The purpose of this study is to confirm the safety and efficacy of ThinkSono Guidance, a software data collection and communication tool designed to collect ultrasound data to help detect blood clots in veins. The ThinkSono system is CE Mark approved in the European Union and in clinical use in Europe. Usually, when an ultrasound is conducted to detect blood clots in veins, a sonographer (trained technologist who conducts ultrasounds) and/or radiologist will conduct the procedure, including a compression ultrasound exam, and the scan may require a bulky cart and ultrasound equipment. ThinkSono Guidance is a mobile software application that enables other healthcare professionals such as nurses, non-radiologist physicians including general practitioners, and other allied healthcare professionals to perform the ultrasound at the point of care using guidance from the software app. This is a multi-site non-randomized, double-blinded, prospective cohort pivotal study.

Deep vein thrombosis (DVT) is well recognized to cause significant morbidity and mortality both at the time of diagnosis and post-diagnosis. With an estimated incidence of 1- 2 per 1,000 people, over to 300,000 people in the US will be diagnosed with venous thromboembolism (VTE) per annum and two-thirds of these will be DVT. However, positive cases only represent 12-25% of the total number of patients who present with suspected DVT. In other words, between 75-88% of suspected DVT cases, when fully investigated, are negative. Estimates have placed the yearly cost of VTE to the US healthcare system at up to $10 billion.

It is notoriously difficult to diagnose a DVT by clinical acumen alone. The standard approach to making a diagnosis of proximal DVT currently involves an algorithm combining pre-test probability assessment through the Wells score, and compression ultrasonography. Handheld ultrasound probes have recently become available. These probes have enabled 'app-based' compression ultrasonography to be performed without the need for bulky cart or laptop-based ultrasound machines. These new machines have a small form factor, meaning only the ultrasound probe is required for diagnostic purposes in conjunction with a smartphone or tablet.

At present, although the new handheld probes are smaller and are better suited for point of care diagnosis, they still require an experienced radiologist or sonographer to perform the compression exam. This means that these devices can only be used wherever sonographers/radiologists are based most often i.e. hospital radiology departments. However, due to recent advances in "machine learning", software is now being developed for these 'app-based' probes that can assist non-radiology specialist healthcare professionals (e.g. nurses, non-radiologist physicians, general practitioners and other allied healthcare professionals) to carry out the compression ultrasound exam with minimal training.

ThinkSono Guidance is a guidance software expected to help non-radiology specialist healthcare professionals produce compression ultrasound image data that meet or exceed the minimal image quality criteria for a remote diagnosis by an expert (e.g. radiologist). The ThinkSono system is CE Mark approved in the European Union and in clinical use in Europe.

This study is a multi-site non-randomized, double-blinded, prospective cohort pivotal study. The purpose of this study is to confirm the safety and efficacy of ThinkSono Guidance as per the intended use defined as:

ThinkSono Guidance is a guidance, data acquisition and communication tool that guides non-ultrasound-trained healthcare staff to collect point of care compression ultrasound data in the proximal deep venous system of the lower extremity for interpretation by qualified clinicians.

Inclusion Criteria:

• The participant is willing to provide written informed consent to participate in this research.
• The participant is over the age of 18
• The participant has symptoms suggestive of a deep venous thrombosis (DVT)
• The diagnostic DVT algorithm indicates that an ultrasound is needed

Exclusion Criteria:

• Patient consent not given or retracted during the study.
• Local imaging specialists fail to scan the patient or fail to produce a conclusive imaging diagnosis.
• Incomplete ThinkSono Guidance scan due to logistical or other issues such as pain, lack of patient cooperation, barriers such as a cast or other physical limitations.
• Pregnant women
• Patients who undergo a local imaging scan and initiation of new treatment for DVT (e.g. lytic agents, therapeutic anticoagulation, or intervention after the local imaging scan) before a ThinkSono scan can be performed.

Data on adverse events were collected from participants while undergoing ThinkSono Guidance scanning on the day of enrollment. Subjects were also given contact information for the study team and were able to contact them to report any adverse events at any point up to 12 months after the date of enrollment.

Though not masked, the study is double blinded. Double blinding will be ensured by the ThinkSono software, as it will not be giving any indication of diagnosis or image quality to the operator. The comparison scan will be done by a qualified imaging specialist as per the standard of care at the local institution, but they will also not know the results of the ThinkSono scan. There will also be "remote qualified clinicians" reviewing the ThinkSono data (separately from local staff) who also will not know the results of the local imaging specialist. Only at the final data assessment upon study completion will outcome assessors be aware of ThinkSono device results and local site imaging results.