coach factory online burberry outlet mulberry outlet cheap oakley sunglasses
  Biomedical Imaging and Intervention Journal Follow BIIJ on Twitter Find BIIJ on Facebook Subscribe to BIIJ RSS feed

Home Current issue Submit a paper Contact us
 
 
 

Editorial Board
Instruction for Authors
Editorial Workflow
Reviewers
Events
Recorded Presentations
Remote Education

Search


Web biij.org
Home > Contents

Article


Biomed Imaging Interv J 2006; 2(4):e60
doi: 10.2349/biij.2.4.e60
© 2006 Biomedical Imaging and Intervention Journal


PDF version Technical Report

Artefacts of PET/CT images

C Pettinato*,1, MS, C Nanni2, MD, M Farsad2, MD, P Castellucci2, MD, A Sarnelli1, PhD, S Civollani1, Tech, R Franchi2, MD, S Fanti2, MD, M Marengo2, MS, C Bergamini1, MS

1 Health Physics Department, Azienda Ospedaliero Universitaria S. Orsola Malpighi, Bologna, Italy
2 Nuclear Medicine Division, Azienda Ospedaliero Universitaria S. Orsola Malpighi, Bologna, Italy



Back to top

ABSTRACT

Positron emission tomography (PET) is a non-invasive imaging modality, which is clinically widely used both for diagnosis and accessing therapy response in oncology, cardiology and neurology.

Fusing PET and CT images in a single dataset would be useful for physicians who could read the functional and the anatomical aspects of a disease in a single shot.

The use of fusion software has been replaced in the last few years by integrated PET/CT systems, which combine a PET and a CT scanner in the same gantry. CT images have the double function to correct PET images for attenuation and can fuse with PET for a better visualization and localization of lesions. The use of CT for attenuation correction yields several advantages in terms of accuracy and patient comfort, but can also introduce several artefacts on PET-corrected images.

PET/CT image artefacts are due primarily to metallic implants, respiratory motion, use of contrast media and image truncation. This paper reviews different types artefacts and their correction methods.

PET/CT improves image quality and image accuracy. However, to avoid possible pitfalls the simultaneous display of both Computed Tomography Attenuation Corrected (CTAC) and non corrected PET images, side by side with CT images is strongly recommended. © 2006 Biomedical Imaging and Intervention Journal. All rights reserved.

Keywords: PET/CT, artefacts, attenuation correction



Back to top

INTRODUCTION

Positron emission tomography (PET) is a non-invasive imaging modality, which is clinically widely used both for diagnosis and accessing therapy response in oncology, cardiology and neurology [1-3].

Because of its very high sensitivity it is an excellent tool to recognise malignant nodules and lesions earlier than their anatomical compromising. The lack of anatomic information in PET images can be compensated by other complementary imaging techniques such as CT or MRI read side by side. Several methods have been developed to register and fuse PET and CT data acquired on separate systems [4-5]. The major problems related with image fusion are the different formats of images of the two datasets and the need to use external markers, visible with both modalities, to be sure to have a good match among corresponding images.

The ideal condition for image fusion is to have the two datasets acquired closely sequentially on the same system [6-7].

It has been well established that the fusion of PET and CT provides information exceeding the sum derivable from the two modalities treated separately [8-17].

The advantages of PET/CT over PET are:

1.        faster and less noisy attenuation correction maps

2.        better diagnostic accuracy especially in disease staging

3.        better ability to identify and localise lesions

4.        shorter transmission acquisition time with a consequent better comfort for the patient and less probability of patient motion.

This paper describes all different artefacts that can be caused by the use of a combined PET/CT system and that can affect the accuracy of PET-corrected images [18-19].



Back to top

PET/CT scanner design

A PET/CT scanner combines PET and CT technology in the same gantry. The patient, lying on the table, undergoes CT and the PET scan sequentially.

The first PET/CT system, developed and installed at the University of Pittsburg, was based on the combination of a spiral CT scan (Somatom AR.SP) with a rotating partial ring PET scanner (ECAT ART) [20].

In all modern commercial systems [21-24] the CT is on the front and the PET is on the back: the patient first undergoes the CT scan and then the PET scan (Figure 1).

No limitations exist on the type of systems employed: the CT can be single or multislice, working in either axial or helical mode while the PET system can use a different crystal material (BGO, LSO, LYSO, GSO). Some PET systems can acquire in either 2D or 3D mode whereas others can only acquire in 3D mode.



Back to top

Acquisition protocols

A PET/CT acquisition protocol has three steps: a) SCOUT acquisition for axial Field of View (FOV) definition, b) CT acquisition, and c) PET acquisition.

Because CT is used mostly to fuse anatomical information to functional PET images and to correct attenuation, low-dose CT protocols can be adopted as a compromise between acceptable image quality and absorbed dose to the patient. This kind of CT images cannot be used on their own for diagnosis.

The common CT protocol uses 100-140 kV and 60-100 mA: the nuclear medicine technologist should modify these values according to the weight of the patient [25]. Additional conservative parameters should be selected for paediatric studies.

The duration of PET scan is about 3-5 minutes/bed position and depends on different factors such as the acquisition mode (2D or 3D), the injected dose and the time between the administration of the activity and the acquisition start time. Because PET image matrix sise is 128x128 and CT is 512x512, CT data need to be rebinned to perform image registration and attenuation correction.



Back to top

Attenuation correction

In conventional PET, attenuation correction is done using transmission scans acquired with external radioactive sources: most systems use 68Ge rods. The transmission acquisition time varies from 2 to 4 minutes/bed position depending on the correction method used (segmented versus measured) [26-27].

The use of CT transmission maps for attenuation correction reduces transmission acquisition time to 1-2 minutes, including SCOUT and whole body CT scans, together with increased accuracy of attenuation coefficients.

Because of the different energy of CT photons compared with the emission photons (about 80 KeV versus 511 KeV) all commercial systems have a scaling algorithm to convert the correction factors from CT to PET [28-29].

All photon attenuation information embedded in the CT data is translated into the PET images because of the attenuation correction. For this reason most of the PET/CT artefacts are related to the CT images and need to be accurately identified to avoid false positive reports.



Back to top

IMAGE ARTEFACTS

PET/CT image artefacts are due primarily to metallic implants, respiratory motion, use of contrast media and image truncation. All these artefacts are visible in both CT alone and in CTAC PET images. The artefacts do not appear in uncorrected PET images, so they may be used as control images for testing doubtful findings.

Metallic implants

The presence of metallic implants, such as dental clogging, dental implants, metallic clips and chemotherapy infusion ports, is visualised by CT images as areas of high density, which cause artefacts on the CT images [30-31]. These high CT numbers correspond to high attenuation coefficients that result in an overcorrection of the PET images, promoting false-positive findings. The uncorrected images can help the nuclear medicine physician to identify these “hot” findings as artefacts.

Figure 2 shows a typical artefact due to the presence of a metallic clip; it is very clear the effect of the higher CT correction on the PET images producing a false-positive finding. A similar artefact can be caused by the presence of a pace maker (Figure 3).

If the metallic implant sise is sufficiently large (for example, a hip implant), the PET images do not present an artefact because the implant area is characterised by the absence of activity in the prosthetics. Therefore, though the CT-derived attenuation coefficients are high, the corrected and uncorrected images are similar and are visualised as “cold” regions [32-34].

To minimise the presence of artefacts due to metallic implants, the technologist should ask the patient to remove before scanning all metallic objects, such as coins, jewels, metallic buttons, belt buckles, bra with iron inserts. Physicians should highlight in the anamnesis the presence of non-removable metallic implants.

CT contrast media

To better visualise vessels and soft tissues and to improve CT image quality, intravenous or oral contrast media are often administered to patients. However, the use of these agents can introduce changes into CT numbers similar to metallic implants, affecting the quantitative and qualitative accuracy of CTAC PET images [35-41]. The effect of contrast media artefacts increases with the concentration of the administered agent and depends on its clearance from patient’s body and the time between administration and CT acquisition. In particular, the tissue concentration of oral contrast agents increases over time, so while their use during a PET/CT protocol gives all the benefits related to a better visualization of CT images without a real compromising of CTAC PET images, particular attention should be taken if the patients had undergone a diagnostic CT scan with contrast few hours before the PET/CT scan.

Several correction techniques are presented in the literature [42]. Nehmeh et al. [43] propose an interesting method to correct for CTAC PET images. This method is performed by contouring the contrast regions, excluding any body structures; transforming the corresponding linear attenuation coefficients, μ(x, E), of contrast correctly from CT to PET energies; and, finally, reconstructing CTAC PET images with the appropriately scaled attenuation map.

Respiratory motion

One of the most significant and frequent artefact in PET/CT images is due to respiratory motion during scanning. Although the use of a combined PET/CT scanner allows the registration of the two datasets in the simplest way, respiratory motion results into mismatch between CT and corresponding PET slices [44-46]. Because of the long acquisition time of the PET scan, the patient is allowed to breath normally during both CT and PET acquisitions. Asking the patient to hold the breath during the CT scan, as it’s normally done in diagnostic CT studies, can lead to artefacts because of the certain mismatch between a specific stage of the breath cycle during the CT and the average of many breathing cycles of the PET images. However, even if the patient is usually allowed to breathe normally during the whole PET/CT study, because of the fast CT, the diaphragm is visualised in a single position that is different from the mean position of PET images or in the course of respiratory motion.

As described by Papathanassiou et al. [47], this phenomenon not only sometimes provokes misregistration of lesions between the two modalities (Figure 4) or disrupts image fusion of normal organs, but also may cause an erroneous attenuation correction. Because of respiratory motion the density of a particular organ could be attributed to an area whose density is different.

For example, the downward displacement of the diaphragm causes an underestimation of correction of the liver dome, leading to a cold area in that zone. It is obvious that particular attention is needed if the patient is suspected for liver metastasis or for nodules at the base of the lung.

The best way to correct for respiratory motion would be to acquire gated images to discriminate different intervals of a breath cycle. Many companies are working to implement hardware respiratory-motion correction on their systems, but none are currently completely validated.

Truncation

The typical transverse field of view (FOV) of the CT scanner in a PET/CT system is about 50 cm, while the PET FOV is 70 cm. The relative small CT FOV can cause truncation of CT images [48]. To avoid truncation artefacts in PET/CT images patients are scanned with arms above their head. However, in obese patients and in scans acquired with arms down, as with some patients with melanoma or head and neck tumours, this kind of artefact is frequently seen.

As described by Mawlawi et al [49] the aspect of truncation artefact in CT images is a bright rim of high attenuation values together with characteristic streaking, reflecting on PET-corrected images as absence of attenuation correction factors in the sections of the PET slices which exceed the CT FOV. The resultant artefact on the attenuation corrected PET images is an overestimation of the activity concentration corresponding to the rim and an underestimation corresponding to the region without attenuation factors.

Several techniques have been proposed and implemented on commercial systems to correct for truncation artefacts and most of them give a recovery of more than 90% of the activity in the truncated regions. Hsieh et al [50] developed an algorithm for truncation correction which extends the CT FOV based on information obtained from untruncated projections of the object and the knowledge that the total attenuation of an object should be the same independent of the projection angle. This technique has been implemented in the GE Discovery ST PET/CT system.

Although the different techniques are effective for normal size patients, images of large or obese patients need a deeper analysis and in all cases corrected SUV measurements must be used carefully.



Back to top

CONCLUSION

PET/CT improves quality accuracy of the image. The use of CT for attenuation correction yields several advantages in terms of accuracy and patient comfort.

Several artefacts are introduced in CTAC PET images due to CT, but their knowledge and the use of proper correction techniques, such as dedicated algorithms, which take into account the presence of high density materials, minimises any source of false findings.

To avoid possible pitfalls, the simultaneous display of both CTAC and non-corrected PET images, side by side with CT images is strongly recommended.

Figure 1 These images show the layouts of the three commercial family systems available on the market: a) Siemens/CTI Biograph, b) GE Healthcare Discovery, c) Philips Gemini.

Figure 2 Focal artefact on CTAC PET images due to the presence of a metallic clip.

Figure 3 Focal artefact on CTAC PET images due to the presence of a pace maker.

Figure 4 Misregistration of CT and PET malignant nodule of the right lung due to respiratory movement.




REFERENCES

  1. Lardinois D, Weder W, Hany TF, et al. Staging of non-small-cell lung cancer with integrated positron-emission tomography and computed tomography. N Engl J Med 2003;348(25):2500-7.   [Medline] [CrossRef]
  2. Kresnik E, Mikosch P, Gallowitsch HJ, et al. Evaluation of head and neck cancer with 18F-FDG PET: a comparison with conventional methods. Eur J Nucl Med 2001;28(7):816-21.   [Medline]
  3. Weber WA, Avril N, Schwaiger M. Relevance of positron emission tomography (PET) in oncology. Strahlenther Onkol 1999;175(8):356-73.   [Medline]
  4. Hawkes DJ, Hill DL, Hallpike L, et al. Coregistration of structural and functional images. Valk P, Bailey DL, Townsend DW, et al., eds. Positron Emission Tomography: Basic Science and Clinical Practice. New York, NY: Springer-Verlag, 181-98.  
  5. Patton JA, Delbeke D, Sandler MP. Image fusion using an integrated, dual-head coincidence camera with X-ray tube-based attenuation maps. J Nucl Med 2000;41(8):1364-8.   [Medline]
  6. Beyer T, Townsend DW, Brun T, et al. A combined PET/CT scanner for clinical oncology. J Nucl Med 2000;41(8):1369-79.   [Medline]
  7. Townsend DW, Cherry SR. Combining anatomy and function: the path to true image fusion. Eur Radiol 2001;11(10):1968-74.   [Medline] [CrossRef]
  8. Charron M, Beyer T, Bohnen NN, et al. Image analysis in patients with cancer studied with a combined PET and CT scanner. Clin Nucl Med 2000;25(11):905-10.   [Medline]
  9. Meltzer CC, Martinelli MA, Beyer T, et al. Whole-body FDG PET imaging in the abdomen: value of combined PET/CT. J Nucl Med 2001;42:35P.  
  10. Meltzer CC, Snyderman CH, Fukui MB, et al. Combined FDG PET/CT imaging in head and neck cancer: impact on patient management. J Nucl Med 2001;42:36P.  
  11. Kluetz PG, Meltzer CC, Villemagne VL, et al. Combined PET/CT Imaging in Oncology. Impact on Patient Management. Clin Positron Imaging 2000;3(6):223-30.   [Medline]
  12. Bar-Shalom R, Yefremov N, Guralnik L, et al. Clinical performance of PET/CT in evaluation of cancer: additional value for diagnostic imaging and patient management. J Nucl Med 2003;44(8):1200-9.   [Medline]
  13. Keidar Z, Bar-Shalom R, Guralnik L, et al. Hybrid imaging using PET/CT with 18F-FDG in suspected recurrence of lung cancer: diagnostic value and impact on patient management. J Nucl Med 2002;43:32P.  
  14. Lardinois D, Weder W, Hany TF, et al. Staging of non-small-cell lung cancer with integrated positron-emission tomography and computed tomography. N Engl J Med 2003;348(25):2500-7.   [Medline] [CrossRef]
  15. Steinert HC, Hany TF, Kamel E, et al. Impact of integrated PET/CT scanning on preoperative staging of lung cancer. J Nucl Med 2002;43:151P.  
  16. Osman MM, Cohade C, Leal J, et al. Direct comparison of FDG PET and PET/CT imaging in staging and restaging patients with lung cancer. J Nucl Med 2002;43:151P.  
  17. Makhija S, Howden N, Edwards R, et al. Positron emission tomography/computed tomography imaging for the detection of recurrent ovarian and fallopian tube carcinoma: a retrospective review. Gynecol Oncol 2002;85(1):53-8.   [Medline] [CrossRef]
  18. Sureshbabu W, Mawlawi O. PET/CT imaging artifacts. J Nucl Med Technol 2005;33(3):156-61; quiz 163-4.   [Medline]
  19. Bockisch A, Beyer T, Antoch G, et al. Positron emission tomography/computed tomography--imaging protocols, artifacts, and pitfalls. Mol Imaging Biol 2004;6(4):188-99.   [Medline] [CrossRef]
  20. Bailey DL, Young H, Bloomfield PM, et al. ECAT ART - a continuously rotating PET camera: performance characteristics, initial clinical studies, and installation considerations in a nuclear medicine department. Eur J Nucl Med 1997;24(1):6-15.   [Medline]
  21. Townsend DW. A combined PET/CT scanner: the choices. J Nucl Med 2001;42(3):533-4.   [Medline]
  22. Bettinardi V, Danna M, Savi A, et al. Performance evaluation of the new whole-body PET/CT scanner: Discovery ST. Eur J Nucl Med Mol Imaging 2004;31(6):867-81.   [Medline] [CrossRef]
  23. Mawlawi O, Podoloff DA, Kohlmyer S, et al. Performance characteristics of a newly developed PET/CT scanner using NEMA standards in 2D and 3D modes. J Nucl Med 2004;45(10):1734-42.   [Medline]
  24. Brambilla M, Secco C, Dominietto M, et al. Performance characteristics obtained for a new 3-dimensional lutetium oxyorthosilicate-based whole-body PET/CT scanner with the National Electrical Manufacturers Association NU 2-2001 standard. J Nucl Med 2005;46(12):2083-91.   [Medline]
  25. Beyer T, Antoch G, Muller S, et al. Acquisition protocol considerations for combined PET/CT imaging. J Nucl Med 2004;45 Suppl 1:25S-35S.   [Medline]
  26. Bettinardi V, Pagani E, Gilardi MC, et al. An automatic classification technique for attenuation correction in positron emission tomography. Eur J Nucl Med 1999;26(5):447-58.   [Medline]
  27. Bengel FM, Ziegler SI, Avril N, et al. Whole-body positron emission tomography in clinical oncology: comparison between attenuation-corrected and uncorrected images. Eur J Nucl Med 1997;24(9):1091-8.   [Medline]
  28. Burger C, Goerres G, Schoenes S, et al. PET attenuation coefficients from CT images: experimental evaluation of the transformation of CT into PET 511-keV attenuation coefficients. Eur J Nucl Med Mol Imaging 2002;29(7):922-7.   [Medline] [CrossRef]
  29. Kinahan PE, Townsend DW, Beyer T, et al. Attenuation correction for a combined 3D PET/CT scanner. Med Phys 1998;25(10):2046-53.   [Medline]
  30. Goerres GW, Hany TF, Kamel E, et al. Head and neck imaging with PET and PET/CT: artefacts from dental metallic implants. Eur J Nucl Med Mol Imaging 2002;29(3):367-70.   [Medline]
  31. Svendsen P, Quiding L, Landahl I. Blackout and other artefacts in computed tomography caused by fillings in teeth. Neuroradiology 1980;19(5):229-34.   [Medline]
  32. Heiba SI, Luo J, Sadek S, et al. Attenuation-Correction Induced Artifact in F-18 FDG PET Imaging Following Total Knee Replacement. Clin Positron Imaging 2000;3(6):237-9.   [Medline]
  33. Goerres GW, Burger CN, Berthold T, et al. Influence of attenuation correction (AC) in positron emission tomography (PET) and combined PET-CT on artifacts of hip protheses. Radiology 2001;221 (suppl):386.  
  34. Goerres GW, Ziegler SI, Burger C, et al. Artifacts at PET and PET/CT caused by metallic hip prosthetic material. Radiology 2003;226(2):577-84.   [Medline]
  35. Antoch G, Freudenberg LS, Beyer T, et al. To enhance or not to enhance? 18F-FDG and CT contrast agents in dual-modality 18F-FDG PET/CT. J Nucl Med 2004;45 Suppl 1:56S-65S.   [Medline]
  36. Antoch G, Freudenberg LS, Stattaus J, et al. Whole-body positron emission tomography-CT: optimized CT using oral and IV contrast materials. AJR Am J Roentgenol 2002;179(6):1555-60.   [Medline]
  37. Antoch G, Freudenberg LS, Egelhof T, et al. Focal tracer uptake: a potential artifact in contrast-enhanced dual-modality PET/CT scans. J Nucl Med 2002;43(10):1339-42.   [Medline]
  38. Antoch G, Jentzen W, Freudenberg LS, et al. Effect of oral contrast agents on computed tomography-based positron emission tomography attenuation correction in dual-modality positron emission tomography/computed tomography imaging. Invest Radiol 2003;38(12):784-9.   [Medline] [CrossRef]
  39. Yau YY, Chan WS, Tam YM, et al. Application of intravenous contrast in PET/CT: does it really introduce significant attenuation correction error? J Nucl Med 2005;46(2):283-91.   [Medline]
  40. Carney JP, Beyer T, Brasse D, et al. Clinical PET/CT scanning using oral CT contrast agents. J Nucl Med 2002;45:57P.  
  41. Cohade C, Osman M, Nakamoto Y, et al. Initial experience with oral contrast in PET/CT: phantom and clinical studies. J Nucl Med 2003;44(3):412-6.   [Medline]
  42. Lonn AHR. Evaluation of method to minimize the effect of X-ray contrast in PETCT attenuation correction. IEEE Nuclear Science Symposium. 2004: 2220-1.  
  43. Nehmeh SA, Erdi YE, Kalaigian H, et al. Correction for oral contrast artifacts in CT attenuation-corrected PET images obtained by combined PET/CT. J Nucl Med 2003;44(12):1940-4.   [Medline]
  44. Goerres GW, Kamel E, Heidelberg TN, et al. PET-CT image co-registration in the thorax: influence of respiration. Eur J Nucl Med Mol Imaging 2002;29(3):351-60.   [Medline]
  45. Goerres GW, Burger C, Schwitter MR, et al. PET/CT of the abdomen: optimizing the patient breathing pattern. Eur Radiol 2003;13(4):734-9.   [Medline] [CrossRef]
  46. Beyer T, Antoch G, Blodgett T, et al. Dual-modality PET/CT imaging: the effect of respiratory motion on combined image quality in clinical oncology. Eur J Nucl Med Mol Imaging 2003;30(4):588-96.   [Medline] [CrossRef]
  47. Papathanassiou D, Becker S, Amir R, et al. Respiratory motion artefact in the liver dome on FDG PET/CT: comparison of attenuation correction with CT and a caesium external source. Eur J Nucl Med Mol Imaging 2005;32(12):1422-8.   [Medline] [CrossRef]
  48. Mawlawi O, Erasmus JJ, Pan T, et al. Truncation artifact on PET/CT: impact on measurements of activity concentration and assessment of a correction algorithm. AJR Am J Roentgenol 2006;186(5):1458-67.   [Medline] [CrossRef]
  49. Beyer T, Bockisch A, Kuhl H, et al. Whole-body 18F-FDG PET/CT in the presence of truncation artifacts. J Nucl Med 2006;47(1):91-9.   [Medline]
  50. Hsieh J, Chao E, Thibault J, et al. A novel reconstruction algorithm to extend the CT scan field-of-view. Med Phys 2004;31(9):2385-91.   [Medline]

Received 9 October 2006; received in revised form 8 November 2006; accepted 24 December 2006

Correspondence
: Medicina Nucleare – Pad 30, Ospedaliero Universitaria S. Orsola Malpighi, Via Massarenti, 9. 40138 Bologna Italy; E-mail: pettinato@aosp.bo.it (Cinzia Pettinato).


Please cite as: Pettinato C, Nanni C, Farsad M, Castellucci P, Sarnelli A, Civollani S, Franchi R, Fanti S, Marengo M, Bergamini C, Artefacts of PET/CT images, Biomed Imaging Interv J 2006; 2(4):e60
<URL: http://www.biij.org/2006/4/e60/>

This article has been viewed 7669 times.


University of Malaya, Kuala Lumpur, Malaysia

Official publication of

ASEAN Association of Radiologists
ASEAN Society of Interventional Radiology
Asia-Oceania Federation of Organizations for Medical Physics
Asian Oceania Society of Radiology
College of Radiology, Academy of Medicine Malaysia
Southeast Asian Federation of Organisations of Medical Physics
South East Asian Association of Academic Radiologists

Published by

Department of Biomedical Imaging, Faculty of Medicine, University of Malaya, Malaysia




   

Biomedical Imaging and Intervention Journal. ISSN 1823-5530 RSS | Facebook | Twitter


 
   
Creative Commons License
Except where otherwise noted, articles published in the Biomedical Imaging and Intervention Journal
are distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited, including full bibliographic details and the URL, and this statement is included.

 
cheap oakleys mulberry outlet burberry outlet online cheap ugg boots oakley sunglasses outlet cheap nike air max louis vuitton australia coach outlet online ugg Boots outlet fake oakley sunglasses cheap ugg boots cheap uggs cheap ugg boots ugg outlet cheap jordans doudoune moncler Michael Kors Outlet Oakley Sunglasses Outlet Cheap Ugg Boots