Image Quality Recovery in Cardiac CT Angiography: How to edit an ECG trace to improve image quality

Barbara L. Kruger, R.T.(R)(CT), Fremont D. Lee, R.T.(R), Natalie N. Braun, B.S., Michael R. Bruesewitz, R.T.(R), Andrew N. Primak, Ph.D., Eric E. Williamson, M.D., Cynthia H. McCollough, Ph.D. CT Clinical Innovation Center, Department of Radiology, Mayo Clinic, Rochester, MN

Background

© 2007 Mayo Foundation for Medical Education and Research

Cardiac Displacement ArtifactsVariations in patient heart rate due to arrhythmia or ectopic beats can cause displacement artifacts. Additionally, missed orextraneous R-waves (as detected by the scanner) can result in an incorrect phase being used for image reconstruction. Thisprinciple is illustrated in Figure 2.

Figure 2: (a) In cardiac CT, different sections of anatomy along the Z axis are imaged during successive cardiac cycles (represented by the colored bars in each of thefour R-R intervals shown). Displacement artifacts occur when one R-R interval is shorter or longer than adjacent intervals (cycle with green bar). The reconstructed datais then at a different phase and discontiguous with the adjacent anatomy. (b) Patient exam demonstrating this principle.

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• Cardiac CT imaging offers a robust, non-invasivemethod of visualizing the coronary arteries, as wellas the morphology and function of the heart.

• Patient preparation is vital in order to avoid imageartifacts. - Proper placement and connection of the ECG leads

are needed to provide a stable, low-noise ECG sig-nal. On the evaluated system (SOMATOMDefinition, Siemens Medical Solutions), severalimportant pieces of information are conveyed tothe operator using the ECG data strip, as shown inFigure 1.

- Prior to scanning, it is essential to coach the patientregarding correct breathing techniques and theneed to refrain from moving or swallowing duringthe scan.

• Artifacts can be classified into two categories: cardiacartifacts and non-cardiac artifacts. Displacement artifacts occur when contiguous sections of anatomy are displaced withrespect to each other, making the tissue appear discontinuous. - Cardiac sources of artifacts include heart rate variability or arrhythmias and errors in the detection of the R-wave due to

a poor quality ECG signal.- Non-cardiac sources of artifacts include patient breathing, patient movement (voluntary and involuntary), and implant-

ed metal medical devices.

Figure 1: The patient’s ECG is recorded by the scanner for use in retrospectively-gatedimage reconstructions. The important information conveyed to the operator is labeledhere.

Heart Rate (bpm) for each R-R Interval

Bullet Point ShowingDetected R-wave

ReconstructedPhase

Scan Time in Seconds

Pink Line:X-ray is On

Purple Region: Scanner Operating at Prescribed

Tube Current

White Region: Scanner Operating at Reduced

Tube Current

Relative Phase (%) orAbsolute Phase (ms)

Non-cardiac Artifacts• Artifacts caused by non-cardiac motion include those caused by breathing, swallowing, or internal motion (bearing

down, Valsalva maneuver). Motion external to the heart may cause displacement artifacts in the cardiac anatomy, whichunfortunately, cannot be corrected with reconstructions at alternate phases of the cardiac cycle or by ECG editing. Thepresence or absence of non-cardiac motion can be determined by:- Looking at the sternum or diaphragm for displacement artifacts outside of the heart region using a sagittal or coronal

view (Figure 3). - Examining the lung tissue for motion blurring using a lung window setting (Figure 4).

• Other sources of non-cardiac artifacts may include metal from surgical clips, artificial valves, pacemakers, defibrillators,or other medical implants, which can cause streak artifacts (Figure 5).

Figure 3: Discontinuities of the sternum or skindemonstrate that extra-cardiac motion hasoccurred. The interpreting physician should reviewthe images to determine whether the extra-cardiacmotion sufficiently compromises cardiac imagequality, such that repeating the exam is warranted.These discontinuities can be identified most easilyusing the sagittal plane.

Figure 4: Lung motion can be recognized by the blurring inthe lung parenchyma, which is best seen using the lung win-dow setting. Some pericardial motion blurring is common.Diaphragmatic motion, however, can cause generalized motionof the lungs and heart that results in blurring of the mid andperipheral lung tissue and displacement artifacts in the heart.

Figure 5: Streak artifacts due tothe presence of metallic cardiacpacing leads. Similar artifacts arecaused by mediastinal clips postcardiac by-pass graft surgery.

Purpose• Learn how variations in patient heart rate or errors in R-wave detection can cause displacement artifacts in

retrospectively-gated cardiac CT. • Provide specific strategies for editing the patient ECG trace in order to decrease displacement artifacts. • Show examples of the suggested processes and the resultant improvement in image quality.

Manual selection of the best phase for recon-struction is a cumbersome process.Retrospective ECG-gated image reconstruc-tion allows the reconstruction of images atany point in the cardiac cycle, providing theopportunity to seek out and use the phasewith the least motion. This is referred to as the“best phase”. However, the best phase canvary from patient to patient, even for similarheart rates. Even for low heart rates, occasion-ally a systolic phase (e.g. 30-40%) yields betterimage quality than a diastolic phase (e.g. 60-80%), although most often end-diastole is bet-ter for heart rates < 70 bpm and end-systole isbetter for heart rates > 70 bpm. Thus, theoperator must determine for each patientwhich is the optimal reconstruction phase.Further, it is not uncommon that the rightcoronary artery looks better at a differentphase (typically earlier) than the left anteriordescending and left circumflex coronary arter-ies, due to the complex motion of the heart.

To determine the best phase, the operator typ-ically selects one anatomic level of interest(e.g. right coronary and circumflex arteriesboth seen, at about mid heart) and performs areconstruction at that level throughout theentire cardiac cycle, typically in 5% incre-ments (e.g. 0, 5, 10% … 95%). This “preview”series is then viewed and the best phaseselected. This manual process is time consum-ing, subjective, and is based on only oneanatomic level of the heart.

An automated technique is available on theevaluated system that reconstructs the entireheart in 2% intervals and computes a figure ofmerit for each phase that assesses the relativemotion at each phase. The algorithm thenselects and reconstructs at full resolution thebest systolic and/or diastolic phases for theoperator (referred to as Best Diastole™ andBest Systole™).

In general, as heart rate increases, the dura-tion of diastole decreases, such that end-systolic images are superior for heart ratesabove 75-85 bpm. Figure 6 illustrates an ECG strip where (a) Best Diastole™ and (b) Best Systole™ weremost appropriate. Table 1 provides manufacturer recommendations for choosing which “best phase” toreconstruct.

The reconstruction phase can be prescribed in either relative terms (percent of the R-R interval) orabsolute terms (a specific time interval, in ms, before or after an R-wave), as illustrated in Figure 7.

Figure 6: Examples of high quality ECG strips where the heart rate wasrelatively constant and the R-waves easily identified. (a) Mean heart rate of51 bpm. The reconstruction phase is most appropriate in the quiescentend-diastolic phase. (b) Mean heart rate of 109 bpm. The reconstructionphase is most appropriate at end-systole due to the decreased length ofdiastole. Note that 3 R-waves were detected by the system (blue arrows)and marked (blue bullets) slightly earlier than the actual R-wave (redarrows). If image artifacts are apparent, these bullets should be manuallymoved to over the respective R-waves. The apparent heart rate variationwould be decreased by proper placement of the R-wave “bullet” identifiers.

A Best Diastole

B Best Systole

Figure 7: (a) A relative recon-struction phase is expressed asa percentage of the R-R interval.For a 1000 ms (60 bpm) R-Rinterval, a 70% reconstructionphase will occur at 700 ms afterthe detected R-wave, which isapproximately end-diastole. Ifthe heart rate increases to 90bpm (667 ms R-R interval), the70% phase will correspond to467 ms after the R-wave, whichwill be close to end-systole. To avoid this inconsistency in reconstructedphase when the heart rate varies considerably, the reconstruction phasecan be prescribed in absolute terms, as the time (ms) either after (b) adetected R-wave (absolute forward phase) or before (c) the next detectedR-wave (absolute reverse phase).

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Heart Rate (bpm) Manufacturer Recommended Reconstruction Phase

<75 Only Best Diastole™

75<HR<85 Both Best Diastole™ and Systole™

>85 Only Best Diastole™

Table 1: Suggested reconstruction phases as a function of heart rate.The Best Diastole™ and Best Systole™ images are automatically selectedusing a motion assessment algorithm.

Selecting the “Best” Reconstruction Phase

Eliminating Undesirable SyncsWhen the system detects an R-wave, a bluebullet is placed on the ECG strip to denote thesynchronization point (R-wave) for that car-diac cycle. These are referred to on the userinterface as “syncs”. Occasionally, noise onthe ECG signal is considered by the system tobe an R-wave. These “extra syncs” do not rep-resent actual R-waves and an extra image willbe reconstructed that is not really at thedesired reconstruction phase. This results incardiac displacement artifacts, as seen inFigure 8a.

To remedy this situation, the extraneous synccan be either deleted or disabled, where dis-abling a sync instructs the reconstructionalgorithm to ignore that sync. To disable async, the operator must right click on thedesired R-wave bullet and select “DisableSync” (Figure 8a). It is better to disable syncsthan to delete them because, once a sync isdeleted, it can only be restored by selectingthe original ECG box. This restores the editedECG to the originally recorded ECG and anyother modifications made to the syncs arelost. In contrast, disabled syncs can be easilyenabled by right clicking on the blue bulletand selecting “Enable Sync”.

After the extraneous sync is disabled, thereconstruction window associated with it isturned a darker shade of blue to denote thatimages were not made there. The new recon-structed images demonstrate that the dis-placement artifact has been eliminated (Figure 8b).

Figure 8: (a) Displacement artifacts (arrows) due to the system mistak-ing noise on the ECG signal as an R-wave. The incorrect syncs (R-waves) aredisabled, resulting in complete recovery of image quality (b).

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Adding New SyncsOccasionally there is insufficientdata to create acceptable imagesdue to the large distancebetween the detected R-waves(Figure 9a). Adding new syncs atappropriate locations can allevi-ate this artifact (Figure 9b). Toadd new syncs, the operatormust right click on the numericalvalue of the heart rate per R-Rinterval on the ECG strip andselect “Insert Sync” (Fig. 9a).

Figure 9: (a) Image quality is severely degraded (arrows) when an insuffi-cient number of R-waves is detected, forcing the system to interpolate databeyond acceptable limits. (b) By inserting syncs at the undetected R-waves,the image quality is completely recovered.

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Moving Existing SyncsAfter inserting new syncs, it isoften necessary to move thelocation of the R-wave bullets tobe positioned directly above theR-wave peak.

Additionally, image reconstruc-tion at different positions in thecardiac cycle (different phases)is possible by moving the R-wave bullet. For relative (%)reconstructions, moving a syncwill affect the reconstructionposition both behind and infront of the moved R-wave. Forabsolute (ms) reconstructions,only the prescribed reconstruc-tion is affected (for forwardphase this would be after themoved R-wave, or for reversephase this would be before themoved R-wave).

In Figure 10a, degradation of aportion of the image occurs dueto insufficient data in the corre-sponding R-R interval. Thisoccurred when the ECG signalwas briefly degraded, invertingthe R-wave, which the auto-detection algorithm thenmissed. Image quality isrestored (Figure 10b) by 1) adding one sync and 2) moving it over the position ofone of the inverted R-waves.

Figure 10: (a) Image quality is degraded by a momentary loss of a goodECG signal. (b) By first inserting a sync and then moving it to an appropri-ate location, image quality is completely restored.

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A Systematic Approach to Image Quality Recovery • Using the described principles of ECG sync editing, we developed and evaluated a systematic approach

to evaluating and resolving cardiac artifacts. The raw projection and ECG data were saved for 40 retro-spectively ECG-gated patient exams where cardiac artifacts were noted by the technologist performingthe examination. Thirty-three of these exams were selected from 242 contiguous, outpatient, ECG-gatedexaminations having one of thefollowing indications: pulmonaryvein stenosis, coronary artery CTangiography, or ECG-gated thorax.Thus, 14% (33/242) of cardiac-gated exams required some evalu-ation for the presence of artifacts.An additional 7 cases were con-tributed from a hospital-basedscanner, where cumulative examcounts were unavailable.

• To assist operators in performingthe necessary ECG editing, a sys-tematic approach was developedby two experienced cardiac CTtechnologists, with assistance froma medical physicist and cardiovas-cular radiologist. The proposedapproach is shown in Figure 11.

• To evaluate the efficacy of the sug-gested approach, two experiencedcardiac CT technologists restoredthe data to the scanner where thedata were acquired (SOMATOMDefinition, Siemens MedicalSolutions) and reconstructed theimages using the original ECGdata in order to document theoriginally observed artifact. Theythen followed the approach shown in Figure 11. Multi-planar reformatted images were reconstructed toevaluate the cardiac region in an oblique sagittal projection and to document the effect of each step in thesuggested approach.

• Clinically, once a satisfactoryreconstruction is obtained, theoperator would not perform addi-tional ECG manipulation or imagereconstructions. For the purpose ofthis evaluation, each step of theproposed workflow was per-formed, regardless of the successof the previous steps. The firstaction in the process that yieldedan acceptable image was recorded,as was the action that yielded thebest image quality (Figure 12a).Finally, the preferred reconstruc-tion phase was recorded for eachexam and evaluated as a functionof mean heart rate (Figure 12b).Motion of the sternum and lungparenchyma was noted on 6/40(15%) and 8/40 (20%) of exams,respectively. In 3/40 exams (7.5%),gross patient motion (breathing)prevented image quality recovery.

Figure 11: Flowchart of systematic approach to image quality recovery.

Heart Rate

>85 <75

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*If artifact is not resolved, see “When All Else Fails.”

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When All Else Fails• If none of the suggested ECG editing strategies resolve the artifact, or if different phases are required to

adequately visualize different arteries, some additional actions can be performed:- Detailed ECG information may be viewed to assist the operator in knowing precisely where to dis-

able, add or move a sync. This feature is enabled using the HeartView configuration menu.- Additional relative or absolute reconstruction phases may be tried. - The physician may use images from different phases for evaluating different coronary arteries. For

example, a systolic phase may be needed to adequately visualize the right coronary artery but maynot be acceptable for the left coronary arteries, which require a diastolic phase. In this case, recon-struct both phases so that the interpreting physician may use the best phase for each specific coronaryartery.

• Unfortunately, there are exams that simply cannot be made diagnostic, typically due to substantial tho-racic/diaphramatic motion, an extremely poor quality ECG signal, or a very irregular heart rate.Improved temporal resolution and/or heart rate control can reduce the frequency of this situation, ascan diligent patient education/coaching before the scan with regard to the importance of holding stilland maintaining a non-strained breathold. In the event of a non-recoverable exam, the responsible physician should be consulted for a decision regarding repeating the exam.

References• Leschka S, Scheffel H, Desbiolles L, et al. Image Quality and Reconstruction Intervals of Dual-Source CT Coronary

Angiography: Recommendations for ECG-Pulsing Windowing. Invest Radiol 2007; 42(8):543-549.

• Ohnesorge BM, Becker CR, Flohr TG, Reiser MF. Multi-slice CT in Cardiac Imaging. Springer-Verlag, Berlin, 2002.