Image Biosensor

3.6. Image Biosensor

It has been proved beyond any reasonable doubt that a plethora of ‘clinical analyses’ are entirely based upon the assay of specific soluble marker* substances duly present in the body fluids, for instance : blood, urine, cerebrospinal fluid etc. However, the direct analyses in situ in cell or tissue level is extremely important in the domain of clinical diagnosis.** In the particular instance of neoplasms (i.e., cancerous tumours) the proper and accurate detection by means of highly sensitive and rapid identifica- tion techniques for such malignant cells are very much needed. It is, however pertinent to state here that the most proper and adequate cell diagnosis is invariably performed exclusively by the ‘visual inspec-

tion’ of duly trained, qualified, and experienced personnels or by the help of ‘flow-cytometery’.***

Quite recently, image analyzing systems has attracted an enormous interest as well as an overwhelming attention in cell diagnosis. It is essentially composed of an image sensor together with

a befitting microcomputer system. However, the image sensors may be classified into two categories, namely :

(a) XY Address Method : Wherein the emanated ‘optical signals’ of each individual address are promptly recorded by switching on the corresponding circuit, and

(b) Charge Transfer Method : It was first and foremost demonstrated by making use of a specific ‘bucket-bridge device’ (BBD) ; and now it has been duly replaced by another much more sophisticated and advanced ‘charge-coupled device’ (CCD).

* An identifying characteristic or trait that permits apparently similar materials or disease conditions to be differentiated.

** Most qualified physicians anchor their exact diagnosis based upon an elaborative clinical test(s) performed on the patient.

*** A sophisticate technique for analyzing individual cells passing through a detector system. The cells may be tagged with a MAB carrying a fluorescent label. They pass through the detector at about 10,000 cells per second. It has several clinical and research applications, including : analysis of cell size, structure, and

BIOSENSOR TECHNOLOGY

Nevertheless, the majority of image sensors being used across the globe are adequately equipped with CCD-Type Sensor.

Fig. 7.8 depicts the schematic diagram of an imaging sensor system in which CCD-type video camera has been utilized.

Personal computer CCD video camera

Controller

Image memory

Printer Microscope

Video

Color

Floppy disk

Fig. 7.8. Schematic Diagram of an Imaging Sensor System.

The CCD is nothing but an ‘integrated semiconductor chip’ which essentially consists of a series of photodiode array tubes and charge transfer circuits. Precisely, the ensuing eletrical charges being accumulated at each phototube are transferred systematically to the output terminal by carefully controlling the electric potential in the chip. However, the output pulse height invariably correlates with the desired brightness aptly observed at the corresponding photodiode. Therefore, ultimately a visual image focused on the CCD may be adequately subject to conversion into a succession of analogue pulses.

Because, the battery of photodiodes are virtually spaced and arranged with a separation of 10 µ m approximately, one would expect the same extent of image resolution. There are, of course, a plethora of distinct advantages to the unique and wonderful solid state CCD image sensor in comparison to a conventional vidicon device.

Advantages of CCD-Type Sensor : The various glaring advantages of CCD-Type Sensor are as follows : • Compactness in dimension (size)

• High degree of sensitivity • Lesser extent of distortion • Afterimage problem eliminated completely • Consumption of reasonably low power • Extended and long operational life span.