Transcript P3 medical Physics - Calthorpe Park Moodle

Welcome To The
P3
Overview
P3 1.1 X-rays
X-Rays
X-rays and Gamma rays are the electromagnetic waves with the
shortest wavelength and highest energy.
X-rays have wavelengths about the width of an atom, and
gamma rays about the width of a nucleus
In an X-ray machine,
VERY FAST
ELECTRONS
are fired in a beam
at a metal target.
Some of their
kinetic energy is
converted into Xrays
X-Ray Photography
X-Rays have enough energy to pass through lots of solid materials such
as skin, muscle and fat, but are absorbed by denser materials like bone.
Most metals absorb X-rays almost completely.
Sheet of
photographic film
X-rays passing through
blacken the film, bones
cast a shadow (looks
white)
“Barium
Meal”
Lead lined
apron
Metal is opaque to long wavelength
9low energy) X-rays
Soft tissue casts a weak shadow if
the x-ray wavelength is long enough
What are the risks from having an X-Ray?
Radiographers use the lowest energy X-rays for the shortest time
possible to get a clear picture. This reduces the radiation risk to the
patient to the minimum.
Radiographers themselves wear lead lined protective clothing and
operate the machine from a distance.
Safety !
SECURITY
Airport baggage scanners make use of
false colour images. Operators must be
trained to check each image in a few
seconds.
Body scanners
These use ‘scattered’ (ie reflected) low energy X-rays to see
beneath clothing
Coming soon to an
airport near you!
Computerised Axial Tomography
‘
(CAT scan or CT scan).
In the 1970s a method was developed to give much
higher quality images including a 3D view of
the patient. This method of scanning is called
Computerised Axial Tomography (CAT or CT).
Computerised axial tomography (CAT or CT)
In the CT scanner there is one X ray source but a large number of detectors and
the patient is placed inside this on a couch. Each detector records an image and
the source and detectors are then rotated around the patient to give views from a
variety of direction.. The couch and patient are then moved along the axis of the
machine and another set of images is taken. few minutes or up to half an hour.
This large number of images (many hundreds) are then combined by a computer
to give a composite detailed 3D image of the organs under investigation.
P3 1.2
Ultrasound
• Have frequencies greater than 20000Hz.
• Too high pitched for human hearing,
• Non-ionising
Medical images from
ultrasound
• The ultrasound is sent into the patients body.
• At each boundary between different tissues or organs
some of the ultrasound is reflected.
• The depth of each layer is calculated using the time
taken for each reflected wave to return.
• The reflected waves (echoes) are usually processed to
produce a picture of the inside of the body on a screen.
In reality, the trace on a CRO
will look more like this.
Each pulse is a reflection off a
different surface.
To determine the
time to receive any
of the echoes…
Pulses B and C indicate echoes
from surfaces inside the
patient.
1. Count the
number of boxes to
a certain pulse,
Pulse A indicates when the
pulse was sent.
2. Multiply by the
given scale.
Use
Speed = Distance
Time
A
BC
D
Breaking down kidney stones and
stones elsewhere in the body
• A high powered ultrasound wave is used
to break down kidney stones and other
stones in the body.
• The stones vibrate until they shake
themselves apart and are then easily
passed out of the body via the urethra.
Before
After
P3 1.3
Refractive index
Refraction of light at a plane surface
(a) Less to more optical dense transition (e.g. air to glass)
AIR
GLASS
normal
angle of
incidence
angle of
refraction
Light bends TOWARDS the normal.
The angle of refraction is LESS than the angle of incidence.
(b) More to less optical dense transition (e.g. water to air)
angle of
refraction
normal
angle of
incidence
WATER
AIR
Light bends AWAY FROM the normal.
The angle of refraction is GREATER than the angle of incidence.
refractive index
n = sin i
sin r
Snell’s Law
Refractive Index for
a vacuum
(and air ) = 1
air
P3 1.4 Critical angle and Endoscope
critical angle
The critical angle is the angle
of incidence in the denser
medium that results in an
angle of refraction of 90º
n=
1
sin c
where:
n is the refractive index of
the denser medium (glass in
the example opposite).
c is the critical angle.
GLASS
AIR
NORMAL
angle of
refraction
= 90º
3. Optical fibres
Optical fibre consists of two
concentric layers of different
types of glass, core and
cladding.
Light entering the inner core
always strikes the boundary of
the two glasses at an angle
that is greater than the critical
angle.
core
cladding
The Endoscope
The medical endoscope contains two bundles of fibres. One
set of fibres transmits light into a body cavity and the other
is used to return an image for observation.
Optical Fibres
is guided by T.I.R . . .
Light in at this end . . .
and comes out here.
Optical fibres are used in communications to carry signals.
(The signals are pulses of laser light)
Optical fibres are used in medicine to look inside the body.
An endoscope is made of a bunch of optical fibres to carry
light into and out of the body.
P3 1.5
Lenses
Convex
FOCUS
axis
F
PARALLEL rays
from distant object
focal length
P=1
f
dioptres
Convex lenses bring the rays together (‘converge’) at a focus.
Convex lenses are CONVERGING LENSES
Converging lens images
1. Object more than twice the focal length distant from a converging lens
object
O
2F
F
Uses: Camera and Eye
The image formed is:
Smaller than the object
(diminished)
Between the F and 2F
Inverted (upside down)
Real
F
2F
image
3. Object nearer than the principal focus
image
F
object
F
observer
Uses: Magnifying glass
The image formed is:
Larger than the object
On the same side of the lens as the object
Upright
Virtual
Concave
PARALLEL rays
from distant object
F
F
Concave lenses spread the rays out (‘diverge’).
Concave lenses are DIVERGING LENSES
Diverging lens images
Objects at all distances from a diverging lens
object
F
observer
image
Use:
Correction of short sight
The image formed is:
Smaller than the object
On the same side of the lens as the object
Upright
Virtual
P3 1.7 The
Eye
Short sighted
Causes:
Shape of eyeball……………………..
Shape of lens…………………………
Focal point…………………………….
Short-sight
Long sighted
Causes:
Shape of eyeball……………………..
Shape of lens…………………………
Focal point…………………………….
Long-sight
Comparison of Eye and Camera
Physics Clinic
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Physics is Fun!