Plastics Help Physicists Test Performance of 3D Digital Breast Imaging Equipment

Over twenty years ago a national breast screening programme was established in England and Wales which invited all women aged between 50 and 70 to have their breasts screened for cancer every three years. The programme saves approximately 1,300 lives a year at an annual cost to the UK’s state-funded National Health Service (NHS) of £96 mn.

Recently, critics have said that the programme has led to over diagnosis. The screening technology highlights cancers, some of which may not have caused a problem if they had not been detected. However, in some cases, the risks associated with this have not been properly explained to patients before they started treatment.

Putting criticisms to one side, the NHS is a pioneer in cancer screening technology. Breast screening was one of the first national screening programmes and the process has been taken up by healthcare providers in other countries.

The commitment by the NHS to breast screening over the last 20 years or so has supported the development of advanced X-ray screening technologies. Most recently, the advent of state-of-the-art three-dimensional digital breast tomosynthesis (DBT), also known as 3D tomo, is helping clinicians achieve even more accuracy during scanning.

Implementing DBT scanning equipment in UK hospitals requires type testing—the process of comparing and testing the technical performance of different systems before recommending they be used in addition to conventional imaging technology in trials to evaluate their clinical effectiveness.

In order to test the equipment, operators use phantoms—flat acrylic plates which are designed to mimic the properties of the breast tissue—to ensure that the equipment is functioning properly before the x-rays can be taken.

A UK-based team of research physicists at The Royal Surrey County Hospital in Guildford, UK, are in the process of type testing new DBT machines. The team is part of the UK’s National Co-ordinating Centre for the Physics of Mammography.

They are using phantoms manufactured specially for 3D DBT machines by UK-based plastics machining expert Carville. Carville have supplied phantoms to the NHS for 2D equipment as well as radiology delivery devices for many years.

The phantom used for the 3D equipment is made from a cast acrylic. Acrylic is ideal for the phantoms because it has very similar attenuation characteristics to human tissue and can be used in various thicknesses—between 20 mm and 70 mm—to simulate human tissue when x-ray performance is being calibrated.

The acrylic is stressed and fully normalised (heat treated) to remove this internal stress. The material is then diamond machined and polished to produce clear flat plates. The latest acrylic phantoms contain 25 aluminium balls with a diameter of 1 mm arranged in a square grid arrangement in the centre of the acrylic plate. The balls are arranged 55 mm apart in a rectangular array, the distance being accurate to within 100 μm (0.1 mm). The reason for specifying this precision in positioning the balls was so that the images could be used to evaluate geometric distortion.

Carville is able to achieve these high levels of accuracy by encapsulating the balls between two plates and then bonding the two plates together using a proprietary diffusion bonding process. According to Carville, the process ensures a seamless joint as if the phantom was produced as a solid block. The dimensions of the phantoms are 300 mm x 240 mm x 5 mm.

These phantoms are being manufactured by Carville now and will be used by 13 regional health authorities in England and Wales.

Carville has also manufactured a block of acrylic containing just one aluminium ball with a diameter of 1 mm. The single ball in the block allows radiographers to test a feature with a particular density and shape in order to perform a regular check on their tomosynthesis images.

Image: Carville manufactures a number of phantoms for the UK’s state-funded National Health Service (NHS). One of the specialist skills here is the inclusion of aluminium balls positioned to the nearest 0.1 mm using Carville’s diffusion bonding process.

Medical Plastics News would like to thank Celia Strudley at the Royal Surrey County Hospital NHS Foundation Trust in Guildford, Surrey, UK.