chapter 1 Flashcards
what is the equation for image resolution?
image resolution (mmpixel^-1)= actual size (mm)/ number of pixels (pixel)
what is a pixel?
the smallest thing you can resolve in a digital image
what is the wave equation?
velocity= frequency x wavelength
how are x-rays produced?
high speed electrons are fired onto a photographic cassette through the body. Bones absorb the radiation more than the less dense soft tissue and so a negative image is shown on the picture
how do MRI’S work?
Magnetic resonance imaging- you lie in a strong magnetic field and radio-frequency waves are directed at your body. The water molecules in your body act was tiny magnets which are very sensitive to magnetic fields- the protons line up in the same direction.Short bursts of radio waves are sent to certain areas of the body, knocking protons out of alignment.when the radio waves are turned off, the protons realign and in doing so send out radio signals which are picked up by receivers. Different tissues form different relaxation times so that areas of the body can be distinguished on the image
how do gamma ray tracers work?
a radioactive source is injected or swallowed into/by the patient. this radiation penetrates the body tissues and can be detected externally. The areas which absorb lots of glucose eg the thyroid emit more radiation and so are detected
how does an STM work?
Scanning Tunneling Microscope. Non- optical microscope that works by scanning an electrical probe top over the surface of a sample at a constant spacing. This allows for a 3D picture of the surface to be created.
wavelength of visible light?
400nm (4x10^-7)-> 700nm(7x10^-7)
red–> violet
protons and neutrons are made of what?
quarks
what is the size of quarks/electrons?
10^-18m
order of the electromagnetic spectrum?
radio waves, microwaves, infra-red waves, visible light, UV, x-rays, gamma
wavelength of microwaves?
10^-1 -> 10^-3 m
wavelength of radiowaves?
10^6–> 10^-1 m
wavelength of infrared?
10^-3–> 10^-6 m
wavelength of uv?
10^-7 –> 10^-8 m
wavelength of x-rays?
10^-8 –> 10^-10 m
wavelength of gamma rays?
10^-10–> 10^-12 m
define period
the time taken for 1 complete wave
frequency=
1/T
how does ultrasound imaging work?
a change in density shows how much will be transmitted and how much will be reflected so you put a gel on the stomach with the same density as the skin so that the ultrasound can pass through and not be reflected. short pulses of sound are sent out from the scanner which is held in contact with the mother’s belly.
the pulses are reflected back where the density of the tissue changes. bone is a good reflector of sound.sound comes back to the scanner from the different parts of the baby, each part producing its own characteristic reflection
what’s acoustic independence (AI)?
dependent on the density of the material in which sound is propagated- the greater the independence the denser the material
where do reflections come from in ultrasound imaging?
from the interface of different AI’s- the greater change pf the AI, the more signal reflected
what’s a transducer?
transmits and receives the ultrasound waves?
what produces ultrasound waves? how?
ultrasound pulse is a pulse containing just a few oscillations produced by a piezoelectric crystal. Ultrasound is generated by rapid electric oscillations applied to a piezoelectric crystal. When the ultrasound pulse returns and vibrates the crystal, it produces a signal that is picked up by the scanner
what type of waves are ultrasound waves?
longitudinal
can you explain how the darkness level on the grey scale is determined by the imaging method?
the number for each pixel decides what shade of grey that pixel will be printed. Darker regions correspond with larger numbers in the array (the date for the image)
what’s the speed of sound in air?
330-340 ms^-1
what is the speed of sound in soft human tissue?
1500ms^-1 (0.67 micro s per mm)
why must the frequency of the ultrasound wave be greater than 1MHz?
because the depths to be ‘sounded’ are only about 100mm, and ultrasound travels at high speed taking about 70 micro seconds to go 100mm in soft tissue. Thus the delay times are short. The pulses must be only about 1 micro second long if returning pulses are not to overlap outgoing ones. The time of an oscillation of the wave cannot be longer than the time the pulse exists so the frequency must exceed 1MHz= 1million oscillations per second
why is ultrasound ultra-inaudible?
because the highest frequency you can hear is about 20KHz
what’s ultrasounds frequency and wavelength?
high f and short wavelength
in binary what does 1 represent?
a signal being on- high voltage
in binary what does 0 represent?
a signal being off- low voltage
what is a single binary digit called?
a bit
what is a group of 8 binary digits called?
a byte - the smallest meaningful units of computer memory
what does the number of bits in a string (I) determine?
how many alternatives that string can code for
how many alternatives does a single bit have?
2 alternatives (0 and 1)
how many alternatives does on byte (8 bits) have?
256 alternatives
number of alternatives=
2 ^ Number of bits or N= 2^I
number of bits=
log2 (number of alternatives) or I =log2N
1 pixel in a black and white image=
1 bit
in a grey scale image 1 pixel=
8 bits
how many shades of grey are there?
2^8= 256
what are the primary colours of light?
red, green and blue
for a colour image how many bits for each pixel?
256/8 = 24
how many different colours are there?
2^24= 16777216
what will add values to the pixel number?
increasing the brightness of an image
what does adding a fixed positive value do to the image?
it makes it brighter,but it doesn’t change the difference between the dark and light regions (contrast)
what does multiplying by a fixed value greater than 1 do to the image?
makes the image brighter and increases the contrast
what reduces noise?
replace each pixel with the median of itself and the 8 pixels surrounding it- the result is that any ‘odd’ (very high or low) values are removed and the image is smoother
what is noise?
unwanted interference affecting a signal. in images this is usually bright or dark spots on the picture
which parts of the eye are involved in bending the light?
cornea - transparent curved front surface and the lens which adjusts the focus to produce a sharp image on the retina
what are the light sensitive elements in the eye?
cones and rods- emit signals that go to the brain. cones detect colours and work best in bright light.rods are sensitive to dim light but do not distinguish colours
how many rods in the eye?
more than 100million
what is the function of the fovea?
only cones, where the eye detects fine detail- central vision eg reading and driving
what does the lens look like when light is from a near object?
short and fat
what does the lens look like when light is from a distant object?
longer and thinner
when light comes from a distant source, what happens to the rays and wavefronts?
they are parallel to each other
draw a ray reflected off a mirror including the normal, incident ray, reflected ray and the wave fronts
look in book
draw a virtual image
look in book
when will the image be at the focal point?
when the light source is distant and wavefronts are parallel
when won’t the image be at the focal point?
if it’s a close source then the image will be further away than the len’s focal point
what happens if the source is at the focal point?
there will be no curvature after and will produce parallel wavefronts and there would be a very distant image of the source which won’t be seen
power of lens (d), Dioptres=
1/f, the focal length has to be in metres as dioptres =m^-1
curvature before the lens=
1/u where u is the object distance - from source to lens
curvature after the lens=
1/v where v is the image distance
1/v=
1/u + 1/f
greater focal length means the lens has less
curvature
how do you find the refractive index of a material that is not air/.
n1sinQ1=nsinqQ2
snell’s law:
n=sini/sinr
refractive index=
speed in free space 3x 10^8m/s / speed in the medium
n= ci/cr
define refraction
when light passes from air into a more dense medium it slows down. this causes the light to bend. the ratio of the speed of light in air to its speed in the medium is called the refractive index of the medium
when a ray passes into a denser medium why does the ray bend towards the normal?
because one edge of the wave will slow down before the others which will keep travelling faster so will change direction
define linear magnification
defined as the ratio of the image height to the object height m=v/u
what does short-sighted mean and how is it corrected?
difficulty focusing on distant objects as they form an image before the retina as it’s adding too much curvature. They need a diverging lens to correct it which removes curvature before the lens.
what does long-sighted mean and how is it corrected?
difficulty focusing on near objects as they form an image after the retina as there’s not enough curvature. Need a convex (converging) lens to focus the image on the retina
what is the laplace rule?
method to finding edges:
1-multiply a pixel by 4
2- subtract the value of the pixels immediately above, below and to the left and right of it
the result is that any pixel not on an edge goes black so you’re left with just the edges
