Download ct physics limitations and more Exams Physics in PDF only on Docsity!
Mahadevappa Mahesh, Ph.D. Johns Hopkins University, Baltimore, MD
Fundamentals of Single and Multiple RowFundamentals of Single and Multiple Row
Detector Computed TomographyDetector Computed Tomography
Mahadevappa Mahesh, Ph.D.Mahadevappa Mahesh, Ph.D.
The Russell H. Morgan Department of RadiologyThe Russell H. Morgan Department of Radiology and Radiological Scienceand Radiological Science The Johns Hopkins UniversityThe Johns Hopkins University Baltimore, MD.Baltimore, MD. USAUSA
OutlineOutline
- • Single row detector helical CTSingle row detector helical CT
- • Multiple row detector helical CTMultiple row detector helical CT −− Four section/rotation scannersFour section/rotation scanners −− Scanners with >4 sections/rotationScanners with >4 sections/rotation
- • XX--ray tube issuesray tube issues
- • Relationship between pitch, dose,Relationship between pitch, dose, noise and section thicknessnoise and section thickness
IntroductionIntroduction
- • A recent survey* of internists rates CT among top 5A recent survey* of internists rates CT among top 5 major medical innovations over the past 30 yearsmajor medical innovations over the past 30 years
- • Two major evolutionary leaps occurred during lastTwo major evolutionary leaps occurred during last decade,decade, spiral or helical CTspiral or helical CT in early 90’s andin early 90’s and multiplemultiple--row detector CTrow detector CT late 90s to presentlate 90s to present
- • CT has evolved considerably since its invention inCT has evolved considerably since its invention in 1972, the progression might be characterized as1972, the progression might be characterized as search toward the 3D radiographsearch toward the 3D radiograph
_Decisions in Imaging Economics, Nov 2001Decisions in Imaging Economics, Nov 2001_**
Conventional XConventional X--ray Imagingray Imaging
NonNon--uniform beamuniform beam exits opposite surfaceexits opposite surface with intensity patternwith intensity pattern due to differentialdue to differential attenuation of raysattenuation of rays along different pathsalong different paths through patientthrough patient
Uniform xUniform x-enters patiententers patient-ray beamray beam
Image receptorImage receptor captures intensitycaptures intensitypatternpattern
XX--RayRay TubeTube
2D Images of 3D Anatomy from Single Projection2D Images of 3D Anatomy from Single Projection
Image due toImage due to differences indifferences in xx--rayray attenuationattenuation along differentalong different paths throughpaths through the patientthe patient
The ProblemThe Problem
- • Resolution >5Resolution >5 lplp/mm/mm
- • Acquisition time <<1 s (stops physiologicAcquisition time <<1 s (stops physiologic motion)motion) But in 2D images of 3D anatomyBut in 2D images of 3D anatomy
- • Tissues are superimposedTissues are superimposed
- • Poor contrast resolution due to highPoor contrast resolution due to high scatter acceptance by image receptorscatter acceptance by image receptor
Mahadevappa Mahesh, Ph.D. Johns Hopkins University, Baltimore, MD
Ultimate Goal: 3D RadiographyUltimate Goal: 3D Radiography
- • Resolution as good as conventionalResolution as good as conventional radiography in all planesradiography in all planes
- • High contrast sensitivity (no scatter)High contrast sensitivity (no scatter)
- • Fast acquisition times to stopFast acquisition times to stop physiologic motionphysiologic motion
- • Can CT get us there?Can CT get us there?
Computed TomographyComputed Tomography
- • Method for acquiring and reconstructing anMethod for acquiring and reconstructing an image of a thin crossimage of a thin cross--section of an objectsection of an object
- • Based on measurements of xBased on measurements of x --rayray attenuation through the section plane usingattenuation through the section plane using many different projectionsmany different projections
Basic data acquisition in CTBasic data acquisition in CT
XX--ray Tuberay Tube
DetectorsDetectors
CT TableCT Table
XX--ray Beamray Beam
CT GantryCT Gantry
Gantry OpeningGantry Opening (60(60--70 cm70 cm diadia.).) Sampled regionSampled regionwhere attenuationwhere attenuation measurements aremeasurements are made (50made (50 ––55 cm55 cm diameter)diameter) Field of View (FOV):Field of View (FOV): user selectableuser selectable location and diameterlocation and diameter within sampled regionwithin sampled region where reconstructionwhere reconstruction matrix is locatedmatrix is located
z
y
x
The CT ImageThe CT Image
Pixel value is measure ofPixel value is measure of xx--ray attenuation inray attenuation in corresponding volumecorresponding volume element (element (voxelvoxel ))
- • Depth dimension ofDepth dimension of voxelvoxel equal to sectionequal to section thickness (1thickness (1 --10 mm)10 mm)
PixelPixel VoxelVoxel w
Limitations of Conventional CTLimitations of Conventional CT
- • Scan plane resolution is ~1Scan plane resolution is ~1--22 lplp/mm/mm
- • Poor zPoor z--axis resolutionaxis resolution −− Section thickness ranges 1 to 10 mmSection thickness ranges 1 to 10 mm −− Volumes underVolumes under--sampled with abutted slicessampled with abutted slices
- • InterInter--scan delayscan delay due to stopdue to stop--start action necessarystart action necessary for table translation and cable unwindingfor table translation and cable unwinding
- • SectionSection--toto--sectionsection misregistrationmisregistration due to variationdue to variation in patient respiratory motionin patient respiratory motion
Mahadevappa Mahesh, Ph.D. Johns Hopkins University, Baltimore, MD
Helical Path of XHelical Path of X--Ray Beam on PatientRay Beam on Patient Technology AdvancesTechnology Advances
- • Interpolation algorithmsInterpolation algorithms −− Projection data is no longerProjection data is no longer in a crossin a cross--sectional planesectional plane −− Interpolation of projectionInterpolation of projection data into plane of interestdata into plane of interest prior to conventionalprior to conventional filtered backfiltered back--projectionprojection
** KalenderKalender WA, et.al. Radiology, 176(1):181WA, et.al. Radiology, 176(1):181--3, 19903, 1990
Helical TrajectoryHelical Trajectory Helical Trajectory
TranslationTranslation z (mm)z (mm) t (s)t (s)
Helical SingleHelical Single--Section ModeSection Mode
Interpolation using samples from single row detector ringInterpolation using samples from single row detector ring Interpolation using samples from single row detector ring
Helical PitchHelical Pitch
Pitch = Table increment per rotation (mm)Beam collimation (mm)
- • Typical Pitch RatioTypical Pitch Ratio -- 0.5, 1.0, 1.5, 2.00.5, 1.0, 1.5, 2.
- • Pitch <1 implies overlapping and higher patient dosePitch <1 implies overlapping and higher patient dose
- • Pitch >1 implies extended imaging and reducedPitch >1 implies extended imaging and reduced patient dosepatient dose
Capabilities of Single Row Detector CTCapabilities of Single Row Detector CT
(SDCT)(SDCT)
- • Large tissue volumes scanned in short timesLarge tissue volumes scanned in short times
- • InterInter--scan delay eliminatedscan delay eliminated
- • Arbitrary section position within scannedArbitrary section position within scanned volume permits overvolume permits over--sampling withoutsampling without increased doseincreased dose
- • Z axis resolution improved by overZ axis resolution improved by over--samplingsampling
- • Up to ~ 2Up to ~ 2 lplp/cm (best case), usually 0.5 to 1.0/cm (best case), usually 0.5 to 1.0 lplp/cm/cm
Limitations of SDCTLimitations of SDCT
- • Large volume scan in short duration is limitedLarge volume scan in short duration is limited
- • Near isotropic resolution only over small volumeNear isotropic resolution only over small volume
- • Poor utilization of XPoor utilization of X--ray tuberay tube
- • Multiple row detector CT (MDCT)Multiple row detector CT (MDCT) offersoffers substantial improvement in volume coverage,substantial improvement in volume coverage, scan speed with efficient use of xscan speed with efficient use of x --ray tuberay tube
Mahadevappa Mahesh, Ph.D. Johns Hopkins University, Baltimore, MD
Multiple Row Detector Helical CT (MDCT)Multiple Row Detector Helical CT (MDCT)
- • Single row ofSingle row of detectors replaceddetectors replaced with multiple rowswith multiple rows
Single multiSingle multi--element moduleelement module Single row detector CT Scanner Multiple row detector CT scanner
X-ray Tube Tube Collimator Collimated Slice
CollimatorDetector 1-Row Detector 8-Row Detector
SDCT versus MDCTSDCT versus MDCT
_Mahesh M,Mahesh M, RadioGraphicsRadioGraphics, 22: 949, 22: 949--962, 2002962, 2002_**
Uniform Element ArraysUniform Element Arrays
LightspeedLightspeed , GE Medical Systems, GE Medical Systems
2 x 0.63 mm 4 x 1.25 mm 4 x 2.5 mm 4 x 3.75 mm 4 x 5 mm 2 x 7.5 mm 2 x 10 mm
Possible section widthsPossible section widths
20 mm20 mm
16 x 1.25 mm16 x 1.25 mm ZZ--axisaxis
NonNon--Uniform Element ArraysUniform Element Arrays
Volume Zoom,Volume Zoom, SiemensSiemens Medical SystemsMedical Systems
2 x 0.5 mm 4 x 1 mm 4 x 2.5 mm 4 x 5 mm 2 x 8 mm 2 X 10 mm
Possible section widthsPossible section widths
5 2.5 1.5 11 1.52.5 5 20 mm20 mm ZZ--axisaxis
Hybrid Element ArraysHybrid Element Arrays
AcquilionAcquilion, Toshiba Medical Systems, Toshiba Medical Systems
4 x 0.5 mm 4 x 1 mm 4 x 2 mm 4 x 3 mm 4 x 5 mm 4 x 8 mm 2 x 10 mm
Possible section widthsPossible section widths
32 mm32 mm
15 mm15 mm 15 mm15 mm
4 x 0.54 x 0.
ZZ--axisaxis
MDCT: Detector Element ArraysMDCT: Detector Element Arrays
GEGE
SiemensSiemens && PhilipsPhilips
ToshibaToshiba
20 mm20 mm
16 x 1.25 mm16 x 1.25 mm
32 mm32 mm
15 mm15 mm 15 mm15 mm
4 x 0.54 x 0.
5 2.5 1.5 11 1.52.5 5 20 mm20 mm
ZZ--axisaxis
Mahadevappa Mahesh, Ph.D. Johns Hopkins University, Baltimore, MD
DAS channels: Four versus EightDAS channels: Four versus Eight
Detector Detector
20 mm20 mm
4 x 1.25 mm4 x 1.25 mm
GE Medical SystemsGE Medical Systems
Switching ArraySwitching ArraySwitching Array 8 x 1.25 mm8 x 1.25 mm
Detector Evolution:Detector Evolution: 4 4 vsvs 16 sections per rotation16 sections per rotation
2X0.5, 4x1, 4x2.5, 4x52X8, 2x 20 mm20 mm
5 2.5 1.5 11 1.5 2.5 5
ZZ--axisaxis
16 x 0.7516 x 0.
24 mm24 mm
4 x 1.54 x 1.5 (^) 4 x 1.54 x 1.
16x0.75, 16x1.5, 8x3,4x6…
SiemensSiemens & Philips Medical Systems& Philips Medical Systems
32 mm32 mm
15 mm15 mm 15 mm15 mm
4 x 0.54 x 0. 4x0.5, 4X1, 4x2, 4x3up to 4x
16x0.5, 16X1, 16x2,up to 8x
ZZ--axisaxis
12 x 112 x 1 32 mm32 mm
16 x 0.516 x 0.
12 x 112 x 1
Toshiba Medical SystemsToshiba Medical Systems
Detector Evolution:Detector Evolution: 4 vs. 16 sections per rotation4 vs. 16 sections per rotation
MDCT Episode II: Attack of the ConesMDCT Episode II: Attack of the Cones
Cone Beam GeometryCone Beam Geometry
- • In MDCT, wideningIn MDCT, widening beam aperture in zbeam aperture in z-- direction increasesdirection increases cone angle, thatcone angle, that results in significantresults in significant cone beam artifactscone beam artifacts
cone anglecone angle
fan anglefan angle
ZZ--axisaxis
Image Reconstruction:Image Reconstruction: Key Problem is Cone AngleKey Problem is Cone Angle
- • Up to 4 sectionsUp to 4 sections dSdS = S, cone angle may be neglected= S, cone angle may be neglected
thicknessthickness^ SectionSection
4 4--sectionsection 8 8--sectionsection
Sectionwidth ‘S’ ‘’Section blurring’’dS Focus
Scan FOV (^) Scan direction
Detector ArrayDetector Array
- • What about more than 4 sections?What about more than 4 sections?
Mahadevappa Mahesh, Ph.D. Johns Hopkins University, Baltimore, MD
- • What happens, if the cone angle of the rays is neglected ?What happens, if the cone angle of the rays is neglected? 41mm12 mm/sec, pitch 1.5 (6) 81mm81mm24 mm/sec24 mm/sec, pitch 1.5 (12), pitch 1.5 (12) 121mm12*1mm36 mm/sec36 mm/sec , pitch 1.5 (18), pitch 1.5 (18) - • Image results for > 4 sections are clinically unacceptable !Image results for > 4 sections are clinically unacceptable!
Key Problem: Cone AngleKey Problem: Cone Angle
161mm161mm48 mm/sec48 mm/sec , pitch 1.5 (24), pitch 1.5 (24)
CourtesyCourtesy SiemensSiemens Medical SystemsMedical Systems
Cone Beam Geometry:Cone Beam Geometry:
Alternate Reconstruction AlgorithmsAlternate Reconstruction Algorithms
- • Advanced Single SliceAdvanced Single Slice RebinningRebinning (ASSR)(ASSR)
- • Adaptive Multiple PlaneAdaptive Multiple Plane Reconstruction (AMPR)Reconstruction (AMPR)
Kohler T, et al., Medical Physics, 29 (1): 51Kohler T, et al., Medical Physics, 29 (1): 51--64, 200264, 2002
- • PiPi,, PiPi--Slant and 3Slant and 3--PiPi methodsmethods
- • HelicalHelical FeldkampFeldkamp withwith weighting function (HFK)weighting function (HFK)
Cone Beam ReconstructionCone Beam Reconstruction (e.g. Adaptive Multiple Plane Reconstruction)(e.g. Adaptive Multiple Plane Reconstruction)
41mm41mm12 mm/sec12 mm/sec, pitch 1.5 (6), pitch 1.5 (6) Standard ReconstructionStandard Reconstruction
161mm161mm32 mm/sec32 mm/sec , pitch 1.0 (16), pitch 1.0 (16) Standard ReconstructionStandard Reconstruction
161mm161mm32 mm/sec32 mm/sec , pitch 1.0 (16), pitch 1.0 (16) AMPR ReconstructionAMPR Reconstruction
CourtesyCourtesy SiemensSiemens Medical SystemsMedical Systems
Helical PitchHelical Pitch -- Reality vs. MythReality vs. Myth Definition, Confusion…Definition, Confusion…
Pitch redefined for MDCTPitch redefined for MDCT
TT -WW -- Table travel (mm)/rotation- Beam width (mm)Table travel (mm)/rotationBeam width (mm)
Multiple Row Detector ArrayMultiple Row Detector Array
TT
DD
WW
Single Row Detector ArraySingle Row Detector Array
TT W=DW=D
Beam Pitch =Beam Pitch = TTWW
Detector Pitch =Detector Pitch = DDTT
Beam Pitch =Beam Pitch = Detector PitchDetector PitchNN
DD -NN -- Single DAS channel width (mm)- Number of active DAS channelsSingle DAS channel width (mm)Number of active DAS channels
Beam Pitch vs. Detector PitchBeam Pitch vs. Detector Pitch
16.516.5 4 x 34 x 3 5.55.5 1.3751.
13.513.5 4 x 34 x 3 4.54.5 1.1251.
7.57.5 4 x 34 x 3 2.52.5 0.6250.
7.57.5 4 x 1.254 x 1.25 66 1.501.
3.753.75 4 x 1.254 x 1.25 33 0.750.
BeamBeam PitchPitch
DetectorDetector PitchPitch
DetectorDetector combicombi
Table SpeedTable Speed (mm/(mm/rotnrotn))
Data for fourData for four--section MDCTsection MDCT
Mahadevappa Mahesh, Ph.D. Johns Hopkins University, Baltimore, MD
XX--ray Tubesray Tubes
- • In helical CT, ZIn helical CT, Z--axis resolution and scanaxis resolution and scan volume place huge demands on tubevolume place huge demands on tube
- • Several technical advances have beenSeveral technical advances have been made to achieve power levels and dealmade to achieve power levels and deal with problems of heat generation, storagewith problems of heat generation, storage and dissipationand dissipation
XX--ray tubes used for Spiral CTray tubes used for Spiral CT
- • Larger anode disks allow higher tube currentsLarger anode disks allow higher tube currents
- • Anodes of graphite based body with tungstenAnodes of graphite based body with tungsten-- rhenium or tungstenrhenium or tungsten--zirconzircon--molybdenummolybdenum** layer deposited by sintering or chemical orlayer deposited by sintering or chemical or physical vapor processphysical vapor process
_ AmmannAmmann E, et al., BJR, 70, S1E, et al., BJR, 70, S1 --S9, 1997S9, 1997_**
XX--ray tubes used for spiral CTray tubes used for spiral CT
- • Metal envelopes with ceramic insulatorsMetal envelopes with ceramic insulators provide higher heat storage capacityprovide higher heat storage capacity
- • Spiral groove bearings improve heatSpiral groove bearings improve heat dissipation requiring shorter cooling periodsdissipation requiring shorter cooling periods and therefore allow continuous rotation withand therefore allow continuous rotation with minimal wearminimal wear
XX--ray tube used in Spiral CTray tube used in Spiral CT
Ceramic insulators
Unique 200 mm anode disk
Direct oil cooling of spiral groove bearing
Compact, all metal envelope Courtesy Philips Medical SystemsCourtesy Philips Medical Systems
Modern CT XModern CT X--Ray TubesRay Tubes
• • Heat storage capacity exceeds >3Heat storage capacity exceeds >3--8 MHU8 MHU
• • No longer the limitations for studiesNo longer the limitations for studies
demanding higher speed and largerdemanding higher speed and larger
volume coveragevolume coverage
NoiseNoise
NoiseNoise
11
vno. of photonsvno. of photons
- • Double the tube current, reduces noise byDouble the tube current, reduces noise by √√ 22
- • Halve the section width, increases noise byHalve the section width, increases noise by √√ 22
Tube currentTube current Scan timeScan time Section widthSection width
Mahadevappa Mahesh, Ph.D. Johns Hopkins University, Baltimore, MD
Noise vs. PitchNoise vs. Pitch
- • For SDCT, noise is independent of pitch forFor SDCT, noise is independent of pitch for constantconstant mAsmAs and section widthand section width
- • However on most MDCT scanners, systemHowever on most MDCT scanners, system software automatically adjust scansoftware automatically adjust scanmAmA perper protocol to obtain comparable image noise asprotocol to obtain comparable image noise as user alters acquisition parametersuser alters acquisition parameters
Effective Section ThicknessEffective Section Thickness
Section and Beam CollimationSection and Beam Collimation
- • SDCT:SDCT: Both are same, influences zBoth are same, influences z--axisaxis coverage per gantry rotationcoverage per gantry rotation
- • MDCT:MDCT: Section thicknessSection thickness** is total beamis total beam collimation divided by number of activecollimation divided by number of active detector channelsdetector channels −− e.g., 10 mm / 4 channels = 4 x 2.5 mme.g., 10 mm / 4 channels = 4 x 2.5 mm
_defined at center of rotationdefined at center of rotation_**
Section ThicknessSection Thickness
- • True thickness of theTrue thickness of the reconstructed image,reconstructed image, measured as full width atmeasured as full width at half maximum (FWHM) ofhalf maximum (FWHM) of slice sensitivity profileslice sensitivity profile Slice Sensitivity Profiles:Slice Sensitivity Profiles: conventional and spiralconventional and spiral acquisitionacquisition
- • Same as beam collimation inSame as beam collimation in conventional scanning butconventional scanning but different in spiral scanningdifferent in spiral scanning
Effective Section ThicknessEffective Section Thickness
- • Measure of slice sensitivity profile at FWHMMeasure of slice sensitivity profile at FWHM
- • Affected by beam collimation, pitch andAffected by beam collimation, pitch and interpolation algorithminterpolation algorithm
- • In SDCT user selects section thickness, but trueIn SDCT user selects section thickness, but true width of reconstructed section is influenced bywidth of reconstructed section is influenced by pitch and interpolation algorithm (180° vs. 360°)pitch and interpolation algorithm (180° vs. 360°)
- • In MDCT user selects beam collimation inIn MDCT user selects beam collimation in combination with desired section width which iscombination with desired section width which is affected by pitch, interpolation algorithm & Zaffected by pitch, interpolation algorithm & Z--filterfilter
Pitch vs. Effective Section ThicknessPitch vs. Effective Section Thickness
- • Increasing pitchIncreasing pitch broadens effectivebroadens effective section thicknesssection thickness
- • Structures outsideStructures outside nominal sectionnominal section thickness willthickness will contribute to imagecontribute to image
