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 1). 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.
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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]. Airtissue (airfluid) 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 fluidair interface in the stomach [4], the pleuralung 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
References