Ultrasound! Flashcards
Name the most common use for ultrasound in ROP? Why is ultrasound useful for this?
Brachy!
Useful because you can see in real-time, no dose delivered.
What frequencies are used in ultrasound?
2-10 MHz… for higher resolution imaging can be up to 50MHz
What does an ultrasound actually measure?
Changes in pressure as a function of time
Sound waves are longitudinal or transverse?
longitudinal. Compression and Rarefaction
What is an elastic media?
One that obeys Hooke’s law F=-kx.
What is compressibility?
measure of the relative volume change of a medium as a response to isotropic pressure. Written as beta.
What is the Bulk Modulus?
inverse of compressibility, k
define the speed of sound in terms of bulk modulus
c=sqrt(k/density)
what is the speed of sound in human tissue?
1540 m/s
Why do we generally assume speed of sounds increases with increasing density, even though they are inverse?
because the bulk modulus also generally increases accordingly
define decibels in terms on intensities and in terms of pressures
dB = 10log(I2/I1) = 10log(P2^2/P1^2)=20log(P2/P1)
What is a half value thickness in terms of decibels?
medium thickness required to reduce the intensity of the beam by half (-3dB)
What is acoustic Impedence?
can be thought of as the stiffness of a compressible medium. How much of an US wave is transmitted or reflected. Z=c*denity
What determines reflection and transmission at an interface?
the idfference in the two acoustic impedences. larges difference, more reflection
When do we see specular relfection or nonspecular?
specular occurs at a smooth surface. this happens when the wavelength of the US wave is much greater than the structural variations in the tissue.. Non-spec occures when the imperfection are about a wavelength
reflection coefficient?
describes what fraction of intensity incident on an interface that is reflected
Ri = Ir/Ii = (Z2cosxi - Z1cosxt / Z2cosx1 + Z1cosxt )^2
Pressure amplitude transmission coef
Rp=Pr/Pi = sqrt (Reflection coef)
Intentisy transmission coef
Ti = 1 - Ri
What percentages of US pressure in reflected at:
Bone-Muscle?
Fat-Muscle?
64%
12%
What is speckle?
deterministic interference pattern formed by a medium containing sub-resolution scattering objects at fixed locations. Does not ocrrespond to structure but to interference patterns as recieved by the transducer.
When a beam is not incident perpendicular, what dictates how w wave will refract?
snells law:
c1sinx1=c2sinx2
AT when angle of incidence does max reflection occur?
Perpendicular
What is the attenuation coef? unit?
relative intensity loss per distance travelled for a given medium. dB/cm.
attenuation pressure formula?
p = p0exp(-alphaz), alpha is attenuation coef.
Rule of thumb for attn coef
1dB per cm travelled per MHz
Do higher frequencies attenuate more rapidly?
yes
how is the focal region defined?
region over which the beam width is no more than two times the width at focal spot.
Near field length=?
d^2/(4*wavelength), d is the transducer diameter
Far field angle of divergence?
sinx=1.22*lambda/d
how waould you produce a shallower focal zone?
by activating the outer elements in the array before the inner ones.
What are “side lobes”?
extra constructive interference away from the central axis.
what is axial resolution?
half the spatial pulse length. spatial pulse length = N*wavelength
at what distance from the transducer is lateral resolution optimized?
at focal region
define elevation resolution?
slice thickness perpendicular to imaging plane
what is pulse repetition frequency (PRF) and pulse rep period (PRP)?
number of pulses per second, and time between sequential pulses
What is Duty Factor?
Pulse Duration/PRP. uaualy around 1%
What is A-Mode Imaging and why do we use it?
Amplitude Mode Imagin! just displays pulse recieved. used to locate interfaces, cracks
What is B-Mode imaging?
typical ultrasouns
What is the frame rate in B-Mode?
number of line scans to complete a single B-Mode image multiplied by the lines per second.
what is M-mode imaging?
Motion mode is the echo from a single beam passing through moving anatomy and displayed as a function of itme.
Which assumptions cause artifacts in US?
- speed of sound is constant
- no scatter
- reflection only comes from along central axis
Name 4 types of US aritfacts
- reverberation
- shadowing/attenuation
- enhancement
- mirroring