Ultrasonographic fata morgana

Vincent M. Brandenburg, Rolf D. Frank, Ulf Janssen, Patrick Wurth, Jürgen Floege and Jochen Riehl

Department of Nephrology and Immunology, University Hospital, Rheinisch-Westfälische Technische Hochschule (RWTH), Aachen, Germany

Keywords: mirror-image artifact; renal transplantation; ultrasonography

Case

A 35-year-old female with end-stage renal disease due to chronic glomerulonephritis received a cadaveric renal transplantation into the right fossa iliaca. Her body mass index was 23 kg/m2. The clinical course after transplantation was uneventful and graft function was stable during follow-up. Ultrasonographic monitoring of the renal transplant was performed regularly with B-scan, colour Duplex and Doppler ultrasonography using a 3.75 MHz curved array transducer and sector transducer (Sonolayer SSA 270A; Toshiba, Tokyo, Japan). On routine examination in the B-scan mode, the patient reproducibly revealed a mirror-image artifact: the entire graft appeared virtually duplicated with the resulting phantom image positioned distally to the real image deeper in the tissue (Figure 1Go). The visceral surface of the ala ossis ilii served as an echodense interface producing the mirror image by multi-path reflections. The interface was visible as a slim echodense line between the virtual and real images. The concave shape of the ala ossis ilii caused distortion of the mirror image. In addition to the B-mode mirror-image artifact, a colour flow map of intrarenal circulation and regular Doppler curves could be generated in both images. The resistance indices measured within the virtual image and the real image were equal and within normal ranges.



View larger version (144K):
[in this window]
[in a new window]
 
Fig. 1.  B-scan mode of the pelvic renal allograft with (a) representing the real image, (b) the virtual duplication (so-called mirror-image) and (c) the ala ossis ilii appearing as slim echodense line separating real and mirror-image.

 

Discussion

Basic principles of image formation in ultrasonography are analysis of the time-of-flight and the angle of incidence of the reflected ultrasound signals. Highly reflective acoustic interfaces may produce multi-path reflections between the target object and the transducer. These multi-path reflections may interfere with the automated process of image formation in double respect. First, the prolongation of the time-of-flight may cause overestimation of the target object depth within the body and secondly, changes of the angle of incidence of the incoming sound signals may modify the calculated target object position [1,2]. Therefore, in case of reflected sound waves partially undergoing straight-line propagation and multi-path reflections at the same time the target object may be malpositioned or even doubled by the ultrasound system software. The resulting virtual image is the so-called mirror-image artifact [27]. Objects immediately adjacent to highly reflective acoustic interfaces are especially prone to mirror-image formation [2,8].

The occurrence of mirror-image artifacts has been described in both B-scan and Duplex sonography [1,611]. Clinical significance emerges from possibly confusing and misleading findings like malpositioning or duplication of target objects [1,7,8]. Air–tissue (air–fluid) interfaces in the human body are predisposed for this phenomenon, since the ultrasound conduction qualities are significantly different leading to great impedance differences [6,7,9]. In clinical practise, these artifacts commonly occur due to reflections from the hepatopulmonary interface that produces virtual malpositioning of intrahepatic structures into the lungs [9]. Furthermore, air-containing abscesses [5], the fluid–air interface in the stomach [4], the pleura–lung apex interface [8] and the urinary bladder [2] have been found previously to create mirror-image artifacts. Concerning Duplex and Doppler ultrasonography, mirror-image artifacts have been described in examinations of the portal vein as well as of the carotid, intracerebral and subclavian arteries [1,6,8,10,11]. Under favourable scanning conditions, spectral Doppler images can be acquired from the mirror image that are comparable with the signals from the real vessel both in direction, strength and velocity [6,8,11,12]. The present case report presents the unusual finding of a pelvic renal allograft, which was completely duplicated as a mirror-image artifact in B-scan and colour Duplex ultrasound. This mirror-image artifact was made possible by the close anatomic relationship of the renal allograft and the ala ossis ilii in a slim patient.

The term ‘fata morgana’ is attributed to the enchantress Morgan le Fay, sister of King Arthur. She gave her name to a periodically appearing mirage in the Straits of Messina. Nowadays the term is widely used for optical delusions based on atmospheric refraction processes.

Notes

Correspondence and offprint requests to: Vincent M. Brandenburg, MD, Department of Nephrology and Immunology, Medical Clinic II, University Hospital Aachen, Pauwelsstrasse 30, D-52057 Aachen, Germany. Email: vincent.brandenburg{at}post.rwth-aachen.de Back

References

  1. Arning C. Mirror image artifacts of color Doppler images causing misinterpretation in carotid artery stenoses. J Ultrasound Med 1998; 17:683–686[Abstract/Free Full Text]
  2. Sandler MA, Madrazo BL, Walter R et al. Ultrasound case of the day. Duplication artifact (mirror image artifact). Radiographics 1987; 7:1025–1028[Medline]
  3. Mihmanli I, Cetinkaya S, Kurugoglu S, Kantarci F, Esen G. Another face of mirror-image artifact. Eur J Ultrasound 2001; 14:183–185[CrossRef][Medline]
  4. Grech P. Mirror-image artifact with endoscopic ultrasonography and reappraisal of the fluid–air interface. Gastrointest Endosc 1993; 39:700–703[ISI][Medline]
  5. Wilson SR, Burns PN, Wilkinson LM, Simpson DH, Muradali D. Gas at abdominal US: appearance, relevance, and analysis of artifacts. Radiology 1999; 210:113–123[Abstract/Free Full Text]
  6. Kremkau FW, Taylor KJ. Artifacts in ultrasound imaging. J Ultrasound Med 1986; 5:227–237[Abstract]
  7. Appelbe AF, Walker PG, Yeoh JK, Bonitatibus A, Yoganathan AP, Martin RP. Clinical significance and origin of artifacts in transesophageal echocardiography of the thoracic aorta. J Am Coll Cardiol 1993; 21:754–760[ISI][Medline]
  8. Reading CC, Charboneau JW, Allison JW, Cooperberg PL. Color and spectral Doppler mirror-image artifact of the subclavian artery. Radiology 1990; 174:41–42[Abstract]
  9. Gardener FJ, Clark RN, Kozlowski R. A model of a hepatic mirror-image artifact. Med Ultrasound 1980; 4:19–21
  10. Parvey HR, Eisenberg RL, Giyanani V, Krebs CA. Duplex sonography of the portal venous system: pitfalls and limitations. Am J Roentgenol 1989; 152:765–770[ISI][Medline]
  11. Mitchell DG. Color Doppler imaging: principles, limitations, and artifacts. Radiology 1990; 177:1–10[ISI][Medline]
  12. Middleton WD, Melson GL. The carotid ghost. A color Doppler ultrasound duplication artifact. J Ultrasound Med 1990; 9:487–493[Abstract]




This Article
Extract
FREE Full Text (PDF)
Alert me when this article is cited
Alert me if a correction is posted
Services
Email this article to a friend
Similar articles in this journal
Similar articles in ISI Web of Science
Similar articles in PubMed
Alert me to new issues of the journal
Add to My Personal Archive
Download to citation manager
Disclaimer
Request Permissions
Google Scholar
Articles by Brandenburg, V. M.
Articles by Riehl, J.
PubMed
PubMed Citation
Articles by Brandenburg, V. M.
Articles by Riehl, J.