CT Teaching Manual: A Systematic Approach To CT Reading Second Edition

198 Pages·2005·55.28 MB·English

by  Matthias Hofer

Matthias Hofer
Second edition
4
I
KeytoAnatomicStructures FrontCoverFlap NormalAnatomyofthePetrousBone(CoronalandAxial) 46
onpages26-73,152/153(head/ neck) NormalVariantsoftheCranium 50
TypicalPartialVolumePhenomenaoftheCranium 52
NotesfortheUser,"Whatyoushouldknow" FrontCoverFlap
CerebralCT,PathologicFindings
NotesfortheUser,ListoftheCTdiagrams IntracranialBleeds 54
CerebralInfarcts 58
ForewordandListof Abbreviations 3 CerebralTumorsandMetastases 59
InflammatoryProcesses 60
PhysicalandTechnical Fundamentals OrbitalChanges 61
GeneralPrinciplesofCT 6 ChangesoftheFacialBonesandParanasalSinuses 62
ComparisonofConventionalCTwithSpiralCT 7
SpatialResolution,Pitch 8 NeckCT
SectionCollimation:ResolutionalongtheZ-axis 9 SelectionoftheImagePlane 64
AdaptiveDetectorDesign 10 CheckiistforaSystemicApproach 64
ReconstructionAlgorithms 11 NormalAnatomyoftheNeck 65
EffectsofkV,mAsandScanTime 12
Three-dimensionalReconstruction Methods: 13 NeckCT,PathologicChanges
Maximum IntensityProjection(MIP) InflammatoryProcessesandTumors 70
MultiplanarReconstruction(MPR) ThyroidGland 71
SurfaceRendering Testyourselfl 72
BasicRulesforReading CTExaminations KeytoAnatomicStructures BackCoverFlap
AnatomicDrientation 14 onpages71,74-149 (thorax/ abdomen)
PartialVolumeEffects
DistinguishingNodularfromTubularStructures 15 ChestCT
Densitometry SelectionoftheImagePlane 74
DensityLevelsofDifferentTIssues 16 SystemicSequentialApproach toInterpretation
DocumentationUsingDifferentWindowSettings 17 ChecklistforInterpretingChestCT 74
NormalAnatomyoftheChest 75
PreparingthePatient Testyourself! 82
MedicalHistory 18
RenalFunction ChestCT,PathologicChanges
Hyperthyroidism AnatomyofthePulmonarySegments 84
AdverseReactiontoContrastAgents HRCToftheLungs:Technique,Effects,Indications 86
Premedication AnatomicVariantsoftheChest 88
OralAdministrationofContrastAgents 19 ChestWall
InformingthePatient AbnormalLymphNodes 89
RemovalofForeignObjects Breast,BonyThorax 90
ControllingRespiration Mediastinum
TumorMasses 91
AdministrationofContrastAgents EnlargedLymph Nodes 92
OralAdministrationofContrastAgents 20 VascularPathology 93
SelectionoftheAppropriateContrastAgents Heart 94
DurationandDose Lung
In avenousInjectionofContrastAgents IntrapulmonaryNodules 95
In avenousAccess 21 BronchialCarcinoma,MalignantLymphangiomatosis 96
:l1ilo'o EffectofContrastAgents Sarcoidosis,Tuberculosis,Aspergillosis 97
~""re .8ReactiontoContrastAgentsandTheirTherapy 24 PleuralChanges,Asbestosis 98
-ayro;o· CrisisanditsTherapy 25 Silicosis,PulmonaryEmphysema 99
InterstitialPulmonaryFibrosis 100
......,,,,,,,1
CT Testyourselfl 100
s.=ec OJ ' ImagePlane 26
'3 _-=- ~:'O 02 toInterpretation Abdomen CT
CranialCT 26 SelectionoftheImagePlane 102
SystemicSequentialApproachtoInterpretation
ChecklistforInterpretingAbdomen CT 10,
27 NormalAnatomyoftheAbdomen 104
32 NormalAnatomyofthePelvis(Male) 11,
it(Axial) 33 NormalAnatomyofthePelvis(Female) 114
- Bones(Coronal) 41
45
Table of Contents
5
AbdomenCT,PathologicChanges Spine
AnatomicVariantsoftheAbdomen 116 Cervical Spine(C-spine) 152
TypicalPartialVolumePhenomena C-Spine, DiskProlapseand Fractures 153
Abdominal Wall ThoracicSpine(T-spine): 15t
Enlarged LymphNodes,Abscesses 117 NormalFindingsand Fracture
SubcutaneousHeparinInjections 118 LumbarSpine(L-spine): 155
AbdominalWallMetastases Normal Findingsand LumbarDiskProlapse
InguinalHernias L-spine,Fractures 156
liver L-spine,Tumors/ Metastases 157
AnatomyoftheHepaticSegments 119 L-spine,Inflammations/InternalFixation 158
ExaminationProtocols 120
SelectionofWindow Display LowerExtremity
Bolus PassageofContrastAgents NormalAnatomyoftheThigh 159
CTPortography NormalAnatomyoftheKnee 160
HepaticCysts 121 NormalAnatomyoftheCalf 161
HepaticMetastases 122 NormalAnatomyoftheFoot 162
SolidHepaticLesions: 123 Fractures ofthe Foot 163
Hemangioma PelvisandThigh:InflammatoryProcesses 166
Adenoma Knee,Fractures,ChecklistFractureDiagnosis 167
FocalNodularHyperplasia
DiffuseHepaticChanges: 124 CT-guidedInterventions 168
FattyLiver
Hemochromatosis ExaminationProtocolsforSpinalCT 169
Cirrhosis
BiliarySystem RadiationProtection
Pneumobilia 124 Radiation Dose/ CancerRisk 174
Cholestasis AutomatedBolusTracking(BT) 176
Gallbladder TubeCurrentModulation 177
Cholecystolithiasis 124
ChronicInflammatoryProcesses 125 CT-Angiography
Spleen IntracranialArteries 178
Enhancement,Splenomegaly 126 Cranial DuralVenousSinus 179
FocalSplenicChanges 127 CarotidArteries 180
Pancreas Aorta 182
AcuteandChronicPancreatitis 128 Heart:CoronaryArteries, 184
PancreasNeoplasms ScreeningforCoronary
AdrenalGlands ArteryCalcifications
Hyperplasia,Adenomas, Metastases,Neoplasm 130 PulmonaryVasculature(PulmonaryEmboli) 186
Kidneys AbdominalVasculature 187
CongenitalVariants 132 IliofemoralVasculature 188
Cysts,Hydronephrosis 133 VascularProtheses,Outlook 189
SolidTumors 134 TestYourself! 190
VascularRenal Changes 135
UrinaryBladder The FundamentalsofInterpretingCT 192
IndwellingCatheter,Diverticula,SolidLesions 136
GenitalOrgans AnswerstoTestYourself 196
Uterus 137
Ovaries, Prostate Gland,Vas Deferens 138 Index 203
GastrointestinalTract
Stomach 139 References BackCoverFlap
InflammatoryBowelDiseases 139
Colon 140 KeytoAnatomicStructures BackCoverFlap
Ileus 141 on pages152-167(spine/ leg)
TestYourself! 141
Retroperitoneum KeytoAnatomicStructures BackCoverFlap
Aneurysms 142 on pages71,74-149 (thorax/ abdomen)
VenousThromboses 143
Enlarged LymphNodes 144
SkeletalChanges
BonyPelvis:Normal Findings,Metastases 145
Fractures 147
HipDysplasia,Necrosisofthe FemoralHead 148
TestYourself! 149
Physical and Technical Fundamentals
6
General PrinciplesofCT
Computed tomography isaspecialtype ofx-rayprocedure that design,and isdefined bythe angle originating atthe focusofthe
involvestheindirectmeasurementoftheweakening,orattenua x-raytubeandextendingtotheouterlimitsofthedetectorarray.
tion,ofx-raysat numerouspositionslocatedaround the patient Typically,images areproduced foreach360°rotation,permitting
being investigated.Basicallyspeaking,allweknowis ahighnumberofmeasurementdatatobeacquired andsufficient
• whatleavesthex-raytube, dosetobeapplied.Whilethescanisbeing performed,attenuation
• whatarrivesatthedetectorand profiles,alsoreferred toas samplesorprojections,areobtained.
• the positionofthex-raytubeanddetectorforeachposition. Attenuationprofilesarereallynothingotherthanacollectionofthe
Simplystated,everythingelse isdeduced from thisinformation. signalsobtainedfromallthedetectorchannelsatagivenangular
Most CTslices areorientedverticaltothebody'saxis.Theyare position of thetube-detectorunit.Modern CTsystems(Fig.6.4)
usuallycalled axial or transversesections.For eachsection the acquireapproximately1400 projectionsover360°,oraboutfour
x-ray tube rotates around the patient to obtain a preselected projections per degree. Each attenuation profile comprises the
section thickness (Fig. 6.1). Most CT systems employ the data obtained from about 1500 detector channels, about 30
continuous rotation and fanbeamdesign: withthis design,the channelsperdegree incaseofa50°fan beam.Whilethepatient
x-raytubeanddetectorarerigidlycoupledand rotatecontinuous table ismoving continuouslythrough the gantry,adigital radio
lyaroundthescanfield while x-rays areemitted and detected. graph ("scanogramm" or "localizer", Fig. 6.2) is produced on
Thus,thex-rays,whichhavepassedthroughthepatient,reachthe whichthedesiredsectionscanbeplanned.ForaCTexamination
detectorsontheoppositeside ofthetube.Thefanbeam opening of thespineor thehead, thegantry is angled to the optimal
ranges from 40° to 60°, depending on the particular system orientation (Fig.6.3).
Angulnlion
Gantry
•
..o
« .
~
~
Table
movement
Fig.6.1 Fig.6.2 Fig.6.3
Multiple-RowDetectorSpiralCT
Multiple-row detector CT (MOCn isthelatest scanner develop
ment. Ratherthan onedetectorrow,multipledetectorrows are
placedoppositethex-raytube.Thisshortenstheexaminationtime
"
I and improvesthetemporalresolution,allowing,forinstance,the
determinationoftherateofvascularenhancement.
Thedetector rowsalong the z-axisoppositethex-ray tubeare
unequal inwidth,withtheouterrowswiderthantheinnerrowsto
provide better conditions for image reconstruction affer data
acquisition(see pages9-11).
Fig.6.4
Physical and Technical Fundamentals
7
ComparisonofConventionalCTwithSpiralCT
Inconventional CT,aseries ofequallyspacedimagesisacquired x-raytube
sequentiallythrougha specificregion,e.g.theabdomenorthe
head (Fig.7.1).Thereisashortpauseaftereachsection inorder Gantry
toadvancethepatienttabletothenextpresetposition.Thesection
thicknessandoverlap/intersection gapareselectedatIheoutset.
Therawdataforeach imagelevelisstoredseparately.The short
pausebetween sections allows theconsciouspatienttobreathe
withoutcausing majorrespiratoryartifacts.
Step-wise
table
However,theexaminationmaytakeseveralminutes,dependingon movement
thebodyregionandthesize ofthepatient.Propertimingofimage
acquisition after Lv. contrastmedia is particularlyimportant for
assessing perfusion effects. CT is the technique of choice for
acquiring complete 20 axial images of the body without the
disadvantages of superimposed bone and / or air as seen in
conventionalx-rayimages.
Fig.7.1
Both single-row detectorCT(SOCT)andmultiple-rowdetectorCT
X-ray lube
~ (MOCl) continuously acquire data of the patient while the
Imaging . examination table moves through the gantry. The x-ray tube
volume "\ / describesanapparenthelicalpatharoundthepatient(Fig.7.2).If
Rotation
tableadvance iscoordinatedwiththetimerequired for a360'
--.~,,- rotation (pitchfactor), dataacquisitioniscompleteanduninter
rupted. Thismodern technique hasgreatlyimprovedCTbecause
respiratoryartifactsandinconsistenciesdonotaffect the single
datasetasmarkedlyasinconventionalCT.Thesingledatasetcan
beusedtoreconstructslicesof differing thicknessoratdiffering
intervals.Evenoverlappingslicescanbereconstructed.
Dataacquisitionfortheabdomen takes only1-2 minutes: two or
threehelices,eachabout10to20seconds,areobtained.Thetime
limit isdetermined bythedurationapatient canholdhisbreath
Continuous
table andthenecessarycoolingofthex-raytubes.Imagereconstruction
movement
takes longer. An assessment ofrenal function following CM will
Fig.7.2
requireashortbreaktoallowforCM excretiontooccur.
One of the advantages ofthe helicaltechnique is that lesions respiration resultsinthem notbeing includedinthesection(Fig.
smallerthantheconventionalthicknessofaslicecanbedetected. 7.3a).Themetastaseswouldappear inoverlappingreconstruc
Smalllivermetastases (7)willbemissedif inconsistentdepthof tionsfrom thedatasetofthehelical technique(Fig.7.3b).
-- -----------
--- --
5
Fig.7.3a ConventionalCT Fig.7.3b Spiral CT
Physical and Technical Fundamentals
8
SpatialResolution
Thereconstructedimagesshouldhaveahightemporalresolution as grey values proportionate totheir attenuation (Fig. 8.1b).In
toseparateeven smallstructuresfromeachother.Thisgenerally reality,however,they are notsquares butcubes (voxel= volume
createsno problem along thex-ory-axis oftheimagesince the element) with their length along the bodyaxis defined by the
selected fieldofview (FOV) typicallyencompasses 512 x512 or sectionthickness(Fig.8.1a).
morepicture elements(pixel).These pixelsappearon the monitor
~"
.:
/ '/ r-,\1
I 'I'
i 129'" \\
122
J..
\ 50 l-1 1,3~ 7
7';;::V
<, v:r.:::v
Fig.8.1a Fig.8.1b
Theimagequalityshouldimprovewithsmallervoxels,butthisonly advantage: The multiplanar reconstruction (MPR) in coronal,
appliestothespatialresolution sinceathinnersection lowersthe sagittalorotherplanesdisplaysthe reconstructed imagesfree of
signal-to-noise ratio.Another disadvantageofthinnersectionsis anystep-likecontour(Fig. 8.2).Usingvoxelsofunequaldimen
the inevitableincrease intheradiationdosetothe patient(see sion (anisotropic voxels) for MPR is burdened by a serrated
page 175). Nonetheless,smaller voxels withidentical measure appearance of thereconstructed images (Fig. 8.3), which, for
ments in all three dimensions (isotropic voxels) offer a crucial instance,can makeitdifficulttoexcludeafracture(Fig.148.5b).
Fig.8.2MPRfrom isotropicvoxels Fig.8.3MPRfrom anisotropicvoxels
Pitch
Pitch= 1 Pitch=2
Bynow,severaldefinitionsexistforthe pitch,which describesthe
~
rate of table increment per rotation in millimeter and section mn
thickness. Aslowly moving table perrotationgenerates atight
acquisition spiral (Fig. 8.4a).Increasingthetableincrement per
rotation without changing section thickness or rotation speed
createsinterscan spacesoftheacquisitionspiral (Fig.8.4b).
a b
Fig.8.4
Themostlyuseddefinition ofthepitch describesthetabletravel
(feed)pergantryrotation,expressedinmillimeters,and selected Tabletravel/rotation
collimation,alsoexpressedinmillimeters. Pitch =
Collimation
Physical and Technical Fundamentals
9
Feed!rotation 24mm! rotation 24mm
Pitch = e.g.: = -- = 1
Collimation 16x 1.5mm 24mm
Sincetheunits(mm)inthenumeratoranddenominatorcancelout, The new scanners give the examiner the option to select e
the pitch is a dimensionless number. For a while, a so-called craniocaudalextension(z-axis)oftheregiontobeexaminedon e
volumepitch wasstated for multiple-row detector CTscanners, topogramaswellasthe rotationtime, sectioncollimation(thinor
whichrelatesthetablefeedtoasinglesectionratherthan tothe thicksections?)and examinationtime (breath-holdingintervals?).
entirearrayof sectionsalongthez-axis.Forthe examplegiven The software, e.g., "SureView®," calculates the suitablepitch,
above, this means a volume pitch of 24 mm! 1.5 mm = 16. usuallyprovidingvaluesbetween0.5and2.0.
However,thereseems tobeatrendtoreturning tothe original
definition ofthepitch.
SectionCollimation: ResolutionAlong theZ-Axis
Theresolution(alongthebody axis orz-axis)ofthe images can (=z-axis),Theexaminercanlimitthefan-likex-raybeamemitted
alsobeadapted totheparticularclinicalquestionbythechoiceof from the x-ray tube by a collimator, whereby the collimator's
thecollimation.Sectionsbetween5and8mmgenerallyaretotal aperture determineswhetherthefanpassingthroughthecollima
lyadequateforstandardexaminationsoftheabdomen. However, torandcollected bythe detectorunitsbehindthepatientiseither
theexactlocalizationofsmallfracturefragmentsortheevaluation wide(Fig.9.1)ornarrow(Fig.9.2),withthenarrowbeamallowing
ofsubtlepulmonarychangesrequirethinslicesbetween0.5 and abetterspatial resolution along the z-axisof the patient. The
2mm.Whatdeterminesthesectionthickness? collimatorcannotonlybeplaced nexttothex-raytube,butalsoin
The term collimation describes how thin orthick the acquired frontofthedetectors,i.e.,"behind"thepatientasseenfromthe
slicescan bepreselected alongthelongitudinalaxisofthepatient x-raysource.
_ x-raytube _ x-raytube
Collimator Q" Collimator ?-
"[ >-
1,1\\ "(" J,t,\
I'll" I I --..1,",1,.\1...- I
I I I \ \
/I II II \\ \\ I"":I1'.~\\,
I J I \ \ ,I:.II:1\\
I , I \ \
, ,II II II \\ \\ , ,II :I ,II •~ \,
J I I \ '\ ,'::,,I ,:';I ;
I :I I \: \\ ~ I ,, I ,, \\
::t/ :tl: :L'\ I I \ ~
::=::t11~...
, , , ,
I Collimator I,, ! ! II \1 I ! -.~.-..1: ! ,1.....-- I
,
II 1 Ir c' . . ,ijB
• • • •
z-axis z-axrs
Fig,9.1 Widesectioncollimation Fig.9.2Narrowsectioncollimation
Dependingonthewidthofcollimator'saperture,theunitswithonly petrous bone with about 0.5 mm sections to detect delicate
onedetectorrowbehindthe patient(singlesection) can generate fracture linesthrough the cranialbaseorauditoryossicles inthe
sectionswithawidthof10mm,8mm,5mmoreven1mm.ACT tympaniccavity (seepages46- 49). Forthe liver,however, the
examinationobtainedwithverythinsectionsisalsocalled ahigh examination is dominated by the contrast resolution since the
resolution CT(HRCT) and,ifthesectionsareatthesub-millimeter question here is the detectabilityof hepatic metastases (here
level,ultrahighresolution CT(UHRCn.TheUHRCTisused forthe somewhatthickersections).
Physical and Technical Fundamentals
10
AdaptiveArrayDesign
Afurtherdevelopmentofthesingle-slicespiraltechnologyisthe Thedetectorrows are notinevitablyequal inwidth.Theadaptive
introduction of the multislice technique, which has not one array designconsistsofdetectorsthatincreaseinwidthfrom the
detectorrowsbutseveraldetectorrows stackedperpendicularto centertotheedge of thedetectorring andconsequentlyallows
thez-axis oppositethe x-ray source. This enables thesimul various combinations of thickness and numbers of acquired
taneousacquisitionofseveralsections. sections.
Adaptive detector design 4-rowunit Variable section thickness
2x0.5mm
4x 1.0mm
, ,, ,
:U0>, , ,///// ,,,, I,,,I,I IIII,IIII, IIIII ,,,,, , 4x2.5mm
ceuos // 5 ,I'2,5Ii-"j;"j ;I"'I\"2'5\\ 5 ,,,, z-axis
// ,, I I I I ', ,
/ " I I I , 2x8.0mm
,
Detect/ors II II II II ,, ~~
I I I I
4x5.0mm !
17mwl I
Resolutionalongthez-axis
+ + + +
adaptabletoclinicalQuestion
Fig.10.1Detectordesignofa4-rowunit,asfoundintheSiemensSensation4
Adaptive detector design 6-rowunit Variable section thickness
Collimator
6x0.5 mm
/ " " I/l1I1l\1I \\ \ -, 6x1.0 mm
e; /.. I I ,,11111\1\\ \ '\
I I I 11/11111\ \\ \ "
---:;~/.~!:;:'~/::'!!:~~~~,.':'~':;:::':JI.'__- - - __:_.
U,L
i l ' . .~ \
.~ /3' 2,ill 1\1\2\ 3 \ z-ans
....~ / J J I "" " ,'I \ \ \ ... 6x2.0mm
/ " / I 1/1JI1'1\ \ \ \ "
.. 11 111111111\\\ \
,
Detect/ors II "I 111/J'I I1I11I1\ I\ \\ \\ II,, ,,,
\ I I '" I I I I I \ \ \
6x3.0mm
Resolutionalongthez-axls
adaptabletoclinicalQuestion
Fig.10.2Detectordesignofa6-rowunit,asfoundintheSiemensEmotion6
Physical and Technical Fundamentals
11
Forinstance,a16-sliceexaminationcanbe performedwith16thinsectionsofahigherresolution (fortheSiemensSensation16,this
means16x0.75mm)orwith16sectionsoftwicethethickness.ForaniliofemoralCTA(seepage 188),itispreferabletoacquireaiong
volumealongthez-axisinasinglerun,ofcoursewithaselected widecollimationof16x1.5mm.
Adaptive detector design16-row unit Variablesection thickness
Collimator x-raytube
_____ /11I '\~ _
~II. ,,,
,f:/, \\~~~"
III/ I \\\ \\,\
~-----A(-If;i ~A-----
/~III '/J/f/rlll'\\\\\~ \\\~\
II ' ,'",1/111111\\\\' \\ \\
> 1111111111l1l1l1\\\~\\\\
o~f-------,1I'•11/•"'/,"."""11"11,1"\\\\\•'.\•"•:.'f:----...
H(1;\. I// 1,5 I IIfII•I'.'IIQ75\•\"\\\\\\ 15''\\\\ z-axis
V~ / / 1. /' I I II III ' \\,\ \\ ' \ \ \
/1 1I IIIIJIII I \\ \\\\ \ \ \\
I I I 1 / 1111,'1111\\\ \\ \ \ \ \ \ 16x1.5mm
I I.' / // / II I I Jl l 1 \\ \\\\ \ \ \ \
Detectors ' 1 1 111 11 111'\\ \ \ \ \ \ \" \ .
I ( 11/ 11111 11 11 \\\\\ \ \ \ \
Resolutionalongthez-axis
adaptabletoclinicalquestion
Fig.l1.1 Detectordesignofa16-rowunit,asfoundintheSiemensSensation16
ThedevelopmentoftheCThardwaredidnotendwith 16slicesandfasterdataacquisitioncanalreadybeachievedwith32-and64-row
scanners.Thetrend tothinnerslicesisassociatedwithhigherpatientexposure toradiation,requiring additional andalreadyintroduced
measuresforexposurereduction(seepages 174-177).
Whenboth liverandpancreasare included,many usersprefera
reducedslicethickness from10mmto3mm toimproveimage
sharpness.This increases,however,the noiselevelbyapproxi
mateiy80%.Thereforeitwouldbenecessarytoemploy80%more
rnAortolengthenthescantime(this increasesthe mAs product)
tomaintain imagequality.
measured
data
ReconstructionAlgorithm
Spiral users havean additional advantage: Inthe spiral image
reconstructionprocess,mostofthedatapointswere notactually tableposition
measuredintheparticularslice being reconstructed(Fig. 11.2). slice
Instead,dataareacquiredoutside thisslice (e)andinterpolated
Fig.11.2 Wide(360°)spiral reconstructionalgorithm
withmoreimportance,or"contibutinn",beingattachedtothedata
located closest tothe slice (X). In other words:The data point
closesttotheslice receivesmore weight,orcounts more,inthe selecting highrnA values,increasethespatial resolution(image
reconstructionofanimageatthedesiredtableposition. sharpness)byreducingslicethickness,andemploypitchtoadjust
the lengthof thespiral range as desired,allwhilereducingthe
This results in an interesting phenomenon. The patient dose patient'sdose!Moreslicescanbeacquiredwithoutincreasingthe
(actually given in mGy) is determined by the mAs per rotation doseorstressingthex-raytube.
dividedbythepitch,and the imagedose isequal tothemAs per Thistechnique isespecially helpfulwhendataare reformattedto
rotationwithoutconsideringthepitch.Ifforinstance150mAs per createother2D views,likesagittal,oblique,coronal,or3Dviews
rotationwithapitchof1.5areemployed,thepatientdose inmGy (MIP,surfaceshaded imaging,seepp.8and 13).
islinearrelatedto100mAs,andtheimagedoseisrelatedto150
mAs.Thereforespiraluserscanimprovecontrastdetectabilityby
Physical and Technical Fundamentals
12
Thedataobtainedatthedetectorchannelarepassedon,profilefor TheInfluenceofkV
profile,tothedetectorelectronicsaselectricsignalscorresponding Whenexamininganatomicregionswithhigherabsorption(e.g.,CT
totheactualx-rayattenuation.Theseelectricsignalsaredigitized ofthehead,shoulders,thoracicorlumbarspine,pelvis,and larger
andthentransmitted tothe image processor. At thisstage,the patients),itisoftenadvisabletousehigherkVlevelsinadditionto,
images are reconstructed bymeans ofthe"pipelineprinciple", orinsteadof, highermA values: when youchoose higherkV,you
consisting of preprocessing, convolution, and back projection arehardeningthex-raybeam.Thusx-rayscanpenetrateanatomic
(Fig.12.1). regions with higher absorption more easily. As a positive side
effect,thelowerenergycomponentsoftheradiationarereduced,
Preprocessingincludes allthe correctionstaken topreparethe whiChisdesirablesince lowenergy x-rays are absorbedby the
measuredscandatafor reconstruction,e.q.,correction fordark patientanddonotcontribute tothe image.Forimagingofinfants
current, dose output, calibration, channel correction, beam orbolustracking,itmay beadvisabletoutilize kV lowerthanthe
hardening,andspacingerrors.Thesecorrectionsarepertormedto standard setting.
furtherminimizetheslightvariationsinherentlyfoundinthetube
anddetectorcomponentsoftheimagingchain. TubeCurrent[mAs]
Thetubecurrent,stated inmilliampere-seconds [mAs],alsohasa
Convolutionisbasicallytheuseofnegativevaluestocorrectfor significanteffectonthe radiationdose deliveredtothe patient.A
smearinginherenttosimpleback projecfion.II, forinstance, a patient with morebody Widthrequires anincrease inthe tube
cylindric water phantom is scanned and reconstructed without currenttoachieveanadequateimagequality.Thus,morecorpulent
convolution,theedges ofthisphantomwillbe extremelyblurry patientsreceivealargerradiationdosethan,forinstance,chiidren
(Fig.12.2a):Whathappenswhenjusteightattenuationprofilesof withamarkedlysmallerbodywidth.
asmall,highlyabsorbentcylindricalobjectare superimposed to Bodyregionswithskeletalstructuresthatabsorborscatterradia
createanimage?Sincethesamepartofthecylinderismeasured tion,suchas shoulder andpelvts,require ahigher tubecurrent
bytwooverlapping projections,astar-shaped imageisproduced than,forinstance,theneck,aslenderabdominaltorsoorthelegs.
instead of what isin reality acylinder. By introducing negative Thisrelationshiphas beenactivelyapplied toradiationprotection
valuesjustbeyondthepositive portionoftheattenuationprofiles, forsome timenow(comparewithpage 177)..
theedgesofthiscylindercanbesharplydepicted (Fig,12.2b).
ScanTime
Back projectioninvolvesthe reassigningofthe convolved scan It isadvantageous toselect a scan time asshort aspossible,
datatoa20imagematrixrepresentingthe section ofthepatient particularlyinabdominal orcheststudieswhere heartmovement
thatisscanned.Thisispertormed profileforprofileforthe entire andperistalsismaydegradeimagequality.OtherCTinvestigations
imagereconstructionprocess.Theimagematrixcanbethoughtof canalsobenefitfromlastscantimesduetodecreasedprobability
asanalogoustoachessboard,consistingoftypically512x512or ofinvoluntarypatientmotion.Ontheotherhand,itmaybe neces
1024 x 1024 picture elements, usually called "pixels". Back sary toselectalongerscan time toprovide sufficient doseorto
projectionresuitsinanexactdensity beingassigned toeach of enable moresamplesformaximalspatialresolution.Some users
thesepixels,whicharethendisplayedasalighterordarkershade mayalso consciously choose longerscantimes tolowerthe mA
ofgrayThelightertheshadeofgray,the higherthedensityofthe settingandthusincreasethe likelihoodoflongerx-raytube life.
tissuewithinthepixel(e.g.,bone).
Simple Back Projection vs. Convolution
~, ---------------------------,,
-+0 -0 -0+0
, ,
Dataacquisitionsystem Preprocessing Convolution Backprojection : Imagedisplay
-----------------------------
b
Fig.12,1 Thepipelineprincipleofimagereconstruction Fig.12.2a Backprojection Fig.12.2b Backprojection
withoutconvolution withconvolution