Vascular Imaging of the Central Nervous System (Physical Principles, Clinical Applications, and Emerging Techniques) || Basic Principles of Computed Tomography Angiography (CTA)

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<ul><li><p>Two Computed Tomography Angiography (CTA)</p></li><li><p>69</p><p>4 Basic Principles of Computed Tomography Angiography (CTA)</p><p>Margareth Kimura1 and Mauricio Castillo21ClinicadeDiagnosticoporImagem(CDPI),RiodeJaneiro,Brazil2DivisionofNeuroradiology,DepartmentofRadiology,UniversityofNorthCarolinaSchoolofMedicine,ChapelHill,NC,USA</p><p>Introduction</p><p>Wheneverapatientneedsimagingevaluationofthebrain,a computed tomography (CT) study is usually the firstexamination. This is due toCTswide availability inmosthospitals, emergency rooms, private imaging clinics, andambulatoryservicesinthemajorityofdevelopedandemerg-ingeconomies. It is inexpensive compared tootherbrain-imaging techniques, quickly performed in most patients,effective in answering the most urgent clinical questions,andflexibleinitsuse.Itsflexibilityisreflectedinitscapacityto give information about not just morphology but alsophysiology.TheintroductionofmultisliceCT(MSCT)inthe1990s expandedCTs limits by allowing fast acquisitionofvolume data with submillimeter spatial resolution. Thesefeatures fulfill thetechnicalconditions forCTangiography(CTA),andthestudyofbloodvesselsandbraincirculationbecamepossibleandisnowconsideredbymanytobethefirstchoiceinseveralclinicalsituations.CTAisnoninvasive,andithaslowercostanddoesnothavetherisksofmorbidity/mortality(1.5%to2%)thatdigitalsubtractionangiography(DSA) has [1]. Compared tomagnetic resonance imaging(MRI),CTArequires less time tobeperformedandallowspatientsincomplicatedhealthsituationstobestudied.CTA identifies most vascular abnormalities and normal</p><p>variants;facilitatesunderstandingofanatomicrelationshipsbetweenboneandbloodvessels[2];permitsdiagnosisoftheeffectsofbluntcerebralvascularinjuries(BCVIs)[3];evalu-ates intracranial vascular disease [4]; characterizes andallows surgicalplanning forarterialvenousmalformations(AVMs) [5]; analyzes, quantifies, and tracks changes ofplaquemorphologyincarotidandvertebralarteries[6,7];andcandepict theocclusionsite,dissection,andcollateralvascularizationinacutebrainstroke[8].CTAsourceimages</p><p>(CTA-SIs) acquired under near-steady-state contrast con-centration provide infarct core size estimations equivalenttothoseseenondiffusion-weightedimages(DWIs),whichisconsideredthegoldstandardmethodforthispurpose[9].</p><p>Fundamentals of the CTA imaging technique</p><p>CTA allows for noninvasive assessment of the vasculaturewiththeuseoftwo-dimensional(2D)andthree-dimensional(3D) postprocessing techniques such asmultiplanar refor-mations (MPRs), maximum-intensity projections (MIPs),volumerendering(VR)andsurfaceshadedrendering(SS)(Figures 4.1, 4.2, 4.3, and 4.4). Volumetric data that areacquiredareisotropic,andthismakespossiblehigh-qualitymultiplanar reconstructions. MSCT units have multipledetector rows, and, depending on the detector configura-tion,morethanoneimageperrevolutionoftheX-raytubecanbecollected.Thethicknessofanindividualdetectorrowrepresents theminimumslice thickness formostscanners.Somescannerswithdoublez-scanningtechnologyofferslicethickness less than the detectorwidth basedon a z-flyingfocalspot.The number of detectors inmodern CT scanners varies</p><p>from64to320.Thetermdetectorconfigurationreferstothenumberofdatachannelsandtheeffectivedetectorrowwidth (e.g., 16 1.25mm). Beam collimation means theX-ray beamwidthor thenumberof data channelsmulti-pliedbytheeffectivedetectorrowwidth.Forexample,ina64-slice MSCT, 64 data channels 1.25mm row width =80mmand64datachannels0.625mmrowwidth=40mm.Beam pitch refers to the table speed per rotation dividedby the total width of the collimated beam (Pitch = Tablemovement per rotation / Detector z Collimation). In</p><p>Vascular Imaging of the Central Nervous System: Physical Principles, Clinical Applications, and Emerging Techniques,FirstEdition.Editedby</p><p>JoanaN.RamalhoandMauricioCastillo.</p><p>2014JohnWiley&amp;Sons,Inc.Published2014byJohnWiley&amp;Sons,Inc.</p></li><li><p>PART Two ComputedTomographyAngiography(CTA)</p><p>70</p><p>ing contrast-induced nephropathy, because even a smallamount of 30mLmay cause it in high-risk patients [10].However, the contrast-induced Nephropathy ConsensusWorking Panel recently suggested guidelines in which acontrastmaterialvolume less than100mL ispreferable inpatients with an estimated glomerular filtration rate lessthan60mL/minper1.73m2[11].RadiationexposureisoneofthedrawbackstoCTA.Radia-</p><p>tion exposure can be reduced by adapting milliampereseconds(mAs)topatientsizeandweight.Automaticexpo-surecontroltechniqueshelpinoptimizingtubecurrentanddosecontrolirrespectiveofotherscanningparameters.Imageprocessingisinitiallydependentonreconstructing</p><p>submillimeterslicesintothickerones.Thisreducesnoiseandnumberofimages,and,duetoisotropicpixelacquisition,itallows for creation ofVRT images andmultiplanar recon-structions images that are then transferred to the picturearchive and communication system (PACS).When in theworkstation,vesselsareanalyzedusingMPR,MIP,orVRT,and the rawdata set is evaluatedagain for any incidentalfindings.ThisinteractivewayofanalyzingCTAoffersmanyadvantagestootherstaticmethods.Itpermitsonetoverifyabnormalities inmanyplanesandperspectives,allowingaprecise diagnosis.Also, it offers thepossibility to calculatevolume, distance, area, zoom, densities by using differentreconstructionalgorithms.Theanalysisofrelationsofbloodvessels to bones and other anatomical structures is veryimportantforsurgicalplanning.</p><p>Artifacts and pitfalls</p><p>Anartifactisanundesiredalterationindataintroducedbyatechniqueorimagedisturbancecreatedbythemultidetector-rowCT(MDCT)technologywhenusedundercertainspecificphysicalconditionsandnormallyinwell-knownanatomicallocations [12] (Figure 4.2). Here, we discuss the mostcommonartifactsthatcanbegeneratedinCTAandattemptto explain how tomanage them in daily clinical practice.Thephysicsfundamentalsandmathematicsunderlyingarti-factcreationarewellexplainedinKalendersbook[13].The beam-hardening effect (Hounsfield bar) is a dark</p><p>bandthatappearsclosetodensestructuressuchasbonesinthe base of the skull, very concentrated iodine contrastmedia, or metallic devices. These artifacts to filtration oflower energy photons and high-energy ones burn thetissuesandcanberemovedusingsoft-tissuealgorithmspro-videdbymostCTsystems[12].Partial-volumeartifactsoccurattheedgesofobjectswith</p><p>very high attenuation coefficients such as bone ormetal.Theycanbeavoidedbyusingthincollimationbecausecon-toursaremorepreciselydelineated.ModernMDCTsystemsprovide scan modes with submillimeter collimation that</p><p>otherwords,thepitchvaluetellsuswhetherdataarebeingcollectedwithagappitch(p&gt;1)orwithoverlap(p</p></li><li><p>CHAPTER 4 BasicPrinciplesofCTA</p><p>71</p><p>Figure 4.2 Different artifacts on computed tomography angiography (CTA). (A to C) MIP images show artifacts of movement (A) in the neck and (B) intracranially, resulting in a double vessel appearance in the latter. (C) Partial-volume effect shows both arteries and veins included in the slice and a </p><p>metallic dental artifact (arrow). (D) Spiral artifacts in a study done with half collimation value, which created starlike artifacts around the posterior bone </p><p>margin of the orbital roofs in the reconstructed images. The increment from 0.9 to 0.8 mm should have been used instead to avoid these artifacts. (E) </p><p>The source image (similar level) in the same study does not have such spiral artifacts. (F) Left-sided metallic aneurysm clip producing metal artifacts in </p><p>source image, obscuring arteries. The artifacts were reduced by using thicker reconstructions and working with proper filters, and on the 3D reformation </p><p>(G) the arteries are clearly seen. (A, B, and F courtesy Gisele Matheus, MD, Charleston, SC.)</p><p>D E</p><p>F G</p><p>A B C</p></li><li><p>PART Two ComputedTomographyAngiography(CTA)</p><p>72</p><p>never the half-collimation values, and to perform thickerreconstructions[12].Motionartifactsproducestreaksandblurredcontours,so</p><p>itisessentialtorestrainpatientsproperlyusingheadholdersand straps. Rotation times as brief as 0.33s per 360 areavailableandcanavoidmotionproblems,butinthecaseofthe CTA technique, slow gantry rotation is necessary toenable high resolution, and motion degradation may beinevitableinsomepatients[12].(SeeTable4.1.)Coneartifactsoccurintheperipheryofimageswhenthe</p><p>numberof slices increases extensively, and shoulder arti-facts happenwhen patients are larger than the scanningfield. For both of these kinds of artifacts, modernMDCTscanners provide, respectively, cone beam reconstructionandextrapolation-typealgorithms toautomatically correctthem.</p><p>should be used in CTA protocols [14]. Spiral artifacts areverysimilartopartial-volumeones.Thedifferencereliesontheirshape,whichseemstorotatearoundtheirsources.Thiscanbemanaged indifferentways,eitherusingz-sharpordoublez-sampling technologyorworkingwith increment,collimation, and reconstruction slice thickness. Tips foravoiding themare touse increments from0.9 to 0.8mm,</p><p>Figure 4.3 CTA (A) with venous contamination on an axial maximum-intensity projection (MIP); and (B) on the 3D view. If venous contamination is present, detailed evaluation of the arteries may be difficult. (Courtesy Gisele Matheus, MD, Charleston, SC.)</p><p>A B</p><p>Figure 4.4 Sinus pericranii. This unusual venous anomaly is characterized by communication between dural sinuses and extracranial </p><p>veins. In this patient, the CTA 3D images (A: inner view, and B: skull </p><p>outer surface view) show the right-side bone defect carrying veins (arrow) </p><p>that anastomose with a scalp vein (arrow). (Courtesy Luciano Farage, MD, </p><p>PhD, Brasilia, Brazil.)</p><p>A B Table 4.1 Technical aspects that help avoid metal artifacts</p><p>Thinnest collimation Avoid partial-volume artifacts.</p><p>Smooth-reconstruction algorithms Reduce streaks.</p><p>Patient position and gantry tilt Exclude metallic objects from </p><p>the scanner area.</p><p>Change in scan plane orientation* Reduce artifacts around the </p><p>implant.</p><p>* Perpendicular to the major axis of the metallic device.</p></li><li><p>CHAPTER 4 BasicPrinciplesofCTA</p><p>73</p><p>rior head can bemissed. The direction of the scanning isfromcaudaltocephalic.Slicethicknessisdirectlyrelatedtoresolution, meaning that slices will range normally from1mmtothethinnestoptionavailable.Gantryrotationcanvary, depending on the scanner being used, from 0.4s to0.7s in the neck where larger and transversely orientedvessels are present. However, the brain contains smallvessels,andmanyareoriented in-planeandarebestana-lyzedusing1.0s scans. Pitch also is important in termsofimage resolution. Inorder tomaintain a balance betweenresolutionandcoverage,thepitchshouldbekeptatabout1.Scandelaycanbechosen,andthearbitrarydelayforthebrainisusually18sandfortheneck15s[12,14,15].Thesevaluesareacceptableforscandurationsof515sandinjec-tiondurationsof1520sinpatientswithnormalcirculation.However, scan delay can vary significantly depending onindividualphysiologicalorpathologicalconditions.Adjustedscan delay uses bolus tracking or automatic triggeringmethodstomeasurecontrastmaterialarrivaltime.ForCTAof theneckandbrain, respectively, the aortic archwitha50100HUtriggerandthemid-ICAwith100HUdeterminea reliable basis for beginning the scan [1416]. Contrastopacificationofbloodvessels isoneofthemost importantandessentialaspectsofCTA.CTAattenuationof250350HUis desirable in routine studies [15]. Factors involved incontrast-medium enhancement are high-concentrationcontrast, a higher intra-venous injection rate, injectionduration, injectionpattern,viscosity, and salineflush.Thevolume of contrast necessary is basically governed by thespeedof thescannerand/orscanduration.However,con-trast concentration required forCTAnormallyvaries from350to370mgofiodinepermilliliter.Ingeneral,forCTAoftheneck,a100mldoseisrequiredforfour-slice,eight-slice,or16-slicescannersanda60mldosefor64-slicescanners;andforbrainCTA,adoseof75mlisrequiredforfour-sliceandeight-slicescanners,and50mlfor16-sliceand64-slicescanners,butcontrastvolumesvary slightlydependingondifferentprotocols[14].Thesizeofthepatientcandecreasethevolumeofcontrastneededorincreaseitdependingonweight.OnthebasisofresultsofMonyeet al.[15],acon-sideration should be made to adjust for weight. Theyobservedthatuseof60mLofcontrastmaterialfollowedbya 40mL saline bolus chaser in patientsweighing 75kg orless, and use of 80mL of contrastmaterial followed by a40mL saline bolus chaser in patientsweighingmore than75kg,resultedinminimumdesirableattenuationsofmorethan 250 and 200HU, respectively. Viscosity of the high-concentrationcontrastmediacandelayitsdelivery,turningenhancement weaker [1719]. This characteristic may bemanagedbyusingaphysicalfactor:temperature.Itispos-sible to reduce viscosity by elevating the temperature ofcontrastmedia[20,21].Warming thecontrastmediumtobody level temperature (35C) improves patient toleranceandallowsforbetterenhancement[22].Asalineflushthat</p><p>Pitfalls</p><p>SomecommonpitfallsthatingeneralcanbepresentinCTAsarelistedinthischapter,andsomespecificonesrelatedtospecificpathologicconditionsarealsodiscussed.Swallowing,arterialpulsations,artifacts,andpoorarterial</p><p>enhancement due to suboptimal timing of the scanningcanobscureormimicinjuries.Thepresenceofoverlappingvenousstructuresandtortuousvesselsconstitutesapossiblepitfallwhentheyareseenonasingleimageandonasingleplane.Inordertodifferentiatewhetherasmallvesselrep-resents an artery or a vein, it is helpful touse interactivevisualization(Figure4.3).Whenevaluatingpossibleocclusioninaninternalcarotid</p><p>artery(ICA)withCTA,itisimportanttoavoidapitfallthatcan happen if one acquires images too fast and does notvisualizetheocclusion.Itisnecessarytobeawarethatcol-lateralsfromanexternalcarotidarterymayreconstitutethedistal ICA, giving the initial impression that it is patent.Another aspect that should be observed when evaluatingocclusionwithCTA is that it isnecessary touseanarrowwindowtodistinguishbetweenthepossibilityofhigh-gradestenosisandanocclusion.Inthepresenceofcalcifiedplaque,MIPreformationismoreaccuratethanVRTfordefiningthepercentageofstenosis[15].Regarding the study of CNS vasculopathy, the involve-</p><p>mentoflargervesselscanbedetectedbutsmall-vesselvas-culitismaynot.Insuchcases,DSAoffersmoreconspicuity.CTAcanbeusedtoevaluateAVMs;nonetheless,itmaybedifficult to seeall feedingarteries and the relativeflow ineachbranch.</p><p>Imaging protocols</p><p>OneofthekeyadvancesofMSCThasbeentubetechnology,whichimprovesheatunitdissipation,andtheuseof120kVp,which improves relative opacification of contrast media[12].ThetubeloadrequisitionvariesinmAs,anditdependson the gantry speed, patient weight and size, and regionscanned.ForCTAofthebrain,imagequalityneedstobesethigher with lower noise. Neck studies permit a slightlyhighernoisesetting.NeckCTAissetupfromtheaorticarchtotheskullbase,</p><p>to1cmabovethetopofthedorsumsella,ortothevertexof the skull if informationabout intracranial circulation isdesired.CTAofthebrainbeginsattheC2orC1levelandgoesto1cmabovethedorsumsellaortothevertex.Nor-mally, a field of view (FOV) of 20cm is enough. The useofasmallerFOVprovidesbetterresolution,and10cmfortheneckcanbettersuit thisanatomical location[15].Thepatient position is neutral inside the unit, and scans areacquiredwithnogantrytiltorthevasculatureoftheante-</p></li><li><p>PART Two ComputedTomograph...</p></li></ul>

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