Topic 3 - system optimisation

Question 1

What features make a quality ultrasound image?

Answer 1

An ultrasound image that clearly and accurately displays anatomy and pathology with minimal or no artefact.

Question 2

What are the three categories on b mode image quality?

Answer 2

• Spatial Resolution
• Contrast resolution
• Temporal resolution

Question 3

What is spatial resolution?

Answer 3

• The ability to define two separate objects as distinct objects rather than as one single larger object.
• This type of resolution relates to the frequency and focal point of the transducer.

Question 4

What is contrast resolution?

Answer 4

• a B-mode image in a vascular study should contain a range of shades of grey from pure black through to pure white, without an excessive amount of any one shade dominating the screen.
• Structures within the image can be
o pure black (which represent structures that reflect no ultrasound)
o pure white (which represent structures that reflect all or nearly all of the ultrasound)

Question 5

What is temporal resolution?

Answer 5

• the ability to view moving objects in real time
• is usually monitored by the frame rate of the image.
• If the image frame rate is too slow, then the image will appear in slow motion and colour flow may not be displayed adequately because the screen takes too long to update.

Question 6

What defines a good quality colour Doppler image?

Answer 6

• The quality of a colour image depends on the purpose of using the colour.
• clearly visible and contained within the image of the vessel.
• Colour must also show the direction of flow, presence of flow and areas of focal velocity increase.

Question 7

What is the most important part of spectral Doppler image quality?

Answer 7

• used to make quantitative measurement of velocity and to help document the direction of blood flow.
• Since measurement of the fastest velocities in the systolic and diastolic phases are the primary measurement to quantify stenosis, then the clarity of the outer spectral envelope is of critical importance.
• The clarity of the spectral envelope outline is the primary criteria which defines a good quality spectral waveform.

Question 8

Describe the appearance of a good quality spectral trace

Answer 8

• Filling of the spectral window is often used in the description of a waveform
• But it can be produced by a number of factors which do not relate to pathology
• therefore it is only of limited value and should only be considered if other pathology is evident.
• adjustment of the spectral Doppler to eliminate spectral broadening should never sacrifice the quality of the spectral envelope.
• The spectral waveform should therefore show a clear spectral envelope with a range of grey-scale that shows full white and a significant number of shades of grey.

Question 9

Which controls affect B mode image quality?

Answer 9

o Transducer frequency (spatial resolution)
o Output power (contrast resolution)
o System gain (Contrast resolution)
o Dynamic range (Contrast resolution)
o Post Processing (image maps: Contrast resolution)
o Compound imaging (Contrast resolution)
o Harmonic imaging (Contrast resolution)

Question 10

Which controls affect colour Doppler image quality?

Answer 10

• Transducer frequency
• Sensitivity (ensemble length)
• Scale (Pulse repetition frequency)
• Colour box size

Question 11

Which controls affect spectral Doppler image quality?

Answer 11

• Spectral map
• Gain
• Gate size
• Filter
• Transducer frequency
• Angle of insonation
• Sweep speed

Question 12

How does transducer frequency affect bmode image quality?

Answer 12

Higher frequency means greater Spatial resolution
• Higher frequencies are much more sensitive to flow but cannot not penetrate to deeper vessels
• The size of a blood vessel and its depth are the primary determinants of the transducer frequency.

Question 13

How does output power affect b mode image quality?

Answer 13

(contrast resolution)
• useful in reducing the level of artefact such as reverberation.
• Adjusting power must done with some care
• reducing it too much may remove subtle pathology such as thrombus which is producing low level echoes.

Question 14

How does system gain affect b mode image quality?

Answer 14

(Contrast resolution)
• controls the amplification of all signals
• gain is defined as the ratio of output power to input power.
• Although reducing gain will reduce artefact to a certain extent, it will often sacrifice the overall Bmode quality of the image and make the image too dark (black) to identify pathology adequately.
System gain should be used to reduce gain if the image is predominantly white, with little or no grey or black. Similarly it can be increased if there is little or no white in the image and the majority of the image is black and lacks detail.

Question 15

How does TGC affect b mode image quality?

Answer 15

The Time Gain Compensation (TGC) or Depth Gain Compensation (DGC) controls are the most commonly adjusted during an examination.
These control the gain of the returning echos in a specific region of the image and compensate for the loss of signal in the deeper tissue.

Question 16

How does dynamic range affect b mode image quality?

Answer 16

(Contrast resolution)
• the ratio (in dB) of the maximum input signal that can be displayed without reaching saturation, to the smallest input signal that can be seen visually above noise level.
• can be reduced to minimise the effect of artefacts such as reverberation
• care should be taken to ensure that subtle pathology is not removed from the image as well.
• it can be increased if the detail of a structure or pathology shows subtle changes in grey-scale level.
• Identifying the characteristics of a plaque or highlighting a thrombus which is made of even and low level of returning echo.

Question 17

How does compound imaging affect b mode image quality?

Answer 17

(Contrast resolution)
• allows the image to be created from multiple directions so that noise can be reduced or eliminated.
• it may eliminate subtle grey-scale detail such as vein thrombus or arterial thrombus in some instances.

Question 18

How does harmonic imaging affect b mode image quality?

Answer 18

(Contrast resolution)
• In vascular studies, the effect does not improve diagnostic detail in many circumstances but may be advantageous for imaging deeper structures such as the aorta or areas where there is excessive noise.

Question 19

How does transducer frequency affect colour Doppler image quality?

Answer 19

• usually dictated by the need to obtain appropriate depth or resolution in the Bmode image
• but a higher frequency may be selected to improve detection of slower blood flow
• and a lower frequency may be used to help minimise aliasing.
usually other controls are adjusted first

Question 20

How does sensitivity affect colour Doppler image quality?

Answer 20

(ensemble length)
• The colour sensitivity (ensemble length) is increased to gather more data from slower flowing or small diameter vessels.
• Increasing the ensemble length is usually shown as a ‘sensitivity’ or ‘flow optimisation’ control where changing the setting to ‘low’ will make it more sensitive to slower flow conditions.
• Increasing the sensitivity may however cause a reduction in the frame rate of the image and this must be considered when the sensitivity is changed.

Question 21

How does scale affect colour Doppler imaging?

Answer 21

• changes the pulse repetition frequency
• this controls the level of slower velocities which will be displayed and the velocity at which aliasing begins.
• The colour should generally be adjusted so that there is either no aliasing or minor aliasing in the central part of the vessel so that velocity changes can be easily distinguished.
• If the scale is too high, the dynamic range is too large and low velocity signals are missed simulating an area of thrombosis, particularly in low flow vessels, such as the portal vein.
• If the velocity scale is too low, the dynamic range is too small to display the high-velocity signals accurately and aliasing result

Question 22

How does colour box size affect colour Doppler image quality?

Answer 22

• The colour box size primarily affects frame rate
• the control of colour box size will have a significant impact on slowing the frame rate if it is too large.
• It is important to remember that the WIDTH of the colour box will affect frame rate
• NOT its depth.

Question 23

How does angle of insonation affect colour Doppler image quality?

Answer 23

• has a significant effect on the presence of aliasing.
• The lower the angle of the colour box to the vessel
• the greater the received frequency shift
• and aliasing will occur at a lower velocity.
• Colour boxes can be steered electronically
• The angle of the beam can also be changed by heel toeing
• Although this technique is used on a daily basis, it does have the disadvantage of reducing the B-mode spatial resolution of the vessel being imaged.
• If absolute B-mode quality is required in a particular image, then colour box steering would be a better option as this allows the image to remain at 90 degrees to the transducer.
(for eg carotid artery study)

Question 24

How does the wall filter affect colour Doppler quality?

Answer 24

• Vessel wall motion may interfere with the clarity of the colour
• increasing the wall filter will eliminate the low frequency components associated with colour flash in the tissues outside the vessel.
• However it should be stressed that the filter may also remove some of the low velocity signal from the vessel and produce anechoic areas within the lumen.

Question 25

How does baseline affect colour Doppler image quality?

Answer 25

* Changing the baseline will change the velocity at which the forward and reverse flow will start to aliase. * Increasing the baseline will reduce the level of aliasing for flow toward the probe and lowering the baseline will reduce the aliasing velocity away from the transducer.

Question 26

How does colour priority affect colour Doppler image quality?

Answer 26

* The priority controls the level of grey-scale which the colour will overwrite. * If the priority is set high on the scale (in the brighter white range) then colour will be written on the grey-scale to that level of B-mode brightness. * If the priority is too high, then it may overwrite and hide pathology or cause unwanted noise to be displayed on the soft tissue grey-scale. * If the priority is too low, then it may not write colour over artefact in the vessel lumen and give a false appearance of pathology such a thrombus or even make some vessels appear occluded.

Question 27

How does the spectral map affect spectral Doppler image quality?

Answer 27

* Similar to the Bmode map. * Each map will change the way that the returning echoes are assigned to a brightness value. * In general, a map which gives the greatest contrast is the most useful for vascular studies so that the spectral envelope can be measured with greater confidence.

Question 28

How does gain affect spectral Doppler image quality?

Answer 28

* Again, gain is all too often used inappropriately and it must be remembered that it controls the amplification of all returning spectral frequencies. * Therefore it will not change the contrast of the spectral waveform but will help show a weak returning signal. * Naturally increasing the gain will bring with it a level of spectral noise and the balance of noise to signal must be judged in each case

Question 29

How does gate size affect spectral Doppler image quality?

Answer 29

• The sample volume is the three-dimensional space from which the Doppler frequency shifts are measured. • The size of the spectral gate determines what area that will be sampled to create the spectral waveform. • The gate is generally kept small so that the central area of a vessel can be sampled without other velocities being included that will contribute to unwanted noise. • there are some cases where a wide spectral gate is desirable o such as an artery that has little or no flow o or where the fastest velocity in a stenosis is difficult to locate.

Question 30

How does the filter affect spectral Doppler image quality?

Answer 30

* Vessel wall motion may interfere with the clarity of the spectral trace * increasing the wall filter will eliminate the low frequency components that they produce * it should be stressed that the filter may also remove some of the low velocity signal from the waveform, so the filter should be used sparingly and with caution.

Question 31

How does transducer frequency affect spectral Doppler image quality?

Answer 31

* Like colour, the transducer frequency will change the velocity at which aliasing occurs. * Using a high frequency transducer will be more sensitive to slower flow and give a higher frequency shift than a low frequency transducer. * Similarly, changing to a lower frequency transducer will help increase the velocities that can be recorded before aliasing occurs, allowing them to be accurately measured.

Question 32

How does the angle of insonation affect spectral Doppler image quality?

Answer 32

* The Larger Doppler angle, the greater the correction is that needs to be done and the greater chance for error; * therefore, the Doppler beam angle must always be kept as low as possible. * Ideally, it should be less than 60 and always less than 70 * because the errors associated with the angle correction increase up to 20% to 30% with higher Doppler angles

Question 33

How does sweep speed affect spectral Doppler image quality?

Answer 33

* The settling of the sweep speed depends on the task you are performing. * A high sweep speed will give increased detail of the spectral shape of an individual cardiac cycle * but the waveform patterns over an extended period of time will not be displayed clearly.

Question 34

What are the three broad categories of Doppler artefacts?

Answer 34

1. artifacts caused by technical limitations 2. artifacts caused by patient anatomy 3. artifacts caused by machine factors

Question 35

What are the artifacts caused by technical limitations?

Answer 35

i. aliasing ii. improper Doppler angle with no flow iii. indeterminate Doppler angle iv. blooming v. partial volume artifact

Question 36

What are the artifacts caused by patient anatomy?

Answer 36

i. mirror image artifact ii. flash artifact iii. ‘‘pseudoflow’’

Question 37

What are the artifacts caused by machine factors?

Answer 37

i. edge artifact | ii. twinkle artifact

Question 38

What do these artifacts primarily relate to?

Answer 38

the generation of Doppler signals by nonvascular structures or fluids. The key to their recognition is 1) knowing that they can occur 2) knowing the common locations and causes for their generation 3) identification of the nonvascular Doppler spectrum they generate, which clinches the diagnosis.

Question 39

How does aliasing occur?

Answer 39

* an inaccurate display of color or spectral Doppler velocity * occurs when the velocity range exceeds the scale available to display it

Question 40

How can you eliminate spectral aliasing when it occurs?

Answer 40

• The solutions to spectral aliasing o first to drop the baseline or increase the velocity scale (ie, the PRF) to increase the available velocity range o If the scale is still inadequate, decrease the Doppler frequency shift by using a lower insonating frequency or by increasing the Doppler angle

Question 41

Why does power Doppler have no aliasing?

Answer 41

because it has no directional or velocity component.

Question 42

When can aliasing be useful?

Answer 42

color Doppler aliasing can be useful because it quickly localizes the highest velocity region within a vessel for spectral sampling identifies abnormal high-velocity vessels, which are often invisible on gray-scale. In particular, arteriovenous fistulae, a common sequelae to renal or hepatic biopsy are often undetectable on gray-scale.

Question 43

How does blooming artifact occur?

Answer 43

* ‘‘color bleed’’ * the color spreads out from within the vessel and ‘‘bleeds’’ beyond the wall into adjacent areas * can occur because the color US image is actually two images superimposed * the color and the gray-scale * depending on how the parameters are set, the color portion of the image can extend beyond the true gray-scale vessel margin

Question 44

Where does blooming artefact usually occur and what is an unwanted result?

Answer 44

* usually occurs deep to the vessels and, most commonly, is caused by abnormally high gain settings * unwanted result is that the information within the vessel (ie, partial thrombus) can be ‘‘written over’’ and obscured

Question 45

What is directional ambiguity?

Answer 45

* indeterminate flow direction * refers to a spectral Doppler tracing in which the waveform is displayed with nearly equal amplitude above and below the baseline in a mirror image pattern.

Question 46

When can directional ambiguity occur?

Answer 46

* This pattern results when the interrogating beam intercepts the vessel at a 90 angle * most common in small vessels, especially those that may be traveling in and out of the imaging plane * should not be confused with true bidirectional flow.

Question 47

How can you differentiate between true bi directional flow and directional ambiguity?

Answer 47

* In bi directional, blood actually flows in two directions, such as in the neck of a pseudo-aneurysm * The clue here is that the flow is first in one direction, then in the opposite, all within a single cardiac cycle * The difference between true bidirectional flow and an indeterminate direction spectral tracing is that bidirectional flow is never simultaneously symmetric above and below the baseline. * The flow direction varies within the cardiac cycle. * True bidirectional flow is not an artifact. * In the visceral arteries it is always abnormal and must be recognized to make the correct diagnosis

Question 48

What is partial volume artifact?

Answer 48

* Partial volume artifact results from a slice thickness that is not infinitely thin. * Echoes and Doppler signals can be acquired from objects that may be within the slice and partly outside of it * echoes can appear within anechoic structures and Doppler signals are acquired in an area in which no vessels are perceived on gray-scale

Question 49

Give an example of partial volume artifact?

Answer 49

* On color flow imaging, an example of partial volume artifact is visualization of a portion of the iliac artery within the ovary giving the impression of abnormal cyst wall flow. * Spectral analysis of this vessel shows the high resistance waveform typical of an iliac artery and imaging from the 90 plane clearly shows the vessel separate from the ovary.

Question 50

What can cause partial volume artefact?

Answer 50

* Partial volume artifact may be produced by grating lobes or side lobes, which generate information outside the expected path of the main beam * Side lobes occur close to the primary beam whereas grating lobes can be far removed from the central beam * These transducer related artifacts are seen mainly with the high frequency, tightly curved, convex, linear arrays used in endocavitary probes

Question 51

What is pseudoflow?

Answer 51

* Pseudoflow is defined as presence of flow of a fluid other than blood * Eg ureteric jets, ascites, amniotic fluid

Question 52

What is flash artifact?

Answer 52

* Flash artifact is a sudden burst of random color that fills the frame, obscuring the gray-scale image. * This artifact may be caused by object motion or transducer motion * most commonly seen in the left lobe of the liver (as a result of cardiac pulsation)

Question 53

How can flash artifact be useful?

Answer 53

• This artifact is the imaging equivalent to an auditory bruit or palpable thrill

Question 54

What is mirror image artifact?

Answer 54

* The mirror image artifact displays objects on both sides of a strong reflector, though they are located only on one side of it. * Eg the diaphragm, pleural surface, or aortic wall * directs some of the echoes to a second reflector before it returns them to the transducer, resulting in a multipath reflection * Mirror images may be produced with gray-scale, color, power, and spectral Doppler. * A carotid ghost is the term for a mirror image of the common carotid artery.

Question 55

What is edge artifact?

Answer 55

* refers to the Doppler signal generated at the margin of a strong, smooth, specular reflector * displayed on imaging as persistent color along the rim of calcified structures, such as gallstones or cortical bone * may mimic vascularity unless the spectral tracing is obtained * may be generated by any echogenic surface, including manmade structures (eg, catheters and foley balloons

Question 56

What is the diagnostic feature of edge artifact?

Answer 56

• The diagnostic feature is the Doppler spectrum, a straight-line pattern, equal above and below the baseline, and representing noise, not flow

Question 57

What is twinkle artifact?

Answer 57

* color Doppler signals that imitate motion or flow behind a stationary strongly reflecting interface * can be seen behind any granular (irregular or rough) reflecting surface * Twinkling artifact is believed to be caused by a narrow band of intrinsic machine noise called phase (or clock) jitter * Similar to an acoustic shadow, twinkling does not occur 100% of the time

Question 58

What are the most common artifact problems in vascular sonography?

Answer 58

* Too much flow, which may obscure thrombi | * too little flow, giving the false diagnosis of thrombosis

Question 59

How can you recognise if there is 'too much flow' artifact?

Answer 59

* seeing color bleed * seeing aliasing in a vessel that normally does not have it * This problem can be corrected by increasing the scale or decreasing the gain.

Question 60

What is another common imaging problem that leads to false negatives in vascular?

Answer 60

uninterrupted flow is imaged from a segment of a vessel and flow is assumed the same throughout the rest of the lumen • usually occurs in longitudinal vascular imaging whereby a partial thrombus or atheromatous plaque may not be imaged if it is not centered in the imaging plane • The fail-safe if inappropriate settings are not recognized is always to image in two planes;

Question 61

How can you limit the 'too little flow' artifact?

Answer 61

* Doppler angle as small as possible * scale set appropriately * The frequency should be appropriate: * Frequency filters and other algorithms designed to decrease color tissue noise can eliminate display of slow flowing blood if they are set too high * reducing the size of the color box reduces the sample size and increases the frame rate, leading to better overall sensitivity and resolution of the color image * color-write priority may be set automatically or manually. * With a high color-write priority the sensitivity for color flow can be prioritized so color shows up where no vessel was visible on gray-scale imaging, and if the color-write priority is set too low, flow may be missed.

Question 62

State some Guidelines for an optimal colour flow Doppler examination

Answer 62

* Adjust the gain and filter settings to obtain an optimal color signal and minimal color noise. * Adjust the velocity scale (PRF) and baseline according to the flow conditions. * Obtain an optimal Doppler angle by adjusting the beam steering and probe position. * The color flow box should be kept as small as possible to allow better frame rate for better resolution and sensitivity. * Adjust the pulsed Doppler sample volume size appropriately (two thirds of the vessel diameter) to obtain accurate velocities * Avoid transducer motion.

Question 63

List the system controls you would adjust to increase the amount of colour in an image.

Answer 63

Increase colour: • increase colour gain; • lower or higher scale; • decrease or increase the filter; • use < 60-degree Doppler angle to increase colour • Set sensitivity to 'low flow' or increase sample size of the colour. (Controls are Machine brand dependent) • adjust colour write priority (if available) • use a more appropriate transducer frequency.

Question 64

What are the advantages and disadvantages of power Doppler (amplitude Doppler) technique?

Answer 64

Advantages of amplitude Doppler: • Increased flow sensitivity • Not greatly affected as the angle of interrogation nears 90 degrees • No black lines where there is directional change • Not as affected by deceptive colour artifacts which means the signal can be amplified significantly more before being affected. • Can use higher gain settings Disadvantages • Absence of directional information • Absence of velocity information • Susceptible to flash artifacts