Ct Physics 1 Flashcards
First gen Ct scanners
Design
Source
Beam
Geometry
Design: single X-ray source and single X-ray detector cell to collect all data from single slice
Source and detector rigidly coupled
- beam: pencil beam translated across patient to obtain set of parallel projection measurement at one angle
Source / detector rotate slightly and a subsequent set measurements are obtained during a translation past patient
Process is repeated once for each projection angle until 180 projections , across a 24cm fov
Translation and rotation process, this geometry is referred to as a translate / rotate scanner or rectilinear pencil beam scanning
First gen scanner
Name
Year
Time
Image looked?
- Emi mark I scanner (1972)
- earliest version: took 4-5 mins for a single scan and thus were restricted to some regions ( patient motion controlled)
- later version : used series of scans procedure time reduced some what by using two detector so that two parallel sections were acquired in one scan
Contrast resolution of internal structures was unprecedented , images had poor spatial
Resolution very poor
Second generation scanner
Design
Source
Generation name?
Design: multiple detectors because X-ray source emits radiation over large angle, the efficiency of measuring projections was greatly
Source and array of detectors are translated as in first gen systems but since the beam measured by each detector is at slightly different angle with respect to object , each translation step generates multiple parallel ray projections
Multiple projections obtained during each transversal past the patient this scanner is significantly more efficient and faster than the first one.
This generation is translate / rotate scanner or rectilinear multiple pencil beam scanning
Second generation scanner
Detectors
Early version
Later versions
First to allow type scans?
Adding detectors angularly displaced, several projections could be obtained in single translation
Early versions: 3 detectors each displaced by 1 percent , since each detector viewed the X-ray tube at different angle, a single transition produced 3 projection.
This system could rotate 3 degrees to the next projection rather than 1 degree making only 60 translations instead of 180 to acquire a complete section
Scan time as much as 3x
Later versions : up to 53 detectors . Fast enough ( tens of thousands of seconds) to permit acquisition during single breath hold.
First designs to permit scans of the trunk
Third gen scanner
Improve
New design
Type movement
Type beam
How projections obtained
What motion is called
Pure rotational scanning motion could be used , then it would be possible to use high power, rotating anode X-ray tubes and thus improve scan speeds in thicker body parts
New design: larger array of detectors ( 300- 700 detectors ., usually arced shape
Slam bang translational motion was replaced with smooth rotational motion and higher output rotating motion and higher output rotating anode X-ray tubes
X-ray tube is collimated to wide X-ray beam( fan beam)
Directed toward an arc shaped row detectors
Tube and detector array rotate around patient
Different projections are obtained during rotation by pulsing X-ray source or by sampling the detectors at a very high rate
This motion is referred to as continuously rotating fan beam scanning
In third gen scanners the apex is at the ?
Source
Third gen scanners
Improvement
Sampling
Scan times
Improvement from detector and data acquisition technology detector array with enough , high spatial resolution cells allow ,measurement of a fan beam projection of entire patient cross section
Sampling considerations required scanning an additional arc of one fan angle beyond 180 degree , although most scanner rotate 360 degree for each scan
Current helical scanners are based on modifications of rotate- rotate designs
Scan times= few seconds or less and recent versions are capable of sub second scan times
Imaging process is significantly faster than 1st or 2nd generation systems
Fourth generation scanner
Design
Source
Beam
Limitations
Design: also eliminated translate rotate motion. Circular array of fixed detectors
Source only rotates within a stationary ring of detectors
Larger fan beam
Shorter scanning time
Early versions: had some 600 detectors
Later versions: had up to 4,800
Limitations: less efficient use of detectors , less than 1/4 are used at any point during scanning
Only the X-ray generator and tube rotate at 360, thus shortening the scanning time even more
3rd gen vs 4th gen
3rd gen- apex tube side
4th gen- apex detector side
5th gen scanner
Design
Use
Scan time and number slices
Called
Design: X-ray tube is a large ring that circles patient, opposed to detector ring
Use: for cardiac tomographic imaging “ cine Ct”
X-rays produced = high energy electron beam
No moving parts to this scanner gantry
It is capable of 50 millisecond scan times and can produce 17 Ct slice/ second
Stationary/ stationary geometry
X-ray tube
Housing?
Filament current?
Cathode vs anode
Melting point?
Vacuum accelerating electrons
Electron will travel faster
Filament alternating current
Thermal electrons
Cathode(-) filament plate with tiny slit
Connected to high voltage battery source
Target / anode (+) electrons collide with target
Produce X-ray
Must have high melting point ( tungsten)
Cine Ct
No mechanical scanning motion
X-ray detector and tube anode are stationary
Anode, is very large semicircle ring that forms an arc around the patient scan circle
Source of X-rays is moved around the same path as fourth gen Ct scanner by steering an electron beam around the X-ray anode
Terms millisecond Ct, ultrafast ct and electron beam Ct have also been used , although latter can be confusing since the term suggests that the patient is exposed to an electron Beam
Cine Ct system
Very fast scanner , data collection for 1 slice is 50-100 ms
Requires no mechanical motion to acquire data
Sweeps an intense electron beam across large stationary anode target which surrounds the patient
X-rays are emitted from the point where electrons strike target
X-rays transmitted through object are measured by stationary array of detectors
Cine Ct systems, have higher noise level and lower spatial resolution but are ideal for some clinical application cardiac imaging with and without the use of contrast agent , lung imaging , and pediatric studies
Sixth generation scanner
1990, significant advancement in technology
Allowed 3D imaging with single breath hold
Spiral helical 6 th gen ct scanner
Design: X-ray tube rotates as patient is moved smoothly into X-ray scan field
Simultaneous source rotation, table translation and data acquisition
Produces one continuous volume set data for entire region
Data for multiple slices from patient acquired at 1 sec/ slice
Spiral/ helical vs axial scans
Spiral/ helical is same scan
Axial scan one slice at a time with small interscan delay
Head scan done?
Axial
Spiral/ helical scan the tube has a slight?
Angle
Advantages of spiral Ct
Speed- patient movement continuous , shorter exam time entire abdomen or chest : 30 s (18h)
Improved detections :differences bh is standard Ct. small lesions fall out of plane each continuous slice
Improved contrast- image a region in short period . Contrast can be timed
Improved reconstruction and manipulation- volume data collected transverse data can be reconstructed in any plane strip away skin muscles , etc
Contrast improved in Ct ( resolution) shows different ____
Greys
Resolution
Ability of scanner to define small structures
High resolution- high bit depth
Low res- low bit depth
Spiral/ helical Ct
Three technological developments
Slip ring technology gantry designs
Very high power day tubes
Interpolation algorithmns to handle projection data
Interpolation
Takes volume of data and mathematically takes average of bit depth pixels if missing ones in between
Slip ring technology
Alternative to cable system= slip ring
1989 kalender
Electromechanical devices: circular electrical conductive rings and brushes
Transmit electrical energy across moving transfer to a signal
All power and control signals from the stationary parts of the scanner system are communicated to the rotating frame through slip ring
Allow scan frame to rotate continuously with no need to stop between rotations to rewind system cables
What goes to slip ring?
Fov
Slip ring
Electromechanical device that allows the transmission of power and electrical signals from stationary to a rotating structure. Can be used in any electromechanical system that requires rotation while transmitting power or signals
High power X-ray tube
Thermal load in Ct
1 and 2nd, stationary tube( low heat, slow scan)
Oil cooling thermal system around tube, fast scans
Scan time vs heat capacity increased by 5x
Thermal depends on X-ray tube
Tubes with much higher thermal capacities were required to withstand continuous operation over multiple rotations
New design ; ceramic insulation , oil cooling of bearing, compact metal envelop
Expected life of tube 10,000-40,000 hrs vs 1000 regular one
Interpolation algorithmns
Kalender developed interpolation methods to generate methods to generate projections in a single plane
Overlapping sections generated by math , not beam , improve z axis with no increase in dose
Improved image quality
Seventh generation Ct scan
New technology , single row had its limitations
Design: multiple detector array
Collimator spacing is wider and more of the X-ray that are produced by the tube are used in producing image data. Opening up the collimator in a single array scanner increases slice thickness , reducing spatial resolution in the slice thickness dimension
With multiple detectors array scanner , slice thickness is determined by detector size not by collimator
7th gen Ct scanner
Turbo charged spiral
Up to 8 rows of detectors
4 rows, large volume of patient scanned 1 breath hold( thorax, abdomen , pelvis) at once
Allows 1 mm sections thorough chest in 20 sec
Improvement in details
Problem with pace , stain on storage system
7 th generations
Cone beam and multiple parallel row detectors
Widened ( z direction) X-ray beam and detector array to acquire multiple (4-64 slices simulatanously)
Advantage: reducing scan time/ increase z- resolution
Disadvantage: less scatter rejection compared to single slice , very expensive
Multiscanning
Relates to the technique of double or triple rotation of the tube and detectors around the same axial plane
Provides double of triple the volume per slice, upon which the final image can be derived
In practice each rotation produces its own of raw data bank
Hence motion which may occur during one rotation can be averaged out from data of the remaining two rotations
Multiscanning therefore reduces motion artifacts and consequently improves image quality
Multiscanning ct
1998 all major manufactures introduced multislice Ct, simulatanous acquisition of four slices , providing great improvement
4 slice, not fast enough to avoid venous overlay assuming a cerebral circulation time less than 5 sec
In 2000, 8 slice Ct were presented, followed by 16 slice in 2001
Most modern generation of msct is 64 slice per rotation, enabling a whole body cta with 1500 mm scan range in 22-25 sec
Future announced 256 slices
Complements: tubes and detector measurement systems , large influence on performance
Development in software and computer capacity lead to processing and reconstruction in short time
1st gen
Single X-ray tube.
Pencil beam
Single detector
2 gen ct
Single X-ray tube
Fan beam source
Multiple detector
3 rd gen scanner
Single X-ray tube
Fan beam
Many detectors
