P3 Topic Two - X-rays and ECGS

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• Created by: Bethfr
• Created on: 24-02-13 21:21

Electron Beams

Thermionic Emission - releasing electrons from a Surface to produce electron beams

Filament acts as cathode, is heated so more energy is given to its electrons. When have enough, the electons 'boil off': thermionic emission. The electrons are then accelerated toward the anode by a potential difference (voltage) between the two electrodes. To produce x rays, fire the electrons at a metal anode, and some kinetic energy is converted into x rays. Glass tube contains a vacuum to prevent the electrons colliding with air particles which would knock them off target and decrease their energy. A lead casing surrounds the tube to absorb some of the x rays so they only aim at the object needed.

kinetic energy of electrons =electron charge (e) x accelerating potential difference(V)              (in joules, J)        (coloumbs, C)              (volts, V)

KE = 1/2 x m x velocity squared = e x v            electron charge = -1.6 x 10^-19 C

current (I) = no. of particles per second (N) x charge on each particle (q)

(amperes, A)        1/second 1/s                        coloumbs, C

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X ray Intensity and Absorption

X rays have high frequency - high energy - ionising to body cells (remove electrons).

Intensity of radiation depends on the distance from source. E.g. if you move twice as far from the source the radiation is being spread over four times the area, so you only recieve one quarter the intensity. This is an inverse square relationship.

Generally, the more dense a material, the more radiation it absorbs. Absorption is affected also by the thickeness of the material its travelling through. Lead and concrete are used to reduce exposure to x rays like in hospitals.

e.g. x ray tube casing is used to absorb the xrays. The thicker the material, the greater the reduction in x ray intensity passing out the other side of the material - more is absorbed.

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X ray imaging

X ray flouroscopes use x-rays to create moving images of the inside of the body:

Basic flouroscopy: placing a patient between a flourescent screen and an x ray source. Different amounts absorbed by body. Intenisty varies depending on what they passed through. X rays hit screen and absorbed and flouresces (gives off light) to show live image on a screen. Image works thanks to difference in x ray intensity - exposing the different bits of the screento different amounts of xrays. Higher the intensity of xrays, brighter the screen.

Modern flouroscopy  uses an image intensifier to increase the brightness of image up to 5000 times more - means a lower does of x rays needs to be given to the patient. The screen is now attached to a pc so images can be recorded.

Flouroscopy is used to diagnose problems in the way organs are functioning e.g. looking at movement through the gastrotestinal tract or blood flow. Because xrays pass easily through soft tissue, the patient is given a contrast medium (by ingestion or injection) which improves the contrast of the image by making the soft tissue more visible.

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More X ray Imaging

CAT scans produce an image of a 2D slice through the body.

An xray beam rotates around the body and is picked up by the thousands of detectors on the machine. Computer then works out how many of the x rays are absorbed, and produces a high quality image - useful for diagnosing subtle problems. CAT scans can also make 3D images by stacking the individual 2D slices. CAT scans used for tumors and cancer.

Xrays are harmful - ionising. Still used though as often best choice for treatment and diagnosis. Provide clear diagnosis - risks are better than treating a patient wrong. Non invasive procedure and quick.

Safer non ionising techniques are used for imaging and treatment where possible e.g. ultrasound, however image quality usually worse so not used if diagnosing medical conditions.

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Electricity and the Body

Muscle cells generate potential difference. Between the inside and outside of a muscle cell, there is voltage/potential difference. The potential difference across a cell membrane of a muscle cell at rest: resting potential. can be measured with tiny needle electrodes. Muscle resting potential is -70mV (millivolts) ish.When a muscle cell is stimulated by an electrical signal, the potential difference changes from -70mV to +40mV - action potential - this passes down the cell, making it contract.

Electrocardiographs measure the action potentials of the heart. When the heart beats, action potenetial passes through the atria (atriums) making them contract. Split second later, another action potential passes through the heart's ventricles, so they contract. Once the action potential has passed, the muscle relaxes. These action potentials produce a weak electrical signal on the skin, which can be measured with an electrocardiograph: electrodes stuck to chest, arms, or legs. Patient has to lie/sit and relax for accuracy.

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Pacemakers and Pulse Oximeters

The hearts natural pacemaker directly controls heart rate, a group of cells in right atrium wall which make electrical signals that pulse 70 times per min. these signals spread through atria and make atria contract with action potential. the signals then pass on through ventricles, they contract too. If natural pacemaker isnt fast enough/pulses irregularly/problems with the electrical signals, then get fitted with artifical pacemaker: keeps heart beating steady using small electric impulses, sent via electrodes attached to heart to stimulate it. Only need small incision in minor surgery to insert. however, battery powered. Many modern pacemakers settings changed externally without surgery, and can monitor breathing and temp - use this info to adjust your heart rate to your activity.

Pulse oximetry measures oxygen level carried by haemoglobin in blood, monitors health before/after surgery. Haemoglobin carries 02 from lungs to cells, pigments blood. Haemoglobin changes colour depending on 02 content - rich in O2-bright red-oxyhaemoglobin; gievn up 02-purply-reduced haemoglobin. A pulse oximeter (finger clips) has transmitter which emits 2 beams of red light, and photo detector to measure light, both placed on either side of a thin body part e.g. finger. Light beams pass though tissue, some light absorbed by blood, reducing light amount detected. Amount of light absorption depends on blood colour-oxyhaemoglobin content. Arteries:healthy people have 95% oxyhaemoglobin and no more than 5% reduced haemoglobin. Reflection pulse oximetry reflects light off red blood cells instead of shining light through body part.

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