Case of the Month – June

Thyroid Volumetry using freehand 3D ultrasound (tomographic Ultrasound): exemplary comparison with 2D model and MRI

Authors:

Markus Krönke, Resident Physician, Nuclear Medicine, Klinikums rechts der Isar, Technische Universität München, Munich, Germany
Dr. Thomas Wendler, Senior Scientist, Computer Aided Medical Procedures and Augmented Reality, Technische Universität München, Munich, Germany

Introduction

Thyroid volumetry plays an essential role in the calculation of the applied dose of radioiodine in patients with Morbus Basedow and local autonomies. Currently, three ultrasound (US) measurements taken by hand in two different planes are used to calculate volume using different mathematical models. Literature shows that these measurements do not only show high intra-observer and inter-observer variation, but may also underestimate the overall volume of the thyroid by up to 30%. Tomographic ultrasound (tUS) might be a user-independent alternative that yields smaller errors. In order to validate this, we compared tUS volumetry with the conventional 2D US approach and calculations based on magnetic resonance images (MRI) as the gold standard.

Case description

A patient undergoing MRI of the neck was recruited to receive 2D US and tUS examination. MRI (T1 vibe sequence with 0.625×0.625x1mm3 resolution, 1.5 T) and tUS were segmented using an interactive segmentation software that is based on Graph Cuts (Figure 1 & Figure 2). The conventional Brunn formula was used to calculate the thyroid volume with 2D US. The calculated volumes of the thyroid were 13.0, 13.1 and 12.8 cm3 for MRI, tUS and the 2D US respectively. The intra-observer variation in this patient was 5% for 2D US, based on three consecutive measurements.

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Discussion

Thyroid volumetry with tUS and MR showed an excellent match with a difference smaller than 1%. In this particular case, also 2D US showed a small difference to MRI with an average difference of <2% and a standard deviation of 5%. If these results are confirmed on a larger patient collective, tUS could have a major impact on dosimetry and possibly on the effectiveness of radioisotope therapy.

Case of the Month – May

Carotid Plaque Volume (CPV) as an indicator of high-risk plaque.

Authors:

Miss Alison Phair, Vascular Specialist Registrar and Clinical Research Fellow. BSc (Hons), MBBS (Hons), MRCS. Department of Academic Surgery, University of Manchester.

Introduction

The degree of stenosis has been the method of defining the necessity of surgery in symptomatic carotid stenosis since the NASCET and ECST randomized trials of the 1990’s. But a greater degree of stenosis does not correlate to a greater risk of stroke. Atherosclerotic plaque burden in the coronary and carotid arteries is reported to be a more important risk factor for future event than severity of stenosis. Measuring carotid plaque volume (CPV), as a measure of atherosclerotic burden, may be of more importance in predicting likelihood of further event than degree of stenosis alone. The case presented highlights the added benefit of assessing CPV prior to surgery to appreciate the risk of stroke.

Case description

A 75-year-old male presented with a history of left arm weakness and left facial droop which lasted 20 minutes and had fully resolved. His past medical history included Atrial Fibrillation currently treated with Apixiban and adequately rate controlled. A computed tomography (CT) scan of the brain showed no evidence of an acute infarct so the diagnosis of a single Transient Ischaemic Attack (TIA) was made. Carotid Doppler examination showed a right internal carotid artery stenosis of >70% based on grey scale measurement and peak systolic velocity.
The plaque was noted to be composed of echo-lucent material on standard duplex (Figure 1). 3-D tomographic ultrasound (tUS) followed by contrast enhanced tomographic ultrasound (CEtUS) assessment was performed as part of recruitment to an ongoing research study. On tUS the full extent of the large, echo-lucent plaque could be appreciated (Figure 2). Measurement of the CPV was performed based on these assessments.
The patient underwent a right carotid endarterectomy (CEA) within 24 hours of admission. Intraoperative findings confirmed a plaque composed of liquefied lipid with intra-plaque haemorrhage (Figure 3). The surgeon commented in the operative notes that this was a plaque with extremely adverse features at high risk of causing a future event had surgery not been performed promptly.
The CPV measured from the endarterectomy specimen was exceptionally high at 1.754cm3. Carotid plaque volume as calculated by tUS was accurate to within 0.1cm3 of the endarterectomy specimen.

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Discussion

Based on the clinical history of TIA and the identification of significant carotid stenosis contralateral to the patient’s symptoms, prompt surgical management was unarguably the correct course of action in this case. However, the standard two-dimensional duplex did not fully appreciate the high-risk nature of this plaque, as unequivocally seen at endarterectomy.
In this unit, carotid plaque volume has been measured in 400 patients prior to endarterectomy. There are statistically significant differences between the volumes measured in symptomatic and asymptomatic plaques; symptomatic plaques having higher plaque volumes (Vascular Society, Nov 2018). This carotid plaque was in the highest quartile for volume, a feature which correlates with high risk plaque features, as was proved by the visual assessment completed by the surgical team.
Adding CPV to the information provided by standard duplex could supplement the clinical team’s understanding of the risk of stroke from a plaque. Furthermore, tUS calculation of Carotid plaque volume can be accurately performed prior to surgery. CPV could be used to predict those symptomatic plaques at greatest embolic potential pre-operatively so that expedited surgical management can be arranged.

CR-05-2019-Figure_3

Case of the Month – April

Colour Duplex Ultrasound combined with 3D Tomographic Ultrasound as a potential sole imaging modality prior to lower limb arterial reconstruction

Authors:

Dr Cleona Gray PhD Chief Vascular Physiologist, Mater Misericordiae University Hospital, Dublin 7
Mr Edward Mulkern PhD FRCSEd, Consultant Vascular Surgeon, Mater Misericordiae University Hospital, Dublin 7

Introduction

The limitations and complications associated with conventional angiography (CA) and MR Angiography (MRA) prior to lower limb revascularization have led to an increased need for a suitable alternative imaging modality.

CR-04-2019-Figure 1

Colour Duplex Ultrasound (CDU) with 3D Tomographic Ultrasound may be an appropriate alternative modality. In patients with limb ischemia a well-performed CDU offers several advantages over traditionally used imaging modalities. CDU is noninvasive and does not require the use of nephrotoxic agents. It is readily available with color flow and waveform analysis providing a better estimation of the hemodynamic significance of disease unlike MR angiography which risks overestimation of disease. It allows visualization of the entire artery and not only of the lumen of the vessel, enabling plaque characterization. Unlike conventional Angiography Color flow and power Doppler techniques have the ability to identify patent native arteries subjected to chronic disease with low flow states. CDU with 3D Tomographic Ultrasound performed by a skilled Vascular Physiologist may represent an alternative to conventional angiography for patients with lower limb ischemia.

Case description

65-year-old female presented in December 2018 with rest pain in the left lower limb. Ankle brachial pressures were found to be 0.35 on the right and 0.38 on the left. The patient went on to have a catheter angiogram. The supra-inguinal vessels and common femoral artery were widely patent. The superficial femoral artery was occluded from its origin and reconstituted at the level of the adductor canal. The popliteal artery was thought to be significantly diseased. The anterior tibial artery was diseased (Figure 1) and the posterior tibial artery was found to be occluded. CDU and Tomographic Ultrasound were used to quantify the disease in the infra-inguinal vessels and identify a suitable target vessel for bypass. The Popliteal artery and the Anterior Tibial artery were found to be widely patent. The Anterior Tibial artery was patent throughout the calf and crossed the ankle (Figure 2).

CR-04-2019-Figure 2

The posterior tibial artery was occluded throughout its’ length. The Long Saphenous vein and its tributaries was also mapped prior to the surgery using tomographic ultrasound (Figure 3).

CR-04-2019-Figure 3The patient underwent left common femoral artery to above knee popliteal bypass surgery using the mapped long saphenous vein as a conduit. Despite the angiography suggesting significant popliteal disease, a decision was made to use the above knee popliteal artery based on the duplex imaging combined with 3D Tomographic Ultrasound.
Post-surgery the graft was found to be widely patent throughout. The patient was discharged free of rest pain.

Discussion

CDU is already considered to be a suitable alternative to conventional angiography in selected cases, the addition of 3D tomographic ultrasound allows for safe high-quality imaging with the advantage of images similar to conventional angiography. CDU along with 3D Tomographic Ultrasound may be an alternative imaging modality prior to lower limb reconstruction and may give additional information over and above that obtained by more traditional forms of angiography, particularly in relation to the flow characteristics in target vessels.