Physics Chapter 10 and 11

Resolution

the ability to create accurate images

(image accuracy is the essence of ultrasound)

two forms of resolution

spatial and temporal

Spatial resolution

ability to distinguish two points as separate in space

• Axial

• Lateral

• Elevational (talk about this in later chapters)

Temporal resolution

ability to precisely position moving structures from instant to instant

• Accuracy in time

Axial Resolution

The ability to distinguish two structures that are close to one another and parallel to the sound beam’s main axis.

Axial resolution is AKA

LARRD:

• Longitudinal resolution

• Axial resolution

• Range resolution

• Radial resolution

• Depth resolution

Axial resolution units, determination, adjustability

• Units: of distance (mm)

• Determined by: the spatial pulse length (medium and source)

• cannot be adjusted since the SPL cannot be adjusted

• does not vary along the sound path

What type of pulses improve axial resolution?

shorter pulses improve axial resolution

How is axial resolution related to pulse duration ?

short duration pulses also have shorter lengths

_____ numbers mean shorter pulses and ____ image quality

lower numbers mean shorter pulses and better image quality

typical values of axial resolution

0.1-1.0 mm

black: whats in the body

whats showing up on the image:

green: longer SPL

red: shorter SPL

solving for axial resolution

axial resolution is ½ the SPL

SPL equations

SPL = #cycles x 1.54/frequency

SPL = #cycles x wavelength

Example: If the pulse length is 1mm, what is the axial resolution ?

1/2=0.5

so, structures need to be 0.5 mm or more apart from each other to be seen as two separate structures

Axial resolution in soft tissue equation

axial resolution = 0.77 x #cycles in a pulse / frequency

Why do some transducers have better axial resolution ?

axial resolution is determined by pulse length:

shorter pulses = better image

How are short pulses created?

• Decreased ringing

• Higher frequencies

• Add damping material

Less Ringing

short pulses means there are few cycles in a pulse

Transducers often have pulses of

transducers often have pulses of 2 or 3 cycles because dampening the ringing after the elctrical signal excited the crystal making the pulse

(fewer pulses mean better axial resolution)

Higher Frequency

• Shorter wavelengths mean higher frequency sounds (wavelength and frequency are INVERSELY RELATED)

What frequency of pulses have better image quality ?

pulses of higher freqnecies have better image quality

Better Axial Resolution

• Shorter spatial pulse length

• Shorter pulse duration

• Higher frequencies (shorter wavelengths)

• Fewer cycles per pulse (less ringing)

• LOWER numerical values

Worse Axial Resolution

• Lowest Frequency

• Greatest number of pulses

• HIGHER numerical values

Lateral Resolution

the ability to distinctly identify two structures that are very close together when they are perpendicular to the sound beam’s main axis

• how well can you tell two structures apart when they are side by side?

• What is the minimum distance that two side by side structures can be from one another and still be identified as two separate structures?

Lateral Resolution

• Small is better- the smaller the number, the better the image.

• Units: of DISTANCE (mm)

• Determined by: width of the sound beam (Narrower beam widths = better resolution)

Lateral resolution AKA

LATA

• Lateral

• Angular resolution

• Transverse resolution

• Azimuthal resolution

• The green dots represent the actual reflectors along the path of the sound beam.

• The black represents what shows up on the image.

• Notice: where the beam is narrowest- the reflectors can be differentiated.

Lateral Resolution and Width of Sound Beam

• Width of sound beam determines the lateral resolution

• Diameter of sound beam varies depending on the depth

• The narrower the sound beam, the better the lateral resolution

• The beam is narrowest at the focus

What width of the beam has the better lateral resolution ?

The narrower the sound beam, the better the lateral resolution

What do you think happens when two structures are side by side and closer to one another than the width of the sound beam?

They will be represented on the image as one structure.

What do you think happens when two structures are side by side and farther from one another than the width of the sound beam?

They will be appropriately represented on the image as two separate structures

What is lateral resolution approximately equal to ?

lateral resolution is approximately equal to the beam diameter:

lateral resolution = beam diameter

Since we know beam diameter changes with depth…

lateral resolution also changes with depth

two structures lying side by side must be at least _____ to be images as two separate structures

a beam widths apart

Where is lateral resolution the best?

Lateral resolution is best in the focal zone where beam diameter is the narrowest

Lateral vs. Axial

• Lateral Resolution is not as good in clinical imaging as Axial resolution.

Why??

• Pulses are shorter than the beams width

• Pulse duration determines axial resolution

• Beam width determines lateral resolution

Slice Thickness Resolution

• Measured perpendicular to the imaging place

• AKA: elevational axis, Z-axis, section thickness, and out of plane focusing

• Accomplished using lens with fixed focus- curve of lens goes from front to back

How can we improve elevational resolution?

switch to a 1.5 D transducer

Focusing

• Concentrates the sound beam into a narrower beam.

• improves lateral resolution.

• Internal and External can be used on single element transducers

Three methods

1. External focusing

2. Internal Focusing

3. Phased Array focusing

Phased array focusing can only work for …

array transducers (multiple active elements)

Fixed Focusing

• AKA conventional or mechanical focusing

• Includes both internal and external focusing techniques;

The focal depth is determined by….

the manufacture and cannot be changed by the sonographer (unless they change the transducer)

External Focusing

• The lens

• A lens is placed in front of the piezoelectric material (fixed focus)

• The amount of curvature in the lens and therefore the focusing, is

predetermined by the manufacturer

The more curved the lens…

the greater the degree of focusing, creating a narrower beam width at the focus.

• Remember narrower beam width means improves lateral resolution

lens (external focusing)

Internal Focusing

• Curvature of the actual piezoelectric crystal

• This is the most common form of FIXED focusing

• Again, the greater the curvature, the greater the degree of focusing, creating a narrower beam width at the focus.

• The amount of curvature in the piezoelectric element, and therefore the focusing, is predetermined by the manufacturer

PZT (internal focusing)

Electronic Focusing

• Used in phased array transducers (phased array literally means adjustable or multiple focus)

• Array transducers have multiple elements

• Electronics within the system focus the sound beam

• Focus can be adjusted by the sonographer

• Cannot be used on single element transducers

Change #1 that the beam undergoes

Beam diameter in near zone and focal zone narrows

Change #2 that the beam undergoes

The focus is moved closer to the transducer- this reduces the length of the near zone

Change #3 that the beam undergoes

Beam diameter beyond focal zone widens

The focusing improves lateral resolution in the near zone and focal zone BUT degrades lateral resolution in all areas beyond focal zone

Change #4 that the beam undergoes

The size of the focal zone is reduced

Display Modes

the way the ultrasound system interprets and displays ultrasound information on the screen.

Amplitude Mode (A-mode)

• One dimensional display or image

• Appears as upward spikes

• Displayed spikes is a relative indication of the intensity or strength of the returning echoes

• Returned reflection processed with upward deflections graphs proportional to the amplitude (bigness) of the echo.

Tall spikes =

strong reflectors

short spikes =

weak reflectors

no spike =

no reflector

x-axis in A-mode represents

depth

• Distance along baseline the corresponds to reflector depth

Y-axis in A-mode represents

amplitude of echoes

• Height= amplitude or strength of the echoes

What is amplitude used for ?

• to accurately determine depth of a structure or thickness of structures

• Commonly used in ophthamology

• Best to use when needing very precise measurements

• No grayscale image given, just spikes

amplitude mode

Brightness mode (B-mode)

• Grey scale mode

• Pulse emitted from transducer (scan line), once echo returns, a brightness is assigned to the dot/pixel (Dot on screen corresponds to transducer position)

• shade of gray corresponds to the strength or amplitude of each echo

• Creates 2-D image

In original mode, pixels could be on or off. What were on and off displayed as?

On: white

off: black

Brighter dot =

stronger reflector

weaker reflection =

dark grey dot

What can brightness mode also be desscribed as?

a line of dots of varying brightness

x-axis in B-mode =

depth (found by time of flight)

z-axis in B-mode =

amplitude/brightness

• An oscilloscope isused to measure this (an instrument that can measure constantly varying signals – amplitude!)

What was the first type of gray scale imaging ?

B-mode

B-mode (left is old machine, right is new)

Motion mode (M-mode)

• Depicts motion over time

• Appears as a group of horizontal wavy lines.

• When a pulse is emitted, reflections move at a constant pace from right to left on the display screen.

• Gives information about an objects (reflectors) changing location with respect to time.

What do the lines in M-mode represent?

• “squiggly” lines represent the changing depths of the reflectors

• As the line moves up and down, M-Mode is showing that a structure is moving nearer then farther from the transducer.

• A horizontal straight line indicates a stationary structure.

the x-axis in M-mode =

time

the y-axis in M-mode =

depth

very high sampling rate =

to PRF of the system

What is M-mode usually used for ?

• mostly for cardiac evaluation, to document viability of the fetus and evaluate lung motion

• No grayscale image

M-mode