Labs Flashcards
coulter principle
the ability to count and measure the size of particles using an electric circuit. particles are moved through a small aperture (capillary) their presence causes a narrowing of the aperture, which leads to an increase in resistance.
By using constant current, according to ohms law (URI), where R increases, U increases as well and a pulse is generated.
parts and functions of the coulter-counter
- two spaces with a small aperture in between, -capillary. contains the electrolyte solution with the cell.
- Measuring electrode, one in each of the spaces (closing the circuit and measuring the voltage between the two spaces).
- pump- draws the solution into the smaller space and out again through the capillary )
- Auxilary electrode- signals when a certain volume is reached, to initiate pumping out of the solution.
- internal discriminator- filter out noises.
- differential discriminator - , map of the size distribution of the particles.
how does the voltage pulse amplitude depend on particle size in the coulter counter. ?
it’s proportional.
Bigger particles will for, higher resistance and hence higher voltage pulses.
how do you separate the red blood cells versus white blood cells signals in a coluter counter?
when you want to measure RBC. it’s not necessary to separate them from the RBC because the numbers of WBC in negligible.
also- by hemolyze RBC and measured them separately photochemically and subtracted the result from the coulter counter measurement.
how do you separate the red blood cells versus white platelets signals in a coluter counter?
By internal discriminator since the size of the platelets is much smaller.
what is the role of a differential discriminator in coulter counter?
mapping the size distribution of the particle
why is the dilution of blood is necessary for coulter counter?
since the cells themselves do not conduct well, we dilute them with an electrolyte solution so changes in resistivity are noticed. also for practical reasons ( we don’t want the solution to dry out.
what solution is used for blood dilution in Coulter counter?
Saline. for optimal osmotic pressure, they keep the RBC in its natural size.
and also they are good conductors.
recommended ratio: 1: 63,000
Definition and components of impedance
a special type of resistance in an AC circuit that has frequency-dependent elements and contains ohmic resistance.
Electric model of the skin
for simplification, in DC only resistors play a role.
Auxillary electrode is neglected due to large A» low R.
In AC, current mostly flows through the capacitors, and resistors are neglected. Auxillary is again neglected due to large A»_space; large C»_space;small capacitive resistance.
Definition and unit of capacitive reactance
The resistance of capacitor in an AC circuit, inversely proportional to frequency, unit: ohms.
specific resistance of the skin
can be measured by multiplying resistance and surface area (note that in the case of the skin, specific resistance includes thickness in its value)
the specific capacitance of the skin
=C/A - note that specific capacitance of the skin includes thickness in its value
what component of the skin impedance dominates in the case of DC versus high-frequency AC?
DC: resistor. Because DC does not run through capacitors.
AC: Capacitors, because most of the current will flow through the capacitors.
what is the difference between the measuring and the auxiliary electrodes in skin impedance measurement?
surface area .much larger in the auxiliary which is neglected in the measurement.
definition and unit of capacitance
the charge storing ability of a capacitor C= E( A/L) unit :farad
what are the parts of a gamma radiation pulse amplitude spectrum
noise, Compton region, and photo-peak.
effect of activity on the pulse amplitude spectrum of gamma radiation isotope
increase the value of the pulse rate ( y-axis )
effect of anode voltage on the pulse amplitude spectrum of a gamma radiating isotope
the pulse amplitude spectrum shifts to the right to higher values (x-axis )
how can you determine the gamma energy of a radioactive isotope with a scintillator counter
the location of the photopeak on the x-axis indicates the energy of the gamma photon. the voltage of the photo-peak is proportional to the energy of the gamma radiation.
because during the photo effect all the energy of the photon is converted to the primary electron.
what kind of discriminator is used for acquiring the pulse amplitude spectrum?
differential discriminator.
(allows us to differentiate the pulse rate values )
which allows us to find the photopeak.
How does the minimum wavelength of the x-ray spectrum change with increasing anode voltage?
decreases.
higher voltage»_space;higher kinetic energy»_space;higher frequency spectrum»_space; Duane-hunt law
λ = k/u
k=hc/e = 1230 pmKV
How does the minimum wavelength of the x-ray spectrum change with increasing anode current?
does not change.
if the accelerating voltage is kept constant but cathode heating hence the number of emitted electrons is increased, then radiation intensity increases, but the energy of the individual electrons remains the same, and the spectrum does not shift along the wavelength axis.
what is the maximum x-ray photon energy at 50 kV anode voltage?
50 KeV
what elements are suitable as an x-ray tube anode material?
High melting point and high atomic number materials,
molybdenum- more common in medical diagnostic.
also tungsten.
what are the x-ray spectral lines characteristic of?
anode material.
why do we need to cool the x-ray tube anode?
Because it heats up due to low efficiency in the conversion of high kinetic energy to x-ray.
How does x-ray absorbance depend on the atomic number of the absorber?
photo effect is the dominant attenuation process.
photo effect mass attenuation coefficient proportional to the third power of the atomic number (Z)
Which part of the x-ray spectrum is attenuated by filtering?
the long-wavelength to void unnecessary radiation absorption, this photons and may hurt resolution due to scattering )
Which attenuation mechanism dominates in x-ray diagnostics?
photo effect
which is a better x-ray absorber, Al or Ag?
elements with a higher atomic number :Ag
