Final Practice exam Flashcards
(a) On a typical spectral Doppler pulse as a function of time, (i) what is a typical number of cycles in this type of pulse?
(i) The Doppler pulse should have 10 to 20 fairly uniform cycles. If the bandwidth is too large the frequency shift is less easy to determine in the Doppler ultrasound electronic circuitry. To solve for this in pulse Doppler diagnostic ultrasound, the SPL is increased to about 10 to 20 wavelengths with a resulting significant decrease in the bandwidth, and unfortunately some loss in axial resolution that goes with the larger value of the spatial pulse length. Axial resolution = SPL/2
On a typical spectral Doppler pulse as a function of time, (ii) Describe the pulse duration as it would appear on the diagram of the pulse.
(ii) The pulse duration is the full length, in time, of the whole pulse, that is all 10 or 20 cycles. Starting at the start of the first cycle and ending at the end of the last cycle.
On a typical spectral Doppler pulse as a function of time (iii) How does the number of cycles in a spectral Doppler pulse differ from a B-mode pulse.
(iii) A B-mode pulse has only 3 to 5 cycles compared with 10 to 20 in a Doppler pulse. The b-mode pulse is highly damped and thus has a large bandwidth increasing it’s spatial resolution. The Doppler pulse is less damped with a long SPL resulting in a more narrow bandwidth, that is a more narrow range of frequencies. This makes it easier for the machine to detect frequency shifts required for an accurate Doppler trace but comes at the cost of axial and thus spatial resolution as represented in the formula axial resolution = SPL/2.
(b) Explain the factors that affect spatial pulse length and, in turn, how this affects spatial resolution.
The spatial pulse length is the product of the wavelength and the number of cycles. Thus, spatial pulse length is inversely proportional to the frequency.
So, increasing frequency will reduce the spatial pulse length which, in turn, improves spatial resolution as defined in the formula Axial resolution = SPL/2.
In reality, low frequencies are used in spectral Doppler ultrasound so that an image may be formed. As Doppler imaging focuses on the imaging of red blood cells that scatter the beam in all directions, only a fraction of the beam will echo back to the transducer. Low frequency transducers are used to lessen the attenuation of ultrasound into soft tissue and maximize the return echoes and this comes at the cost of axial and thus spatial resolution.
Similarly, the longer SPL is a product of a pulse that is less damped than that used in b mode imaging. This means there is a far narrower range of frequencies used. This makes it easier for the machine to determine the frequency shift caused by the Doppler effect but also comes at the loss of axial and thus spatial resolution.
(c) (c) Discuss the concept of dynamic range in the following terms. An ultrasound signal processor has an output voltage range of 0.0 to 1.0 volts with a voltage precision of ΔV = 0.001 V. (i) State the first three and the last three voltage steps in this arrangement.
The signal processor output comprises of voltage pulses of height that provide the necessary information about the reflecting interfaces in the patient at given depths. If, after amplification, the peak voltage values are in the range 0.0 - 1.0 V, and a given voltage is precise to 0.001 V (1 mV) then The signal processor output comprises of voltage pulses of height that provide the necessary information about the reflecting interfaces in the patient at given depths. If, after amplification, the peak voltage values are in the range 0.0 - 1.0 V, and a given voltage is precise to 0.001 V (1 mV) then The first three voltage steps will be 0.000 V, 0.001 V, 0.002V, and the last three voltage steps will be 0.998 V, 0.999V and 1.000V
(c) (c) Discuss the concept of dynamic range in the following terms. An ultrasound signal processor has an output voltage range of 0.0 to 1.0 volts with a voltage precision of ΔV = 0.001 V. (ii) State the number of discrete voltages that can be recognized by the system?
The signal processor output comprises of voltage pulses of height that provide the necessary information about the reflecting interfaces in the patient at given depths. If, after amplification, the peak voltage values are in the range 0.0 - 1.0 V, and a given voltage is precise to 0.001 V (1 mV) then there are a total of (ii) 1,001 discrete voltage values that the electronics can recognize.
(c) (c) Discuss the concept of dynamic range in the following terms. An ultrasound signal processor has an output voltage range of 0.0 to 1.0 volts with a voltage precision of ΔV = 0.001 V. (iii) Give the defining equation for dynamic range.
(iii) The defining equation for dynamic range is DR = (Vmax -Vmin)/ ΔV and in this instance is (1000-0) / 0.001 = 1000/1
(c) (c) Discuss the concept of dynamic range in the following terms. An ultrasound signal processor has an output voltage range of 0.0 to 1.0 volts with a voltage precision of ΔV = 0.001 V. (iv) Express the dynamic range in decibels (dB).
(iv) Another way to express this is to take the dynamic range ratio and turn this into a dB value. In this case we have the DR = 1,000:1, or 20 log10(1.000/0.001) = 20 log10(1,000) = 60 dB This number of 60 dB is reasonable for an electronic amplifier. The reason that 20 log10 is used, and not 10 log10, is that these voltages are considered as magnitudes and not power (magnitude squared).
(a) List the features found in a typical arterial spectral Doppler display
B-mode image Scan line Sample volume Doppler angle cursor Spectral broadening Spectral display Intrinsic spectral broadening Spurious spectral broadening
Give a full description of B-mode image
obtained by scanning the area of interest and acquiring an appropriate frame from which the spectral Doppler display will be acquired. This image allows the user to adjust the Doppler angle cursor and define the location and size of the sample volume
Give a full description of scan line
The single scan line on the display down which the pulses are sent to acquire the echo spectrum
Give a full description of Sample volume
The sample volume of a Doppler spectral display is determined by slice thickness, lateral resolution and range gate duration. It is generally shown by a square box placed within the blood vessel along the scan line. Only the Doppler shifted echoes from the soft tissues and blood within the sample volume contribute to the spectral display, all other echoes are ignored. Range gating is when the machine only detects return echoes for a short amount of time (usually one or two microseconds) and it determines the axial limit of the sample volume. Increasing the axial limit of the sample volume will increase the range gate and vice versa. The time elapsed between the transmission of the Doppler pulse and the range gate is such that signals received only come from the required sample volume depth and any echoes that arrive before or after the range gate are ignored by the machine
Give a full description of spectral broadening
Spectral broadening is a Doppler artefact signified by a variance in the Doppler signal and it can be either intrinsic or spurious. Spectral broadening manifests as widening of the trace associated with lengthening of the vertical spectral lines and ‘filling’ of the spectral window that should otherwise be dark in appearance.
Give a full description of intrinsic spectral broadening
caused by the way the machine processes the data it receives, that is, as if there were a single line of sight, a single point of origin and a single Doppler angle. As the Doppler signal received is on a significant area of the transducer, from multiple paths, all with different Doppler angles, there will be an intrinsic range of Doppler shifts in the overall Doppler signal.
Give a full description of spurious spectral broadening
caused by: setting a large sample volume, a large Doppler angle or placing a sample volume too close to a vessel wall where blood flow varies in speed. It may also be real and caused by pathology. If vessel disease causes the velocity profile to change and blood flow becomes turbulent the spectrum will have a larger variance in Doppler