4_CT_generation_(B).pdf

19/01/1442 Dr. Lubna Dr .Bager 1 Lec 4 19/01/1442 Dr. Lubna Dr .Bager 2  Generations 1- 4 used a slice-by-slice image acquisition where the x-ray tube is moved around the patient to collect data from a single slice of tissue followed by table indexing to scan the next adjacent slice....

19/01/1442 Dr. Lubna Dr .Bager 1 Lec 4 19/01/1442 Dr. Lubna Dr .Bager 2  Generations 1- 4 used a slice-by-slice image acquisition where the x-ray tube is moved around the patient to collect data from a single slice of tissue followed by table indexing to scan the next adjacent slice.  This is repeated until data from several adjacent slices are collected. 19/01/1442 Dr. Lubna Dr .Bager 3 Conventional slice-by-slice 19/01/1442 Dr. Lubna Dr .Bager 4 Conventional slice-by-slice had some limitations: • Longer inter-scan delay times due to start-stop action necessary for patient breathing, and table indexing. • Slice-to-slice misregisteration due to patient respiration phase may not be consistent between scans (lesions smaller than 1 cm can be missed due to inconsistent breathing). • • Inaccurate generation of 3-D images and MPR images . Only a few slices can be scanned during contrast enhancement when the technique is administered. 19/01/1442 Dr. Lubna Dr .Bager 5 The small lesion is missed due to inconsistent breathing. This is a common feature of a slice-by-slice scanning. 19/01/1442 Dr. Lubna Dr .Bager 6 • This system has been followed by the spiral (volume) scanner that can do both slice-by-slice and multi-slice techniques. • Problems of inter-scan delays in 1 st – 4th slice-by-slice acquisition scanners were solved by continuous rotation of x-ray tube by a slip-ring technology. Rotation of the tube is combined with the continuous motion of patient along scanner axis to produce a helical (spiral) type of motion geometry. 19/01/1442 Dr. Lubna Dr .Bager 7 6th generation: helical The Spiral (helical) CT scanner Spiral /helical (volume) data acquisition geometry. 19/01/1442 Dr. Lubna Dr .Bager 8 6th generation: helical The Spiral (helical) CT scanner  Spiral CT was introduced to clinical practice 1989.  Spiral CT requires Slip Ring technology for data transfer from the rotating gantry.  Slip Ring Technology:  Slip rings are "electromechanical devices consisting of circular electrical conductive rings and brushes that transmit electrical energy across a rotating interface". 19/01/1442 Dr. Lubna Dr .Bager 9 Slip Ring Technology 19/01/1442 Dr. Lubna Dr .Bager 10  Slip-ring scanners provide continuous rotation of the gantry through the elimination of the long high-tension cables to the x-ray tube used in conventional start-stop scanners (1-4), which must be un wound after a complete rotation.  In conventional scanners, these cables originate from the highvoltage generator, usually located in the x-ray room.  In 6th generation, The high-voltage generators of slip-ring scanners are located in the gantry.  Scanners with either low-voltage or high-voltage slip rings are available based on the power supply to the slip ring. 19/01/1442 Dr. Lubna Dr .Bager 11 reciprocate rotation (A) versus fast continuous rotation using slipring technology (B) 19/01/1442 Dr. Lubna Dr .Bager 12 Advantages of Slip Ring  The major advantage of slip-ring technology is that it facilitates continuous rotation of the x-ray tube so that volume data can be acquired quickly from the patient.  Other advantages are as follows: 1. Faster scan times and minimal interscan delays. 2. Capacity for continuous acquisition protocols in the future. 3. Elimination of the start-stop process characteristic of conventional CT scanners. 4. Removal of the cable wrap-around process. 19/01/1442 Dr. Lubna Dr .Bager 13 Spiral CT Patient 19/01/1442 Dr. Lubna Dr .Bager 14 The small lesion is missed due to inconsistent breathing. This is a common feature of a slice-by-slice scanning. Multi-slice technique reveals the lesion. 19/01/1442 Dr. Lubna Dr .Bager 15 FIG.. With volume CT scanners, the x-ray tube and detectors rotate continuously as the patient moves continuously through the gantry As a result, the x-ray beam traces a path (beam geometry) around the patient.This method of scanning the patient is referred to as helical or spirol CT 19/01/1442 Dr. Lubna Dr .Bager 16 Spiral CT Instrumentation  Not different in external apperence from conventional CT scanner  The difference in several major component such as:  Use of slip ring technology  X-ray tube is larger, energized for longer period and have heat dissipation greater than 3 MHU  Detectors should be solid state  The high voltage generator is high frequency with High power output  Interpolation algorithms to handle projection data 19/01/1442 Dr. Lubna Dr .Bager 17 Effects of transporting the table too quickly: Image degradation by motion artifacts. Patient may suffer motion sickness. • • NB/ • • Reduction of slice thickness by post-patient collimation leads to greater spatial resolution, but produces more dose to a patient for series of adjacent slices. It also leads to image noise. Increase of slice thickness increases the low contrast image resolution, leading to an artifact. 19/01/1442 Dr. Lubna Dr .Bager 18 Pitch factor in spiral (volume) scanners: Pitch: is a Ratio of the distance the patient’s table moves (in mm) during one revolution of x-ray tube, to slice thickness (beam collimation). Pitch = Table feed or travel (mm) / slice thickness e.g., for a table travel of 10mm/s and slice width 10mm, the pitch will be 10/10 = 1 Typical pitch values are in the range 1 to 2 (depends on required spatial resolution in the z direction. 19/01/1442 Dr. Lubna Dr .Bager 19 • • • Pitch ratio = 1:1 (called pitch 1) when the table travels a distance equal to slice thickness or beam collimation. pitch (1) in spiral CTs results in the best image quality. Pitch can be increased so as to increase the volume coverage and speed up the scanning process. Volum coverage: Volume coverage = (pitch) x (slice thickness). Higher pitch also produces lower dose, less x-ray tube load, and less motion artifacts. 19/01/1442 Dr. Lubna Dr .Bager 20 Pitch increase 19/01/1442 Dr. Lubna Dr .Bager 21 • More coverage: Pitch increase has 3 main effects: • Scan times will be reduced • Radiation is less concentrated, and exposure dose will be reduced. • No more (detail) in the image. Its quality will be reduced. 19/01/1442 Dr. Lubna Dr .Bager 22 Pitch = 1  Pitch = 1 means patient moves exactly one slice thickness per revolution of tube table motion during one tube rotation Slice Pitch = ---------------------------------------------slice thickness Beam positions when tube directly above patient 19/01/1442 Dr. Lubna Dr .Bager 23 Pitch <1  Pitch < 1 means patient moves less than a slice thickness during one tube rotation  Can improve visualization of objects table motion during one tube rotation Slice Pitch = ----------------------------------------------slice thickness Beam positions when tube directly above patient 19/01/1442 Dr. Lubna Dr .Bager 24 Pitch >1  Pitch > 1 means patient moves further than slice thickness during one tube rotation table motion during one tube rotation Slice Pitch = ---------------------------------------------slice thickness Beam positions when tube directly above patient 19/01/1442 Dr. Lubna Dr .Bager 25 Pitch & Dose  Dose inversely proportional to pitch  Pitch = 0.5 => Dose doubles  Pitch = 2 => Dose cut in half 19/01/1442 Dr. Lubna Dr .Bager 26 Advantages and Limitations  Advantages: Scans complete (volume) of tissue in very short scan times (30 s for the chest or abdomen) because of the continuous data acquisition, elimination of inter-scan delays, and continuous patient transport. 2. The Examination time is reduced because of the elimination of the ISD. 3. Gapless scanning is possible because a volume of tisse is scanned 4. Artifact caused by patient motion are reduced 1. 19/01/1442 Dr. Lubna Dr .Bager 27 5. Slice can be obtained for any position with the 6. 7. 8. 9. 10. volume. The effect of different levels of respiration are removed. New perspectives in contrast medium studies are possible. Improved detection of small lesions and contrast enhancenhancement. Greater accuracy in MPR and 3D imaging processing Multidimentional imaging including CT fluoroscopy, 3D imaging, CT angiography and CT endoscopy 19/01/1442 Dr. Lubna Dr .Bager 28 Limitations  Although CT scanning in spiral offers significant advantages over the use of conventional slice – by – slice CT scanning it also has following drawback: X-ray tubes with higher thermal capacity and cooling rates are required (expensive). 2. Unsharp (streaks) artifacts appear on images due to partial volume averaging, or patient motion. 1. 19/01/1442 Dr. Lubna Dr .Bager 29 Limitations of the spiral 19/01/1442 Dr. Lubna Dr .Bager 30 7th generation The multi-slice (MS) CT scanner  helical CT was very hard on x-ray tubes.  As a result, the more rotation of x-ray tube, creates huge amounts of heat which effects the ability of the scanner  A straightforward solution to this heat issue, of course, is to develop x-ray tubes with a higher heat capacity 19/01/1442 Dr. Lubna Dr .Bager 31  Another approach is to more effectively use the available x-ray beam: if the x-ray beam is widened in the z-direction (slice thickness) and if multiple rows of detectors are used, then data can be collected for more than one slice at a time  This approach would reduce the total number of rotation and therefore the total usage of the x-ray tube needed to cover the desired anatomy. This is the basic idea of MSCT. 19/01/1442 Dr. Lubna Dr .Bager 32 MULTISLICE SPIRAL CT  Introduced at the 1998.  They are based multiple detector. rows ranging between 8, 16, 24, 32 and 64 depending on the manufacturer.  The overall goal is to improve the volume coverage speed performance.  Complete x-ray tube/detector array rotation in less than 1s.  Partial scan images can be obtained in approximately 100ms. 19/01/1442 33 MSCT Detectors:  The primary difference between single-slice CT (SSCT) and MSCT hardware is in the design of the detector arrays  In MSCT, each of the individual, monolithic SSCT detector elements in the z-direction is divided into several smaller detector elements, Rather than a single row of detectors encompassing the fan beam, there are now multiple, parallel rows of detectors 19/01/1442 Dr. Lubna Dr .Bager 34 advantages: multi-slice CT scanner uses multi detector array. This has the following advantages: • Improved volume coverage and faster scan with (50% reduction in scan times). • Improved spatial resolution (provides very thin slices – This is known as isotropic imaging. • Higher pitch ratio (large volume coverage) improving the image quality with the whole beam used to get 4 slices/ 360 rotation (4-slice multi-section scanner can provide 19/01/1442 Dr. Lubna Dr .Bager 35 Increase whole life of the x-ray tube. • Reduces radiation exposure by 40% for the same image quality as compared to a single-slice unit. • Improves accuracy in the needle placement in CT fluoroscopy because of the multiple images produced. 19/01/1442 Dr. Lubna Dr .Bager 36 Cone beam and Fan beam geometry:  Because of larger number of detectors used, the x-ray beam becomes a wider diverging cone beam (not a fan beam).  The number of rows greatly affects slice’s thickness and volume coverage.  A cone beam geometry produce more beam divergence along the z-axis direction compared with fan beam geometry  Additionally, the number of detectors rows plays an important role in slice thickness selection and volume coverage 19/01/1442 Dr. Lubna Dr .Bager 37 MSCT CONCEPTS: differences between MSCT AND SSCT Slice Thickness: Single Detector Array Scanners:  The slice thickness in single detector array CT systems is determined by the physical collimation of the incident x-ray beam with two lead jaws. Slice Thickness: Multiple detector Array Scanners:  The slice thickness of multiple detector array CT scanners is determined not by the collimation, but rather by the width of the detectors in the slice thickness dimension 19/01/1442 Dr. Lubna Dr .Bager 38 Clinical application of MSCT scan:  CT angiography  CT fluoroscopy  multislice CT, where up to 64 (128 - 256) slices can be collected simultaneously  3-D CT and CT endoscopy  Cardiac image 19/01/1442 Dr. Lubna Dr .Bager 39 Pitch in dual-slice CT scanner: For a multi-slice scanner, increasing the pitch by a factor of 4 allow for (increased volume coverage) in (less time) with no loss of image quality. 19/01/1442 Dr. Lubna Dr .Bager 40 COMPUTER TOMOGRAPHY REFERENCES 1. Euclid Seeram.Computed Tomography Physical Principle, Clinical Application, and Quality Control, 2nd edition, W.B. Saunders Company, 2001. 2. 3- Stewart C. Bushong. Radiologic Science for Technologist, 7th edition, Mosby, Inc, 2001 3.  Text book of radiographic positioning and related anatomy; by Kenneth L.Bontrager,6th edition. Useful Links : www.Xray2000.com  www.CTisUS.com 19/01/1442 Dr. Lubna Dr .Bager 41