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Cardiac misalignment recognition and correction of artifacts using PET/CT systems

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PET/CT Marketing Philips Healthcare
PET/CT Clinical Science Philips Healthcare

Several factors affect the quality and accuracy of cardiac studies acquired using PET/CT scanners. One key factor is possible misalignment between the CT study and the PET study, both acquired as part of a single PET/CT study. The misalignment is due to either gross patient motion, or patient breathing between the CT acquisition and the PET acquisition. Due to the above reasons, PET myocardial perfusion data (Rb-82 or NH-3) acquired using a PET/CT scanner is frequently misaligned with a CT attenuation correction data derived from the CT study1-6. This misalignment can create artifacts on the attenuation-corrected PET images, which primarily manifest themselves as false positive defects in the anterior and antero-lateral segments7.

Figure 1 Example of mis-registered PET/CT perfusion study.  (a) Misaligned CT and PET.  (b) CT and PET aligned correctly.  Under-correction for attenuation in antero-apical and antero-lateral segments produced could reduce apparent uptake and be falsely assessed as a perfusion defect.
Figure 1
Example of mis-registered PET/CT perfusion study. (a) Misaligned CT and PET. (b) CT and PET aligned correctly. Under-correction for attenuation in antero-apical and antero-lateral segments produced could reduce apparent uptake and be falsely assessed as a perfusion defect.

 

Sources of CT and PET data misalignment include:

  1. CT-based attenuation correction is performed non-simultaneously resulting in a potential patient motion.
  2. Differences in scanning time between PET and CT: the latter may be acquired in a few seconds, covering only a fraction of the respiratory cycle, whereas PET shows an image averaged over several respiratory cycles.
  3. Changes of the heart location due to pharmacologic stress agents.

 

PET/CT mis-registration can be minimized during acquisition with optimized protocols. One can maximize patient comfort with arms-down acquisition and/or short data acquisition protocols. A possible approach is to modify the CT acquisition protocol to reduce its temporal resolution to match that of the PET examination. Proposed implementations include using an ultra slow CT acquisition8,9 or respiration-averaged CT10.

In the GEMINI PET/CT system9, the CT acquisition protocol parameters are set so that the acquisition is slow enough to capture multiple respiratory cycles to allow respiratory motion to be time averaged in the CT scan. This causes the CT to more closely match the PET acquisition, which is acquired over an extended period of time and over multiple respiratory cycles. If the CT acquisition results in data set at the end inspiration, attenuation correction of the PET data may be compromised by this CT image as patients normally spend more time in expiration than in inspiration10.

Figure 2 (a) An example of the result of mis-registration when a part of the lateral walls of myocardium in the PET data are attenuation corrected with the higher attenuation of the heart tissues, when (b) they should have been corrected with the lower attenuation of the lung.
Figure 2
(a) An example of the result of mis-registration when a part of the lateral walls of myocardium in the PET data are attenuation corrected with the higher attenuation of the heart tissues, when (b) they should have been corrected with the lower attenuation of the lung.

 

The CT acquisition protocol approaches can indeed minimize the breathing induced misalignment at the expense of increased radiation dose for the patient or longer acquisition times. However, misalignment resulting from other sources (e.g., patient motion or changes of the heart location due to pharmacologic stress agents) will still not be corrected. Another possible strategy is to perform a second low-dose CT. However, this approach may not be always convenient from the workflow perspective and it slightly increases the radiation dose received by the patient (an additional CT scan). Moreover, the second CT scan could potentially present misalignment as well7.

Therefore, due to the issues described above, the images should be always analyzed for mis-registration between the emission and transmission data. This is best accomplished using an alpha blend image of the transmission and emission reconstructed data. If the misalignment is detected, image registration appears to be a promising solution.

 

The CT data can be realigned manually or automatically with reconstructed PET data and a new PET data reconstruction can be performed using this re-registered attenuation map. Recently published data suggest that manual registration is a preferred option, despite the inconvenience of being time-consuming and observer dependent. Manual realignment resulted in a change in the defect size of >10% of the left ventricle in 6 of 28 studies (21.4%); in 5 of the studies, this resulted in the disappearance of large apparent perfusion defects (15%-46% of the left ventricle), which were fully due to PET-CT misregistration7.

 

Described above quality control and data re-processing steps are implemented on the GEMINI PET/CT systems10,11:

  1. Mis-registration between the emission and transmission data can be visualized using PET/CT viewer or Syntegra (both applications include alpha blending capabilities)
  2. Manual realignment of the CT data can be performed in all 3 spatial directions (as translation between both datasets)
  3. PET data can be re-reconstructed using a registered CT data set as a new attenuation correction data

References

  1. Koepfli P, Hany TF, Wyss CA, et al., CT attenuation correction for myocardial perfusion quantification using a PET/CT hybrid scanner. J Nucl Med. 2004; 45:537-542.
  2. Osman MM, Cohade C, Nakamoto Y, Marshall LT, Leal JP, Wahl RL, Clinically significant inaccurate localization of lesions with PET/CT: Frequency in 300 patients. J Nucl Med. 2003;44:240-243.
  3. McLeish K, Hill DLG, Atkinson D, Blackall JM, Razavi R. A study of the motion and deformation of the heart due to respiration. IEEE Trans Med Imaging. 2002;21:1142-1150.
  4. Loghin C, Sdringola S, Gould KL, Common artifacts in PET myocardial perfusion images due to attenuation-emission misregistration: Clinical significance, causes, and solutions. J Nucl Med. 2004;45:1029-1039.
  5. McCord ME, Bacharach SL, Bonow RO, Dilsizian V, Cuocolo A, Freedman N., Misalignment between PET transmission and emission scans: its effect on myocardial imaging. J Nucl Med. 1992;33:1209-1214.
  6. Matsunari I, Boning G, Ziegler SI, et al., Effects of misalignment between transmission and emission scans on attenuation-corrected cardiac SPECT. J Nucl Med. 1998;39:411- 416.
  7. Axel Martinez-Moller, Michael Souvatzoglou1, et al., Artifacts from misaligned CT in cardiac perfusion PET/CT studies: frequency, effects, and potential solutions. J Nucl Med 2007; 48:188-193.
  8. Brunken RC, DiFilippo FP, Bybel B, Neumann DR, Kaczur T, White RD., Clinical evaluation of cardiac PET attenuation correction using ''fast'' and ''slow'' CT images [abstract]. J Nucl Med. 2004;45(suppl):120P.
  9. Philips PET Customer Support, GEMINI TF - Instructions for Use. Published by Philips Medical Systems (Cleveland), Inc.
  10. Pan T, Mawlawi O, Nehmeh SA, et al., Attenuation correction of PET images with respiration-averaged CT images in PET/CT. J Nucl Med. 2005;46:1481-1487.
  11. Philips PET Customer Support, Improving PET cardiac studies: Ensuring alignment of GEMINI TF PET/CT cardiac studies. Philips Technical Bulletin, May 2007.


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Gemini GXL / LXL systems, Gemini TF 16, Gemini TF 64
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