Results Background and objective Methods Conclusions Results
Optimising the Angle of Lithotripsy Vikram Nagarajn, 1, Naomi L Sharma2, Benjamin W Turney2, Robin O Cleveland3 1University of Oxford Medical School, UK;; 2Oxford Stone Group, Department of Urology, Nuffield Department of Surgical Sciences, University of Oxford, UK; 3InsOtute of Biomedical Engineering, University of Oxford, UK
Background and objective
Results
Results
Shockwave Lithotripsy (SWL) is a highly effecOve first line technique used for the fragmentaOon of kidney stones. It is a non-‐invasive procedure that can be performed without anaesthesia and has few side effects. In this study, computer modelling was used to assess the energy loss of the shockwave at differing angles of entry into the body.
A 2-‐D CT image in Mimics® so\ware with the 0 and 45 degree lines in place, between which acousOc aborpOons were calculated at 50 increments.
Methods Non-‐contrast CT scans from 50 paOents booked for SWL were analysed using a customised MatLab program. Each voxel was segmented into: bone, fat, air and other so\ Ossue. CalculaOons were performed for the Storz Modulith SLX F2 lithotriptor with a focal length of 165 mm and full width aperture angle of 84.50. The acousOc absorpOon of the 3-‐dimensional shockwave cone before it reached its focal point (the site of the stone) was calculated. AbsorpOon was compared as a funcOon of shock wave angle starOng from an angle perpendicular to the back (0°) and increasing by up to 45° on the flank. A one-‐way ANOVA analysis with Bonferroni correcOon was used to calculate the opOmal angle in which to direct the shock waves to minimise acousOc absorpOon. y mm
x mm
A representaOve model with the 3-‐ D cone overlaid, using Mimics® so\ware. IniOal analysis showed that differences in the acousOc properOes of so\ Ossue were minimal and it was the presence of bone and the thickness of so\ Ossue that dominated energy delivered to the stone, therefore the 3-‐D model did not require details of so\ Ossue type. Only the voxels proximal to the stone and inside the body were included in the absorpOon calculaOon.
The acousOc absorpOon at each angle for each paOent was standardised to loss at 0° to allow for the results from different paOents to be directly comparable. It was found that a shock wave absorpOon was at a minimum at 35° (28.5% reducOon in absorpOon). However, there was significant spread in the data with the esOmated absorpOon at 35° varying from 30% to 125% of that at 0°; of parOcular note was that 5 paOents (10%) suffered more aienuaOon at 35° than at 0°. These 5 paOents all had stones in the kidney and no other predicOve parameter was found, such as amount of fat or muscle.
Conclusions Using a computaOonal model based on CT scans and a Storz Modulith lithotriptor, these results suggest that the angle at which a shock wave is directed towards a kidney can have a large impact on the acousOc energy delivered to a kidney stone with the effecOve absorpOon varying by up to a factor of four. The effect is highly paOent-‐dependent and suggests that shock wave placement should be paOent-‐specific in order to best direct the shock wave on the stone and consequently improve fragmentaOon.
Background and objective
Results
Results
Shockwave Lithotripsy (SWL) is a highly effecOve first line technique used for the fragmentaOon of kidney stones. It is a non-‐invasive procedure that can be performed without anaesthesia and has few side effects. In this study, computer modelling was used to assess the energy loss of the shockwave at differing angles of entry into the body.
A 2-‐D CT image in Mimics® so\ware with the 0 and 45 degree lines in place, between which acousOc aborpOons were calculated at 50 increments.
Methods Non-‐contrast CT scans from 50 paOents booked for SWL were analysed using a customised MatLab program. Each voxel was segmented into: bone, fat, air and other so\ Ossue. CalculaOons were performed for the Storz Modulith SLX F2 lithotriptor with a focal length of 165 mm and full width aperture angle of 84.50. The acousOc absorpOon of the 3-‐dimensional shockwave cone before it reached its focal point (the site of the stone) was calculated. AbsorpOon was compared as a funcOon of shock wave angle starOng from an angle perpendicular to the back (0°) and increasing by up to 45° on the flank. A one-‐way ANOVA analysis with Bonferroni correcOon was used to calculate the opOmal angle in which to direct the shock waves to minimise acousOc absorpOon. y mm
x mm
A representaOve model with the 3-‐ D cone overlaid, using Mimics® so\ware. IniOal analysis showed that differences in the acousOc properOes of so\ Ossue were minimal and it was the presence of bone and the thickness of so\ Ossue that dominated energy delivered to the stone, therefore the 3-‐D model did not require details of so\ Ossue type. Only the voxels proximal to the stone and inside the body were included in the absorpOon calculaOon.
The acousOc absorpOon at each angle for each paOent was standardised to loss at 0° to allow for the results from different paOents to be directly comparable. It was found that a shock wave absorpOon was at a minimum at 35° (28.5% reducOon in absorpOon). However, there was significant spread in the data with the esOmated absorpOon at 35° varying from 30% to 125% of that at 0°; of parOcular note was that 5 paOents (10%) suffered more aienuaOon at 35° than at 0°. These 5 paOents all had stones in the kidney and no other predicOve parameter was found, such as amount of fat or muscle.
Conclusions Using a computaOonal model based on CT scans and a Storz Modulith lithotriptor, these results suggest that the angle at which a shock wave is directed towards a kidney can have a large impact on the acousOc energy delivered to a kidney stone with the effecOve absorpOon varying by up to a factor of four. The effect is highly paOent-‐dependent and suggests that shock wave placement should be paOent-‐specific in order to best direct the shock wave on the stone and consequently improve fragmentaOon.
