectors placed opposite to the x- ray tube differences in tissue contras These detectors are quantitative and can measure very small differences in tissue contrast. In addition, the data will be process in digital form by computers that used special algorithm to reconstruct the image of tissue cross section. 11  1 ST GENERATION It is a parallel beam geometry that defined by a set of highly collimated parallel x-rays that generate a projectile profile. A pencil beam system required 180 translations, which were separated from one another by a 10 rotation It collected 160 measurements across the 24 cm scan field (FOV) 12  1 ST GENERATION D ATA A C Q U I S I T I ON Translate-rotate principle across the patient to collect transmission data. After 1 translation, the tube and detector rotate by 10 and translate again to collect reading from different direction. This method of scanning referred as rectilinear pencil beam scanning “A total of 180 x 160 = 28,800 rays were measured” 13  ADVANTAGES & DIADVANTAG ES OF 1 ST GENERATION A D V A N TA G E S D I S A D VA N T A G E S Pencil beam allowed very efficient scatter reduction Data acquisition took about 5 minutes to complete a 1 single image. Scatter that was deflected away from the pencil ray was not measured by a detector 1 2 Patient motion during scan: degrade image quality 3 2 With regard to scatter rejection, the pencil beam geometry used in 1st generation CT 3 scanners was the best. 4 Constraints of cardiac and respiratory motion 4 (involuntary movement) Only practical for head scans 14  2 ND GENERATION D ATA A C Q U I S I T I ON Translate-rotate principle across the patient to collect transmission data. After 1 translation, the tube and detector rotate by large increment depending on the detector no. and translate again to collect reading from different direction (1800). This method of scanning referred as rectilinear multiple pencil beam scanning Acquisition time decreased is in inversely proportional to The angle between the pencil beams is 1 0 the number of detectors ( detectors,  acquisition time) 15  3 R D GENERATION D ATA A C Q U I S I T I ON Fan-shaped beam geometry that rotate continuously 360° X-ray tube coupled to a curved detector array (fixed) that subtends an arch 30-55° that enough to encompass the whole of the test object in FOV No more translate and rotate - allows all of the readings that make up a view to be recorded instantaneously and simultaneously This method of scanning referred as continuously rotating beam scanning 16  ADVANTAGES & DISADVANTAGES OF 3 R D GENERATION A D V A N TA G E S O V E R 1 S T & 2 ND G E N D I S A D VA N T A G E S O V E R 4 TH G E N Reduced scan times Fixed source-detector 1 Frequent occurrence of ring artifacts because Reduce artifact resulting from patient the same bank of detectors is used repeatedly, motion 1 2 even a very small misalignment of a single detector will result in visible ring artifact. 3 2 Fixed source-detector allow the beam to be highly collimated, which greatly reduces scatter radiation, thereby 4 improving image quality. 17  3 R D GENERATION EARLY M ODEL VS LATER MODEL E A R LY M O D E L L AT E R M O D E L Both the x-ray tube power and the detector signals were transmitted by cables. Used slip-rings technology for power and 1 data transmission. Limitations on the length of the cables forced the gantry to rotate both clockwise and counter 1 2 clockwise to acquire adjacent slices. Since the gantry could rotate at a constant 2 speed during successive scans, the scan time The acceleration and deceleration of the gantry, 3 was reduced to 0.5 sec or less. which typically weighed several hundred kilograms, restricted the scan speed to roughly 2 sec per rotation. 4 These caused interscan delay due to momentum (3 to 15 seconds per slice). 18  4 T H GENERATION D ATA A C Q U I S I T I ON Wide fan-shaped beam geometry that rotate continuously 360° rotating x-ray tube The x-ray tube rotates within the fixed detector array and produces a fan-shaped beam geometry. The number of detectors in use at any one time is controlled by the width of the beam. This method of scanning referred as continuously rotating beam scanning 19  ADVANTAGES & DIADVANTAG ES OF 4 T H GENERATION A D V A N TA G E S O V E R 3 RD G E N D I S A D VA N T A G E S A higher sampling density-eliminate potential aliasing artifacts. Increased patient dose – tendency of overscan 1 technique will increase the radiation dose to the patient Free from ring artifact 1 2 High scattered radiation - detector cell receive x- ray photons over a wide angle 3 2 Reduces the stability requirements of the detector 3 4 Expensive due high number of detectors required to form a complete ring (complete single slice 480 Reduce scanning time detector) 20  5 T H GENERATION: ELECT RON BEAM CT SCAN NER (EBCT) D ATA A C Q U I S I T I ON This is clearly very challenging for 3rd or 4th gen. Classified as high-speed CT scanner – acquire scan scanners due to the enormous centripetal force data in miliseconds placed on the x-ray tube and the detector. Built for cardiac applications to “freeze” cardiac A complete set of projections must be collected motion within 20-50ms 21  5 T H GENERATION: ELECT RON BEAM CT SCAN NER (EBCT) D ATA A C Q U I S I T I ON 3 2 4 When the electron beam collides with the tungsten Use electron beam technology target - x-ray beam produce. Large electron gun as its x-ray beam source that generate This beam accelerated (high-speed), focused and deflected at 130kV electron beam the prescribe angle by electromagnetic coil to strike 1 of 4 adjacent anode tungsten target ring. 22  5 T H GENERATION: ELECT RON BEAM CT SCAN NER (EBCT) D ATA A C Q U I S I T I ON 5 7 3 2 6 4 IN TARGET RING - the rotation of the electron Output of the detector is sent to the data beam is provided by the sweeping motion of the acquisition system (DAS) – sample and digitize electron beam (by deflection coil) the output signals from the detector The system contains two rows of detectors, each forming a The detector ring and the target ring are offset to make room for 216 arc with a radius of 67.5 cm the overlapped portion. 23 5 T H GENERATION: ELECT RON BEAM CT SCAN NER (EBCT) D ATA A C Q U I S I T I ON 24  SPIRAL/HELI CAL CT S CANNER D ATA A C Q U I S I T I ON X-ray tube is continuously rotate while the the patient move through the gantry aperture during scanning to cover entire volume of tissue Permits continuous data acquisition, using slip- ring technology. Acquired data while table is moving, producing helical pattern around the patient being scanned A rotary movement of the projection system around the longitudinal axis (Z) of the patient. Often referred to as volumetric scanning 25  SPIRAL/HELI CAL CT S CANNER CHARACTERISTIC Continuous Memory Continuous rotating scanner Mass memory buffer to store the vast amount data collected 06 01 Continuous Algorithm 05 02 Continuous couch movement Occupied with spiral/helical algorithm for 04 reconstruction 03 Cooling Load-ability Increased cooling capacity of the x-ray tube Increase load-ability of of x-ray tube, able to deliver 200 milliamperes per revolution continuously 26  SI NGLE S LICE CT (SSCT) SSC T D I S A D VA N T A G E S 1ST design of spiral tomography device, used an array of X- ray detectors arranged in a single row. Excess stress on the x-ray tube 1 Fan-shaped beam geometry that rotate 1 continuously 360° 2 Limited the ability to perform thin slice imaging 2 3 Tube current is limited to 100 mAs for this slice imaging 27  MULTI SLI CE CT (MSCT) Adding additional rows of detectors Reduce Scanning Time 1 Practical MSCT devices contain between 4 and 1 320 rows of detectors 2 Increased resolution and reduced image noise 3 2 Acquired projections simultaneously for the subsequent reconstruction of up to 4 slices 3 Effective use of the power of x-ray tube 28  DUAL CT dual-source dual-energy single-source dual-energy single-source dual-energy scanner with a dual detector with a single detector with a dual-layer detector 29  DUAL ENERGY DUAL DETECTOR The dual-source CT scanner has two X-ray tubes that are positioned 90◦ apart, which can be operated at different kV 1 In practical application, one detector array covers the entire scan FOV in approximately 2 50cm, whereas the 2nd detector is limited to a smaller (26cm) central FOV 3 The kV can be quickly modulated between low and high kV while a single circular or helical acquisition is performed. 4 30  DUAL ENERGY SIN GLE SOURCE The dual-source CT scanner has one X-ray tubes that use fast kVp switching method. The tube voltage follows a pulsed curve, and projection data is collected twice for every projection, one at high and one at low tube voltage Able to archive reasonable quality but speed will be compromised due to kVp switching Constant mAs quantity leading to higher dose 31  DUAL LAY ER A single-source dual-energy scanner with a dual-layer detector uses a single source and a single detector The detector has two layers that are used to distinguish between high and low energy photons The kVp from source 120 or 140 and fix mAs quantities through out scanning Fast scanning kVp switching technology 32  APPLI CATION OF DUAL SOURCE CT C O M PA R E D T O M S C T Applications of dual-energy CT include: 1. Increased scan speed 2. Elimination of beam hardening artifacts, 3. Automatic segmentation, for example automatic bone removal 4. Retrospective generation of (virtual) monochromatic images at any possible energy, 5. Tissue characterization, 6. Virtual unenhanced images. Dual kV examination with contrast agent allows an image to be produced as if there were no contrast agent. This eliminates the need for two scans, i.e., a pre- contrast (unenhanced) and a contrast scan. 33  ADVANTAGES OF ADVAN CED CT 1. Faster scan times and elimination of start-stop process: minimal interscan delay require less contrast media resulting in decreased patient risk of contrast media reactions reduce motion artifact increases patient throughput 2. Eliminating overlap or missed areas 3. More diagnostic information especially when using contrast media 4. More clinical applications especially with volumetric data acquisition 5. Reconstruction section thickness can be smaller than acquisition section thickness. 34  ADVANTAGES OF CT OV ER CONV EN TIONAL RADIOGRAPHY 1. Excellent low contrast resolution is possible 2. Capability to manipulate the window width (WW) and window level (WL) in image window 3. The image contrast scale of the image can be manipulated and varied to suit the preference of the observer (radiologist) 4. Capability in volume scanning in spiral/helical CT scan 5. Allow development of variety technique that intended to facilitate the diagnostic process such as DYNAMIC CT,CT PERFUSION, PET CT 6. Allow image manipulation for analysis 35  ADVANTAGES OF CT OV ER MRI 1. Speed - much faster than MRI scans, making them the preferred option in emergency settings 2. Bone Imaging - CT excels at imaging bones and detecting fractures, especially in complex anatomical regions like the spine and skull, whereas MRI is better suited for soft tissue. 3. Less Sensitivity to Motion:CT is less sensitive to patient movement compared to MRI. 4. Cost effective - CT is generally less expensive than MRI, making it a more cost-effective solution for many clinical scenarios. 5. Better for Patients with Implants:CT can be used in patients with metal implants (pacemakers, cochlear implants) 36  ADVANTAGES OF CT OV ER USG 1. Penetration and Resolution:CT has a much higher spatial resolution and better tissue contrast than ultrasound. It’s particularly useful for deep structures 2. Less Operator Dependency:CT provides more consistent and reproducible results, whereas ultrasound is highly operator-dependent and can vary based on the skill of the technician. 3. Imaging of Gas or Bone:Ultrasound struggles to image structures obscured by gas (e.g., bowel gas) or bone, while CT can easily image through these structures 4. Comprehensive Whole-body Scans:CT is ideal for whole-body trauma scans, allowing rapid identification of multiple injuries. 37  DISADVANTAGES OF CT 1. Spatial resolution is notably poorer (VS MRI) 2. The radiation dose is generally much higher for the same anatomical region 3. CT is limited to transverse axial scan because of the hardware of the scanner, although the gantry can be angle to take image of slice up to 30 to transverse section (VS MRI) 4. Difficult to image an anatomical region in which the soft tissues are surrounded with large amount of bone eg; posterior fossa, spinal cord, pituitary and intrapetrous space (VS MRI) 5. The presence of metallic object might produce metallic artifact on CT image. In addition, CT also produced other artifact compare to radiography. 38 DISADVANTAGES OF CT OVER CONVENT IONAL RADIOGRAPHY 1. Higher Radiation Exposure - CT scans involve significantly higher doses of ionizing radiation compared to conventional X-rays. While X-rays are sufficient for many diagnostic purposes (e.g., fractures, chest imaging), CT delivers much higher radiation, which may not be justified for simple cases. 2. Higher Cost - CT scans are much more expensive than traditional radiography, which limits their use in routine, lower-risk diagnostic scenarios. 39  DISADVANTAGES OF CT OVER MRI 1. Limited Soft Tissue Contrast - MRI offers superior soft tissue contrast compared to CT, particularly for imaging the brain, muscles, ligaments, and soft tissues of the joints. 2. No Functional Imaging - MRI provides functional data, such as blood flow, through techniques like functional MRI (fMRI) and diffusion-weighted imaging. 3. Radiation Exposure - MRI uses magnetic fields and radio waves, so it does not expose the patient to ionizing radiation, unlike CT. 4. Safety with Contrast Media - MRI contrast agents (e.g., gadolinium) are generally considered safer than iodine-based contrast agents used in CT, particularly for patients with kidney issues. 40  DISADVANTAGES OF CT OVER USG 1. Cost and Accessibility - Ultrasound is a cheaper and more portable imaging modality than CT. 2. Radiation issues - Ultrasound is entirely radiation-free, making it safer for pregnant women, children, and frequent follow-up imaging. 3. Real-time Imaging:Ultrasound offers real-time imaging, which is valuable for procedures such as guiding biopsies or monitoring fetal development. 4. Better for Superficial and Dynamic Structures - Ultrasound is better suited for imaging superficial structures (e.g., thyroid, breast, testicles) and dynamic organs (e.g., the heart, blood flow through Doppler ultrasound) in a real-time manner. CT is static, and repeated scans are needed to assess movement. 41 THANK YOU 42