EJVES Extra (2010) 19, e35ee37

SHORT REPORT

Time of Flight Magnetic Resonance Angiography:A Trap for the Unwary

L. Corfield*, A. Speirs, D.J. McCormack, M. Waltham

Department of Vascular Surgery, Guy’s and St Thomas’ NHS Foundation Trust, Lambeth Palace Road, London SE1 7EH, UK

Submitted 15 December 2009; accepted 17 January 2010

KEYWORDSCarotid dissection;Magnetic resonanceangiography;Time of flight;Artefact

DOI of original article: 10.1016/j.e* Corresponding author.

1533-3167/$36 ª 2010 European Sociedoi:10.1016/j.ejvsextra.2010.01.002

Abstract Introduction: Magnetic resonance imaging is now frequently used to image bloodvessels. This case illustrates a pitfall of this mode of imaging.Report: A 6-year-old girl sustained a severe neck injury and subsequently developed a Horner’ssyndrome. A time-of-flight magnetic resonance scan could be interpreted by the inexperiencedas showing an extensive dissection. However, a contrast-enhanced scan confirmed the pres-ence of a localised carotid injury only.Discussion: Time-of-flight magnetic resonance scanning produces flow voids which can mimicdissection, particularly in high velocity vessels such as the carotid artery. This case isa reminder that whatever imaging modality is used, correct interpretation is essential.ª 2010 European Society for Vascular Surgery. Published by Elsevier Ltd. All rights reserved.

Introduction

Magnetic resonance scanning is being increasing used invascular imaging. However, liked any other modality, itdoes have some pitfalls. This case illustrates one suchpitfall when time-of-flight magnetic resonance imaging isused to assess high flow vessels such as the carotid arteries.

Report

A six-year-old girl was attacked by a Staffordshire terrierwhich locked its jaw on the child’s neck and vigorously flung

jvs.2010.01.025.

ty for Vascular Surgery. Publishe

her around. When the dog released the child she was found tobe bleeding profusely from multiple deep puncture woundson the left neck, lower face and around the left ear. Therewas marked surgical emphysema of the neck. She underwentwound exploration under anaesthesia. The trachea wasfound to be partially transected and was repaired. The lefteleventh cranial nerve was thought to be damaged but wasgrossly intact. No major vascular injury was found. She wasventilated and subsequently required a tracheostomy. Shewas then noted to have a left Horner’s syndrome. A time-of-flight magnetic resonance angiogram (MRA) was arranged toimage any carotid injury (Philips Achieva 1.5T scanner witha Sensor head coil: Tr Z 15, Te Z 3.3 with a flip angle of 20�).This demonstrated a focal area of expansion of the distalcommon carotid artery in combination with a defined area ofvessel narrowing. This was reported as suspicious for a local

d by Elsevier Ltd. All rights reserved.

e36 L. Corfield et al.

dissection although no associated area of high signal wallhaematoma or intimal flap could be clearly identified. Shewas fully anticoagulated with low molecular weight heparinon the basis of a presumed diagnosis of dissection.

Fig. 1 shows the area of abnormality but also demonstratesa catch for the unwary. In addition to the localised abnor-mality there is a laminar flow artefact visible throughout theleft internal carotid artery. This might be interpreted asa dissection by physicians without sufficient MRA experience.

Seven days later a contrast-enhanced (Magnevist 5ml) MRAwas performed that confirmed the localised abnormality in the

Figure 1 Time of flight MRA showing left common carotidinjury (A) and laminar flow void in left internal carotid artery(B). The flow void is also visible in the right carotid arteries (C).

distal common carotid artery (Fig. 2). The left internal carotidis, however, shown to be pristine. Duplex scanning at 6 monthsagain demonstrated a dissection flap localised within thecommon carotid artery causing an 85% stenosis, although thepatient remainedclinicallywellwithnoneurological sequelae.

Discussion

MRA has been reported as having 100% sensitivity andspecificity for carotid dissection.1 The key features ofdissection are vessel expansion, stenosis and intramuralhaematoma, with some studies describing the presence ofintramural haematoma in all cases.2 However, a choice ofimaging modalities for identifying dissection (CT angiog-raphy, duplex, conventional angiography and MRA) is oftenavailable and the clinical team and radiologist have toweigh up the potential complications of each test with theirrespective accuracies. In this case the patient was young soMRA has advantages over CTA as it is non-invasive and thereis no ionising radiation. However, a CT angiogram, with itsimproved spatial resolution can be diagnostic.3 This casedemonstrates that whatever imaging is used, correctinterpretation of the scan is essential. MRA interpretation

Figure 2 Gadolinium-enhanced MRA showing left commoncarotid injury (arrow) but no laminar flow void.

Time of Flight Magnetic Resonance Angiography e37

in particular can be difficult. The flow voids on time-of-flight (TOF) imaging can mimic dissection. This occursbecause TOF depends on saturation differences betweenmolecules flowing into an imaging slice and static moleculeswithin the slice. These molecules have a different level ofradiofrequency excitation which leads to the angiographicdifferences. This occurs because the flowing molecules ina particular slice must experience both the excitatory andrefocusing radiofrequency pulses to produce a signal. Athigh velocities, such as in the carotid arteries, most of theflowing molecules have left the slice before the refocusingpulse is applied and thus a flow void is seen.4 As velocitiesin the centre of a vessel are greater than those at theperiphery, this difference is particularly noticeable in themiddle of the vessel. In effect, the rapid flow in the centreof the vessel escapes the image slice between excitationand signal measurement. Clinicians reviewing scans of theirpatients must have knowledge of such interpretation trapsand liaison with an experienced radiologist is essential.

Conflict of Interest/Funding

None.

References

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2 Kirsch E, Kaim A, Engelter S, Lyrer P, Stock KW, Bongartz G, et al.MR angiography in internal carotid artery dissection: improve-ment of diagnosis by selective demonstration of the intramuralhaematoma. Neuroradiology 1998;40:704e9.

3 Vertinsky AT, Schwartz NE, Fischbein NJ, Rosenberg J,Albers GW, Zaharchuk G. Comparison of multidetector CT angi-ography and MR imaging of cervical artery dissection. Am JNeurorad 2008;29(9):1753e60.

4 Nishimura DG. Time-of-flight MR angiography. Magn Reson Med1990;14:194e201.