**4. Doppler velocity QC**

To carry out periodic quality control on ultrasound equipment used as a Doppler velocity metre (continuous Doppler, pulsed, colour, power, etc.), it is necessary to define a minimum set of significant and measurable parameters to compare with parameters measured at the time of acceptance of the machine itself, or with levels of acceptability. To do this it is necessary to proceed with the measurement starting from a known machine state that can be reproduced in subsequent measurements. The standard configuration set for the test will be called the initial setup and consists of a series of settings and adjustments that must be recorded in the test protocol.

Some parameters, although important for image quality, are intrinsically determined by the setup of the equipment, such as the minimum measurable speed, determined by the choice of the wall filter once the angle has been fixed Doppler, the maximum measurable speed, determined by the choice of PRF, once the Doppler angle has been fixed and so on.

Therefore, no quality control is carried out on these parameters. The parameters tested are the following:


## **4.1 Flow sensitivity**

It represents the minimum flow that produces a statistically different signal from the background and can be variable depending on the average velocity in the vessel. This parameter is measured by simulating a known, gradually decreasing flow in a phantom shown in **Figures 6**–**8** (in this case it is necessary to use synthetic blood), or in a test object (preferably decreasing the diameter of the pseudo-vessel and keeping the average speed constant). In fact, as is known, the laminar flow in a duct at a given instant is equal to the product of its section at a point and the average velocity of the particles passing through that section at that instant. It should be remembered that in laminar conditions, the speeds of the particles close to the axis of the vessel

*Perspective Chapter: Quality Assurance in Diagnostic Ultrasound DOI: http://dx.doi.org/10.5772/intechopen.114115*

are greater than those placed near the walls and that this difference increases as the diameter of the duct decreases. This first level test is performed with the selection of the lowest wall filter and decreasing the velocity until the flow disappears in the image (spectral or Doppler).

In our experience, the lowest detectable velocity was 0.04–0.06 cm/sec in a 3.00 mm tube. Detection was about three to four times less sensitive in a 0.30-mm and 0.05-mm tubes. Amplitude Doppler US sensitivity is only slightly dependent on the angle of incidence and propagation medium.

#### **4.2 Accuracy of speed estimation**

It is obtained by comparing the true (known) average speed inside a pseudo-vessel with the one measured by the velocity meter, for the different speeds of interest.

The parameter in question depends, for a given flow, on the velocity profile inside the duct itself. If we assume that the flow is laminar, and therefore the velocity profile is parabolic, it is possible to estimate the peak velocity vp using the following relationship:

$$\boldsymbol{\nu}\_p = \mathcal{Z}\overline{\boldsymbol{\nu}}\tag{2}$$

If the flow inside the vessel is turbulent, or has characteristics other than laminar ones, estimating the peak velocity is more problematic.

Generally, when a liquid enters a tube it does not present a laminar regime but reaches it after a certain distance whose length depends on the diameter of the tube, the speed and the viscosity of the liquid. In commercially available phantoms there are generally some sections of the pseudo-vessels in which the laminar regime conditions can be assumed to be satisfied. It is therefore necessary to measure the speed within these zones and compare it with that calculated with the following expression:

$$\overline{\upsilon}\left(cm/s\right) = \frac{\wp\left(ml/s\right)}{A\left(cm^2\right)}\tag{3}$$

where φ is the flow rate and A is the vessel cross-sectional area.

In our experience, Doppler methods are capable of good absolute accuracy when suitably designed equipment is used in appropriate situations, with systematic errors of 6% or less (**Figure 7**, steady flow). This is a second-level test.

Particular attention must be taken in order to measure the parabolic region of the flow.

#### **4.3 Precision of speed estimation**

It consists of checking the reproducibility of a speed measurement and is obtained by carrying out subsequent measurements in the same conditions and checking the constancy of the measured value. The precision, taking attention at an angle is generally high. Particular attention must be taken in order to measure the parabolic region of the flow.

### **4.4 Positioning accuracy of the sample volume**

The sample volume delimits the region of the image, and therefore of the object, from which Doppler signals are obtained. The check is carried out by positioning, at different points inside the vessel, the smallest sample volume available and carrying out a flow measurement in points from one wall to the opposite one. The highest speed value must be obtained when the sample volume is found at the centre of the vessel. It is a check equivalent to that of the accuracy of the speed estimate: a misalignment produces a variation in the average speed measured.

The results we obtained were very spread due to the quality of the scanners. The test requires particular precision.

#### **4.5 Spatial resolution in colour**

It represents the system's ability to spatially discriminate two different flows, in conditions of high contrast, for example, two opposite flows, when they are very close to each other.

Typically, it can be checked by simulating two pseudo-vessels of decreasing diameter and increasingly closer (and parallel) to each other, in which two opposite flows flow (therefore one represented in red and one in blue).

It is necessary to evaluate the minimum conditions of flow velocity and distance that produce an image in which the flows begin to be unresolved (**Figures 9**–**12**).

If the phantom does not have two adjacent pseudo-vessels, it is still possible to carry out the check using a flow perpendicular to the ultrasound beam. In this case, the Doppler signal is affected by spectral broadening. If the flow is orthogonal to the beam, the broadening artefact must be the same for both positive and negative speeds and the amplitude of the spectrum must be the same. If the velocity spectrum was obtained in pulsed Doppler mode it must be symmetrical with respect to the axis. The values of vessel diameter and flow velocity that produce a flow not resolved are an index of the second level of spatial resolution in colour. Value obtained is very spread.

#### **4.6 Colour contrast resolution**

It represents the system's ability to discriminate different speeds, even in the same flow.

Typically, it can be checked by evaluating what is the minimum difference between two known speeds that determine two distinguishable speeds in the image. It is a second-level test. Value obtained is very spread.

## **4.7 Colourful noise**

Coloured noise consists of the detection of flow in areas where there is no flow. Typically, it is sufficient to acquire a colour image using a phantom or a test object in conditions of no flow and gradually increase the colour gain until coloured noise appears. The higher the gain at which the coloured noise appears, the better the performance of the ultrasound. It is a second-level test. Value obtained is very spread.

*Perspective Chapter: Quality Assurance in Diagnostic Ultrasound DOI: http://dx.doi.org/10.5772/intechopen.114115*

**Figure 9.** *Scheme of the homemade Doppler phantom for colour resolution.*

**Figure 10.** *Homemade Doppler phantom for colour resolution.*

**Figure 11.** *Doppler phantom image with colour resolved.*

**Figure 12.** *Doppler phantom image with colour not resolved.*
