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Keywords:

  • horse;
  • computed tomographic angiography;
  • aneurysm;
  • parapharyngeal

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Case details
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Manufacturers' addresses
  9. References

Multidetector-row computed tomographic contrast angiography (MDCTA) is routinely employed to investigate vascular masses in human patients but, to date, the use of this technique to investigate an aneurysmal mass has not been reported in an equine case. The potential of MDCTA to investigate a right-sided parapharyngeal mass in a 6-week-old Thoroughbred foal was therefore investigated.

A 4-slice helical computed tomography scanner was used on a superficial, firm, ovoid mass yielding arterial blood on fine needle aspiration. MDCTA enabled identification of the vessels involved thus assisting in forming a diagnosis based on the morphology of the lesion and aided surgical planning. Histology confirmed the presence of mural thrombus and calcification within the smooth muscle wall consistent with a true aneurysm. MDCTA provided additional information to conventional imaging in this case of vascular masses of the head and neck. The technique therefore has the potential to improve diagnosis and assist in the management of such lesions.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Case details
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Manufacturers' addresses
  9. References

The commercial introduction of multidetector-row computed tomographic contrast angiography (MDCTA) to clinical radiology in human practice triggered a rapid development in CT technology and, almost a decade later, MDCTA remains the most rapidly evolving imaging modality in human diagnostic imaging, which has become indispensable in routine clinical practice (Kalra et al. 2004). Multidetector-row technology allows the acquisition of more than one image per gantry rotation and the helical motion of the x-ray tubes around the patient enables faster image acquisition. These 2 functions allow for wider scan coverage and thinner section thickness in a shorter period. This increase in temporal resolution improves contrast utilisation in vascular and cardiac studies and allows the acquisition of isotropic scan data, helping to create exquisite 3D or orthogonal multiplanar images (MPI) (Napel 2004).

The use of contrast enhanced computed tomography (CT) to evaluate bone and soft tissue lesions in horses has focused primarily on the distal limb (Whitton et al. 1998; Widmer et al. 2000; Tietje et al. 2001). Contrast enhanced CT has been described as a means of diagnosing the cause of heel pain in horses (Puchalski et al. 2007, 2009). Various indications for CT evaluation of the equine head have been described (Warmerdam et al. 1997; Chalmers et al. 2006; de Mira et al. 2007; Annear et al. 2008; Chehak et al. 2008; Windley et al. 2009; Kinns and Pease 2009). However, whilst the use of MDCTA is widespread in the investigation of small animal diseases, its use in equine cases is less prevalent and, to the author's knowledge, there are no reports describing the use of MDCTA for the evaluation of an aneurysmal mass in a horse. This report describes the MDCTA technique and findings along with the gross surgical appearance and histopathology in a case of aneurysm in a foal.

Case details

  1. Top of page
  2. Summary
  3. Introduction
  4. Case details
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Manufacturers' addresses
  9. References

Presentation

A 6-week-old Thoroughbred colt foal weighing 111 kg was admitted to Rossdales Equine Diagnostic Centre for further imaging of a right-sided parapharyngeal mass. The foal was otherwise in good health. The mass, present at birth, had been observed to enlarge slowly over the previous 6 weeks. The mass was firm to the touch and not palpably pulsatile. Initially thought to be a parotid cyst, a fine needle aspirate had yielded arterial blood prompting further investigations.

Transcutaneous colour flow Doppler sonographic evaluation using a GE Logiq-e portable ultrasound machine2 with microconvex and linear transducers revealed an encapsulated mass; the capsule measured up to 3 mm thick in places. The mineralisation within the capsule (subsequently revealed on histopathology) was not detectable ultrasonographically. Turbulent, pulsatile flow was present with mural thrombus adjacent to the facial artery of the right hemimandible. No direct communication between the aneurysmal mass and the adjacent vessel was detected ultrasonographically.

Follow-up with MDCTA technique

The foal was anaesthetised and placed in dorsal recumbency with the pharyngeal region in the centre of the CT gantry. Ionic iodine based contrast agent (320 ml: Ultravist1 370 mgI/ml) was manually injected via catheter placed in the right jugular vein at a rate of 5 ml/s. Helical images collimated to a slice thickness of 1 mm of the cranial neck and caudal aspect of the head were acquired using a Siemens Volume Zoom M 4-slice helical computed tomography scanner3 from 15 s after commencing the i.v. injection of contrast agent. The rotation time was 0.75 s and the feed/rotation time was 3.5 mm. The scan length was 20 cm to ensure the vessels of the cranial cervical region and mandibles were included. Scan direction was caudorostral. To observe temporal changes in enhancement the scan was repeated 3 times with a 4 s delay between imaging.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Case details
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Manufacturers' addresses
  9. References

The images were exported into a DICOM workstation and manipulated using Osirix version 3.3.2 32bit4. The 2D data set was reconstructed using a soft tissue algorithm. The presence or absence of contrast medium was identified by subjective visual assessment and the measurement of attenuation in Housfield units using an operator defined region of interest.

Arterial and venous contrast enhancement within the scan field were present on all acquired scans but optimal during the scan, which commenced 39 s after the onset of i.v. injection with contrast agent. Both 2D (Fig 1) and volume rendered images were optimised at this time for definition of the aneurysmal mass and its vascular associations. Volume rendered images enabled better evaluation of the structures involved due to the tortuous course of the facial artery.

image

Figure 1. Axial 2D computed tomographic image. Contrast is present within the aneurysmal mass (AM) and surrounding vessels - the right and left linguofacial veins (small black arrows) and the left facial artery (white arrow).

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The aneurysmal mass was found to originate from the facial artery: the artery was seen to course dorsally and abaxially following its bifurcation with the linguofacial artery before reflecting ventrally, axially and rostrally to the angle of the right hemi-mandible (Fig 2). Here it could be seen to merge with the rostral most compartment of the mass on its dorsal aspect. The vessel could then be seen emerging from the ventral aspect of the most caudal compartment of the aneurysmal mass coursing immediately rostrally in close association with it before assuming its normal anatomical position axial to the right mandible.

image

Figure 2. Ventral view of a 3D volumetric rendered computed tomographic image with the overlying soft tissues removed to the show the vessels. RLV=right lingual vein; RM =right mandible; RLFV =right linguofacial vein; RJV=right jugular vein; AM=aneurismal mass; RFA=right facial artery.

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The lingual and facial veins were displaced axially and dorsally by the mass; and were partially occluded by the mass. There were filling defects in both veins as they coursed caudal to the mass and merged to form the linguofacial vein before draining into the right jugular vein. The right jugular vein was wider in cross-section and more hyperattenuating than the contralateral jugular vein, probably related to the insinuation of contrast agent into this vessel. Therewas minimal contrast enhancement of the wall of the mass and region of mural thrombosis.

Surgical management

The following day the foal was anaesthetised and positioned in dorsal recumbency. The thick walled aneurysmal mass was dissected from surrounding tissues, taking care to avoid the parotid salivary duct, through a 10 cm ventral intermandibular incision. The arterial supply to the structure (which throbbed with a pulse) was identified deep to the lesion, isolated and ligated with 3 metric polyglactin 910 (Vicryl)5. The bilobar mass (approximately 9 × 6 × 2.5 cm) was removed (Fig 3). The incision was closed with 3 metric polyglactin sutures in 2 continuous layers and the skin with simple interrupted sutures of the same material. Recovery from anaesthesia was uneventful.

image

Figure 3. Gross appearance of the mass after surgical excision. Image courtesy of Tim Greet.

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Histology

The tissue was submitted for histology, which revealed degenerative changes and mineralisation within the vessel wall with almost complete loss of the internal elastic lamina and extensive, organising mural thrombus more consistent with an aneurysm than a congenital vascular malformation (Fig 4). Histopathology ruled out a neoplastic process or harmartoma.

image

Figure 4. Photomicrograph of the aneurismal mass. B =blood filled cavity; T =thrombus; arrows =mineralisation of both the vessel wall and the thrombus. Image courtesy of Alastair Foote.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Case details
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Manufacturers' addresses
  9. References

Vascular masses of the head and neck in horses are an extremely rare and poorly defined group of lesions, and little information is available regarding their classification, clinical presentation, natural history and imaging appearance (Cannon and Loh 1982; Platt 1987). Aneurysms have been reported, most frequently at the aortic root and, in the majority of cases, are diagnosed post mortem following rupture and sudden death (van der Linde-Sipman et al. 1985). To the author's knowledge there are no reports regarding the use of MDCTA to investigate an aneurysm in a horse.

Congenital vascular lesions in human infants are classified into 2 major groups: infantile haemangiomas and vascular malformations. Haemangiomas are the most frequent vascular tumour in infancy developing from a hamartomatous growth of capillaries. Vascular malformations are further classified into venous malformations (VM) (low flow) and arteriovenous malformation or fistulae (AVM/AVF) (high flow) (Mulliken and Glowacki 1982). A separate group of vascular masses are acquired, non-neoplastic vascular masses, which are most commonly extracranial carotid aneurysms (McCollum et al. 1979; Haynes et al. 1992). Frequently, secondary to trauma, they may also result from infection, congenital vascular anomalies and atherosclerotic disease (Margolis et al. 1972).

Extracranial aneurysms in the neck are very rare in human patients, accounting for only 4% of all peripheral aneurysms (El-Sabrout and Cooley 2000; Siablis et al. 2004). However, potentially fatal complications have given rise to numerous reports in the literature (Jarvis and Parker 2001; Pourhassan et al. 2007). CTA of these lesions is a routine method of achieving a definitive diagnosis in man, providing many advantages and few disadvantages over magnetic resonance angiography (MRA) and ultrasonography (Randoux et al. 2001; Patel et al. 2002).

Iodine containing contrast agents used in horses are generally small molecular weight substances that mix rapidly in plasma when administered intravascularly (Costa 2004). Although many factors influence the delivery of contrast media to tissues and resulting tissue contrast enhancement (Napel 2004; Kishimoto et al. 2008), the main patient-related factor affecting contrast enhancement in equine cases is body weight, as larger individuals have larger blood volumes, contrast agent administered to the blood compartment is diluted relatively more (Kormano et al. 1983; Heiken et al. 1995). The volume of contrast agent, injection rate and injection duration was extrapolated from that used in human patients (Bae and Heiken 2000) and calculated to optimise the vascular contrast enhancement taking into account the relatively large size and increased blood volume compared to human patients.

In conclusion, MDCTA provided a noninvasive, accurate evaluation of an aneurysm of the facial artery and yielded information regarding the nature of the vascular associations not evident on ultrasonographic images. In this 111 kg foal, contrast enhancement for visualisation of the vascular anatomy within the pharyngeal region was optimal when scan commenced around 40 s after the onset of contrast media injection delivered at 5 ml/s. Post contrast CT images are evaluated for tissue attenuation secondary to an increase in the tissue or blood vessel iodine concentration. The 2D data set can undergo multiplanar reconstruction and volumetric rendering to produce images in orthogonal planes and 3D images of exquisite detail, which reveal the anatomy of the lesion, aid the differentiation of true and pseudoaneurysm and form a basis for surgical planning in this complex anatomical area. The methodology in this case could have been improved by separating the visualisation of the venous and arterial systems. This would have aided the differentiation between aneurysm and arteriovenous malformation to be made from the CT images alone. Increased utilisation of CT in equine imaging is likely to enable improved diagnosis and surgical planning in these and other clinical conditions.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Case details
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Manufacturers' addresses
  9. References

Thanks are made to Tim Greet and Celia Marr for clinical input and to Alistair Foote for that of pathology.

Manufacturers' addresses

  1. Top of page
  2. Summary
  3. Introduction
  4. Case details
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Manufacturers' addresses
  9. References

1 Bayer Schering Pharma, Newbury, Berkshire, UK.

2 General Electric Company, Fairfield, Connecticut, USA.

3 Siemens, Frimley, Surrey, UK.

4 OsiriX DICOM viewer, osirix@osirixviewer.com

5 Ethicon, Livingston, West Lothian, UK.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Case details
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Manufacturers' addresses
  9. References