Construction of The Phantom

In order to assess the performance of the technique, a phantom was constructed to simulate pulsatile blood flow and hence to permit the correlation of flow velocities derived from X-ray angiography with independent flow measurements using an EMF.

The phantom consisted of a variable-speed pump (Bio Medicus, Bio-Medicus Inc., 15307 Industrial Road, Minnetonka, MN 55343, USA), 4-7 m length of flexible polythene tubing, a tubular probe of an EMF (Nycotron Blood Flow Meter 376, A/S Nycotron, PO Box 425-3001, Drammen, Norway) and a solenoid to simulate a pulsatile flow waveform, which included reverse flow (fig. 8.1). Normal saline solution (0.9% sodium chloride by weight) was used throughout the flow circuit. A catheter inserted upstream of the imaging site, by means of a Y-connector, was used to inject contrast material.

Instantaneous flow rates were measured with a 9.5 mm calibre tubular flow probe placed in series, downstream of the imaging site, and a Nycotron EMF connected to a strip chart recorder. An ion chamber was placed to one side of the tube, to record the X-ray exposure times in order to synchronise the EMF flow reading with the X-ray exposure. The ion chamber output was recorded on the same paper trace as the EMF reading. Synchronisation of the frame number of the angiographic run with the EMF trace was obtained by counting pulses derived from the ion chamber on the paper trace. This validation was therefore dependent on the accuracy of the timing of the pulsed exposure from the DSA system. The number of pulses recorded on the paper trace in a ten second sequence were counted and the timing was found to be accurate to within 1 -2%. Although measurable, this error may be considered insignificant with respect to the error of the angiographic flow estimates. The distance between the end of the site of the X-ray measurements and the site of the EMF was about 10 mm. In the case of a rigid tube and incompressible fluid, there would not be any time delay, in blood flow waveforms at the two sites. We used polythene tubing, whose elastic properties will introduce a small time delay but this was ignored when comparing flow data.

The instantaneous blood flow was calculated at 0.04 second intervals, corresponding to the X-ray framing rate, and compared with the output of the EMF flowmeter. The flowmeter zero value and full scale deflection were checked before each experiment in order to compensate for DC drift. The probe was calibrated for zero flow by clamping the flexible plastic tube both up and down stream of the meter.

Paper recorder

Pump

Power Contrast injector

Fluid

reservoir Imaging region

Extension set Solenoid I T Û ] ____ T— 1 Ion Chamber Y-connector Scaling marker 2 00 mm --- 8F catheter Flow EM Flowmeter 13 0 mm Flow

Fig. 8.1. Block diagram of flow phantom.

For phantom studies, Urografin 370 (370 mg of iodine/ml) was injected into the phantom via the catheter with a power injector (Simtrac C, Siemens, Siemens Aktiengeseilschaft, Bereich Medizinische Technik, Erlangen, Germany). The injector delivered contrast material at a rate of 3 ml/sec for a 6.6 mm tube diameter (a total volume of 9 ml per injection) during the image acquisition. This was determined empirically to be the optimal injected flow rate. The point of injection was 130 mm upstream from the imaged section of tubing.

For 2D experiments, a 200 mm section of the flexible plastic tube was laid in the X-ray field of view on a perspex tray in an approximately straight line with its long axis parallel to the X-ray table. The flexible plastic tube was taped to the plastic tray to prevent movement caused by pulsatile flow. The X-ray magnification and image scaling were determined using a small metal disc and metal rods placed every 10 mm alongside the tube. The markers were visible on the X-ray image. The EMF measured flow rates are summarised in table 8.1.

In addition, data from two experiments (mean flow rates of 348 and 708 ml/min) were used to assess the effect of the distance between the catheter tip and the measurement site on the accuracy of the measurement. Data corresponding to a 100 mm section of the tube were analysed for distances ranging from 130 to 230 mm between the catheter tip and the start of the analysis site.

The same data sets were used to investigate the effect on the accuracy of the velocity measurements of reducing the vessel length analysed from 200 to 20 mm.

For analysis in 3D, six experiments were performed with the 3D phantom oriented at an angle of 15°, 33° and 35° (for tubes with internal diameters of 6.0, 4.0 and 3.0 mm respectively) to the imaging plane. The parameters of angle and calibre were purely arbitrary. The experimental details are summarised in table 8.1.

Table 8.1. Summary of phantom experiments for 2D and 3D data processing.

Experiment Data Acquisition Vessel Calibre

(mm)

Vessel Angle to Imaging Plane (°)

Mean EMF Flow (ml/min)

Peak EMF Flow (ml/min)

Peak EMF Back Flow (ml/min) 1 2D 6.6 0 349 1232 -499 II 2D 6.6 0 708 2198 -699 III 2D 6.6 0 1705 4329 -999 IV 2D 4.0 0 339 900 -94 1 3D 6.0 15 586 1295 -70 II 3D 6.0 15 1157 2093 0 III 3D 4.0 33 176 488 -38 IV 3D 4.0 33 271 713 -75 V 3D 4.0 33 687 1763 -225 VI 3D 3.0 35 229 523 -21 -147-