Embolic particles carried by the bloodstream can cause strokes and other circulatory disorders. During surgery, emboli may occur when clots form in the blood, air enters into the bloodstream, or tissue fragments break loose or become dislodged. The blood carries the emboli into increasingly smaller arteries until they become lodged and obstruct the flow of blood. The amount of damage that results depends on the size of the emboli, the point in which it lodges in the blood flow, the amount of blood leaking around the emboli, and how blood is supplied by collateral paths around the obstruction. The resulting functional deficit depends in part on the composition of the emboli. For example, air may be reabsorbed in a short time, clots may dissolve, (particularly if blood-thinning drugs are present), while particles composed of plaque and body tissue may not dissolve at all. Therefore, it is important to have non-invasive instrumentation that can accurately detect the presence of emboli, determine their composition, and estimate their size so that appropriate medical management decisions can be made.
Instrumentation for detecting and classifying emboli based on broadband ultrasound is described in U.S. Pat. No. 5,441,051, the disclosure of which is incorporated here by reference, and in U.S. patent application Ser. No. 11/429,432, filed on May 8, 2006, the disclosure of which is also incorporated here by reference. When an emboli passes through an ultrasound beam, the change in acoustic reflectivity causes a reflection which can be detected by an ultrasound receiver. The number of embolic events can be counted by monitoring the number of reflected echoes that exceed a predetermined threshold. An embolus may be characterized by composition and size in order to classify it for example as a gas or a fat particle based on detailed analysis of the echo signal for each embolus.
Ultrasonic echoes reflected from a moving object are typically processed in order to remove reflections from stationary objects that are of less interest, enhance signal to noise ratio, and reduce false object detections. But the accuracy of the detection and characterization of a moving object based on ultrasonic echo signal processing apparatus depends on how well that apparatus is calibrated. It is known to use “phantoms” to calibrate ultrasonic echo signal processing apparatus. Phantoms are test objects that closely mimic the ultrasonic propagative/reflective characteristics of certain materials to be analyzed such as human tissue, food products, fluids, etc. Phantom test objects typically have well known ultrasonic propagation and/or reflection characteristics. If there is a difference between determined characteristics of reflected signals from phantom objects provided by the ultrasonic echo signal processing apparatus and the well known characteristics, then the ultrasonic echo signal processing apparatus may be adjusted or calibrated to reduce that difference. One or more apparatus parameters may be adjusted such as power level/gain, frequency, phase, etc.
While phantom-based calibration methods may be effective in certain applications, such as phased-array scanners used for imaging tissue structures, the measurement of physical quantities such as the size and composition of emboli flowing through a tube of a heart-lung machine requires a higher degree of precision. For example, phantom-based calibrations are typically performed infrequently and off-line. But this is a problem for applications like blood circuit monitors, where emboli in the blood stream must often be detected on a continuous basis while in use, counted, and classified with high accuracy and speed. For example, the assignee of this application offers an EDAC™ quantifier device that detects individual micro-emboli at rates over 1000 per second, identifies micro-emboli from below 10 microns to up to 1000 microns, and instantly reports relevant data to the user. Accuracy is very important, and as a result, calibration should be performed regularly—continuously would be best, and if possible, calibration should be performed on-line and automatically.
Another issue relates to the shape of the reference object used for calibration. Although different shapes may be used, some shapes require extensive and careful alignment in order for the calibration results to be accurate. In some instances, special alignment procedures and adjustments may be necessary when orienting the reference object for calibration. This kind of precision handling is undesirable in many applications, in particular those where calibration accuracy is critical and/or where the skill set of the user may not include knowledge of proper reference object alignment/orientation.