CT Users Group

GC Stevens¹, NP Rowles¹, RT Foy², RJ Loader¹, N Barua³, J Palmer³

1. Clinical & Radiation Physics, Plymouth Hospitals NHS Trust

2. Medical Physics, Royal Cornwall NHS Trust

3. Neurosciences, Plymouth Hospitals NHS Trust

Abstract

Novel shielding was designed and incorporated into the Ceretom. The scatter distribution was measured with a perspex CT head and chest phantom to mimic patient attenuation and scatter conditions, using a single slice at maximum rated tube current (7 mA), with the shields in situ. The scatter distribution was measured at sequential 1 m distances from the unit's bore to build a matrix showing the secondary scatter over 8 m x 8 m. This matrix was then used to simulate the dose distribution across the ward, and assuming an inverse quare relationship to dose for points outside the measured area. The dose to adjacent patients and staff members was used together with a planning dose constraint of 0.3 mSv to find the number of scans allowed per bed per year. The shielding significantly reduced scatter laterally and to the rear of the unit, without this, a practical solution was impossible. The ward dose distribution simulation showed a maximum of 65 scans per bed per year could be permitted whilst adhering to the planning dose constraint of a 0.3 mSv dose to the public, (4 m bed separation and 60 day maximum patient stay).

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