Ultrasound!

Question 1

Name the most common use for ultrasound in ROP? Why is ultrasound useful for this?

Answer 1

Brachy!

Useful because you can see in real-time, no dose delivered.

Question 2

What frequencies are used in ultrasound?

Answer 2

2-10 MHz… for higher resolution imaging can be up to 50MHz

Question 3

What does an ultrasound actually measure?

Answer 3

Changes in pressure as a function of time

Question 4

Sound waves are longitudinal or transverse?

Answer 4

longitudinal. Compression and Rarefaction

Question 5

What is an elastic media?

Answer 5

One that obeys Hooke’s law F=-kx.

Question 6

What is compressibility?

Answer 6

measure of the relative volume change of a medium as a response to isotropic pressure. Written as beta.

Question 7

What is the Bulk Modulus?

Answer 7

inverse of compressibility, k

Question 8

define the speed of sound in terms of bulk modulus

Answer 8

c=sqrt(k/density)

Question 9

what is the speed of sound in human tissue?

Answer 9

1540 m/s

Question 10

Why do we generally assume speed of sounds increases with increasing density, even though they are inverse?

Answer 10

because the bulk modulus also generally increases accordingly

Question 11

define decibels in terms on intensities and in terms of pressures

Answer 11

dB = 10log(I2/I1) = 10log(P2^2/P1^2)=20log(P2/P1)

Question 12

What is a half value thickness in terms of decibels?

Answer 12

medium thickness required to reduce the intensity of the beam by half (-3dB)

Question 13

What is acoustic Impedence?

Answer 13

can be thought of as the stiffness of a compressible medium. How much of an US wave is transmitted or reflected. Z=c*denity

Question 14

What determines reflection and transmission at an interface?

Answer 14

the idfference in the two acoustic impedences. larges difference, more reflection

Question 15

When do we see specular relfection or nonspecular?

Answer 15

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

Question 16

reflection coefficient?

Answer 16

describes what fraction of intensity incident on an interface that is reflected
Ri = Ir/Ii = (Z2cosxi - Z1cosxt / Z2cosx1 + Z1cosxt )^2

Question 17

Pressure amplitude transmission coef

Answer 17

Rp=Pr/Pi = sqrt (Reflection coef)

Question 18

Intentisy transmission coef

Answer 18

Ti = 1 - Ri

Question 19

What percentages of US pressure in reflected at:
Bone-Muscle?
Fat-Muscle?

Answer 19

64%

12%

Question 20

What is speckle?

Answer 20

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.

Question 21

When a beam is not incident perpendicular, what dictates how w wave will refract?

Answer 21

snells law:

c1sinx1=c2sinx2

Question 22

AT when angle of incidence does max reflection occur?

Answer 22

Perpendicular

Question 23

What is the attenuation coef? unit?

Answer 23

relative intensity loss per distance travelled for a given medium. dB/cm.

Question 24

attenuation pressure formula?

Answer 24

p = p0exp(-alphaz), alpha is attenuation coef.

Question 25

Rule of thumb for attn coef

Answer 25

1dB per cm travelled per MHz

Question 26

Do higher frequencies attenuate more rapidly?

Answer 26

yes

Question 27

how is the focal region defined?

Answer 27

region over which the beam width is no more than two times the width at focal spot.

Question 28

Near field length=?

Answer 28

d^2/(4*wavelength), d is the transducer diameter

Question 29

Far field angle of divergence?

Answer 29

sinx=1.22*lambda/d

Question 30

how waould you produce a shallower focal zone?

Answer 30

by activating the outer elements in the array before the inner ones.

Question 31

What are "side lobes"?

Answer 31

extra constructive interference away from the central axis.

Question 32

what is axial resolution?

Answer 32

half the spatial pulse length. spatial pulse length = N*wavelength

Question 33

at what distance from the transducer is lateral resolution optimized?

Answer 33

at focal region

Question 34

define elevation resolution?

Answer 34

slice thickness perpendicular to imaging plane

Question 35

what is pulse repetition frequency (PRF) and pulse rep period (PRP)?

Answer 35

number of pulses per second, and time between sequential pulses

Question 36

What is Duty Factor?

Answer 36

Pulse Duration/PRP. uaualy around 1%

Question 37

What is A-Mode Imaging and why do we use it?

Answer 37

Amplitude Mode Imagin! just displays pulse recieved. used to locate interfaces, cracks

Question 38

What is B-Mode imaging?

Answer 38

typical ultrasouns

Question 39

What is the frame rate in B-Mode?

Answer 39

number of line scans to complete a single B-Mode image multiplied by the lines per second.

Question 40

what is M-mode imaging?

Answer 40

Motion mode is the echo from a single beam passing through moving anatomy and displayed as a function of itme.

Question 41

Which assumptions cause artifacts in US?

Answer 41

- speed of sound is constant - no scatter - reflection only comes from along central axis

Question 42

Name 4 types of US aritfacts

Answer 42

- reverberation - shadowing/attenuation - enhancement - mirroring