Biplanar X-ray Imaging
What is Biplanar X-ray Imaging?
Biplanar X-ray imaging is a technique employing two X-ray sources coupled to scintillation screens to monitor the motion of radio-opaque objects. The equipment is a medical diagnostic tool used to investigate the condition of heart valves after a radio-opaque dye has been injected into the patient's heart valves. Consequently, the image produced has the heart valves clearly marked as black tubes with a lighter grey for the surrounding portions of the heart. In our research the patient is replaced by other opaque systems (industrial or biological) depicting complex flows and deformations. The figure below is a particulate flow investigation of an experimental tumbling mill system employed in the minerals industry for breaking rock.
How does it work?
X-rays produced within a contained unit, marked with the letter A, pass through the 'patient' (Perspex mill) resulting in scintillations of varying contrast on the input screen of the image intensifier, denoted by the letter B. The scintillations are then recorded by a high speed digital camera, positioned directly behind the image intensifier, which is eventually sampled at effective rates of 12, 25, 50 or 75 frames per second depending on the requirements of the cardiologist. The sampled images are also relayed to an on-line television screen, indicated by the letter C, providing a quick in-situ diagnosis of the heart valves. The usual procedure following the angiographic assessment is to download the relevant X-ray image scenes to a personal computer for further diagnostics. The biplanar mode of the angiographic unit is used to obtain two, nearly simultaneous, images of the heart (denoted by the two perpendicular image intensifiers). The images resulting from biplanar filming satisfies the minimum conditions for reconstructing 3D positions of corresponding points. The asynchronicity of the sampling between the two planes produce images that are rotated in space and time, thus precluding a natural frame by frame comparison.
Much of the x-ray imaging research has been focussed on mineral systems like tumbling mills. The Centre for minerals research (CMR) housed in the department of chemical engineering at UCT is a key collaborator in this work.
3D reconstruction from X-ray images?
The reconstruction of 3D motion from biplanar X-ray images (see above figures) is currently achieved through a semi-automated procedure. Once the key features (points, lines, shapes) are delineated, a mapping procedure governed by photogrammetry is employed to reconstruct the desired 3D features from each image pair. Before using the photogrammetric equations, the parameters governing the mapping are solved for using a control frame. The frame consists of ball bearings distributed across the volume spanned by the intersecting, conical X-ray beams. The spatial distribution of the ball bearing are determined, apriori, to a high degree of accuracy using a Coordinate measuring machine (CMM), see (a). The resulting X-ray image of the control frame is shown in (b). Using the X-ray images of the tumbling mill, (c), and the calibrated mapping equations, the necessary 3D features are reconstructed.
Algorithms for mapping X-ray images of dynamic particulate systems to 3D space has been developed within the group . Extension of these algorithms include:
- reconstruction without control frames,
- multiple particle tracking,
- phase lag correction and
- pincushion distortion modelling
I. Govender, A.T. McBride and M.S. Powell. Improved experimental tracking techniques for validating discrete element method simulations of tumbling mills. Journal of experimental Mechanics, Vol. 44, No. 6, June 2004, pp. 593-607 (2006 Peterson Award)