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Chapter: Medical Electronics - Bio-Chemical and Non Electrical Parameter Measurement

Blood Cell Counter

The blood cell counter count the number of RBC or WBC per unit of volume of blood using either of two method:

BLOOD CELL COUNTER

 

•   The blood cell counter count the number of RBC or WBC per unit of volume of blood using either of two method:

 

–   Electrical method called aperture impedance change

 

–  Optical method called flow cytometry

 

Aperture impedance change

 

         When blood is diluted in the proper type of solution, the electrical resistivity of blood cells (ρc) is higher then thhe resistivity of the surrounding fluid (ρf)

 

         By contriving a situation in which these resistivities can be differentiated from each other, we can count cells

 

Blood cell sensing

 

         The sensor consist of a two-chamber vessel in which the dilute incoming blood is on one side of barrier, and the waste blood to be discarded is on the other

 

         A hole with a small diameter (50μm) is placed in the partition between the tow halves of the cell

         Ohmmeter measure the change on the resistance when the blood cell p ass the aperture

 


1. COULTER COUNTER

 

         Constant current source (CCS) and voltage amplifier replace the ohm meter

•   RA is the resistance of the aperture and will be either high or low, depending on whether or not the blood cell is inside the aperture.

Amplifier convert the current pulse to voltage pulse

 


 

2. FLOW CYTOMETRY CELL COUNTERS

 

Optical flow cytometry sensing

 

         The optical cytometry sensor consists of a quartz sensing sheath designed with a

 

–  hydrodynamic focusing region

–  cell path region that passes only a single cell at time.

 

         Focusing is done by decreasing the diameter of the aperture.

 

         Light source is (He-Ne) Laser

 

         Two Photodetectors (photosensors)

 

–  Photodetector A detects forward scatted light

–  Photodetector B detects orthogonal scatted light

 

         blood sample enters the analyzer

–  Optical counter → WBC count

–  Colorimeter → hemoglobin

–  Optical flow sensor → RBC count


The blood is actually split into different chambers, where in each chamber it is diluted / mixed to differentiate different cell types. WBC and RBC are separated (using lysing)


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