For the simulation of human tissue, a firm ballistic gelatine phantom was used. The gelatine phantom was prepared by dissolving 10 % gelatine in water of 45 °C [
7,
8]. This mixture was cooled to 4 °C before use. Six gelatine phantoms of 1.7 L were prepared. The phantoms were used at 4 °C because at this temperature the density and viscosity of this gelatine is equal to the density and viscosity of muscle tissue [
8]. Three different types of projectiles were used. From the database of the Netherlands Forensic Institute, a projectile was selected with a large amount of ferromagnetic steel. The second projectile had no ferromagnetic steel (negative control). The third projectile was completely made of ferromagnetic steel (see Table
1). The projectiles were placed in the gelatine phantom in two ways. With the first method, the projectile was suspended in the gelatine using a plastic wire while the gelatine phantom was still liquid. After 24 h the wire was removed without moving the projectile. With the second method, the projectile was pushed into the gelatine phantom in order to simulate a trajectory. All phantoms were initially imaged on a MDCT scanner (Somatom Definition Flash, Siemens Healthcare, Forchheim Germany) to assess the position of the embedded projectiles. The gelatine phantoms were placed in the middle of the table and moved in and out of the centre of the homogeneous magnetic field using the patient table. The maximum table speed was 0.2 m/s. After exposure to the magnetic field of the MRI scanner, the phantoms were rescanned with MDCT to detect and measure changes in the position of the projectiles. In between scanning sessions, the gelatine phantoms were stored at 4 °C and after the first and the longest scanning session the core temperature of the gelatine phantom was measured by introducing a thermometer in the gel. The position of the projectiles with regard to main axis (
z-axis) of the static magnetic field of the MRI scanner was either perpendicular or parallel with the long axis of the projectiles. The clinically most prevailing magnetic field strengths were used (1.5 and 3 T). Only in the first experiment an imaging pulse sequence was employed. In order to increase the MRI signal a water phantom was scanned together with the gelatine phantom. For the other phantoms, only the effect of the B0 field was studied. In the first experiment, two gelatine phantoms with embedded projectile type A and a simulated trajectory (see Table
2) were placed in a 3-T MRI scanner (Achieva, Philips Healthcare, Best, The Netherlands). The projectiles were aligned to the magnetic field lines and perpendicular to the magnetic field lines. In the second experiment, the other four gelatines were placed in a 1.5-T MRI system (Intera, Philips Healthcare, Best, The Netherlands). In gelatine 3, the projectile was placed with the first method to test for possible effects of a simulated trajectory. In gelatine 4, 5 and 6, three different projectiles were place in the two positions in the 1.5 MRI scanner (see Table
3). The effect of the magnetic field on the projectiles was illustrated by subtracting the ‘post-MRI’ MDCT from the ‘pre-MRI’ MDCT image after alignment of both images on the outer contour of the gelatine phantom.
Table 1
Description of the used projectiles
A | Sellier & Bellot | 7.62 × 39 mm | 3.59 g |
B | Sellier & Bellot | 9 mm | Not present |
C | Self-made | 9 mm | 7.6 g |
Table 2
Results of the first experiment with the 3-T MRI with gelatine number 1 and 2
1 | A | 3 T | Pushed | Parallel to the z-axis | The projectile moved back through the trajectory parallel to the z-axis |
2 | A | 3 T | Pushed | Parallel to the x-axis | The projectile rotated parallel to the z-axis and created a new trajectory parallel to the z-axis |
Table 3
Results of the second experiment with the 1.5-T MRI with gelatine 3–6
3 | A | 1.5 T | The projectile was suspended on a wire in the liquid gelatine and the wire was subsequently removed | Between the z- and x-axis | The projectile rotated parallel to the z-axis and made a new trajectory along the z-axis |
4 | A | 1.5 T | Pushed | Parallel to the x-axis | The projectile rotated parallel to the z-axis |
5 | B | 1.5 T | Pushed | Parallel to the z-axis | No visible changes |
6 | C | 1.5 T | Pushed | Parallel to the z-axis | The projectile moved a few millimetres along the trajectory |