Chapter 4: Pulsed Ultrasound Practice Flashcards

Introduction to Pulsed Ultrasound

• Conceptual Difference between Sound Waves:
  • Continuous Sound Waves: These waves do not generate anatomical images in diagnostic ultrasound.
  • Pulsed Sound Waves: These waves do generate anatomical images of tissue boundaries, typically represented in shades of grey (grey scale) or B-mode (2D).
• Localization of Reflectors: Sound returning from the body must be timed to determine the distance of reflectors from the transducer. This is referred to as "go-return time" or "time of flight."
• Defining a Pulse: A pulse is a collection of cycles that travel together. It is composed of a period of "on" time (the pulse duration) and a "dead" or "off" time (listening time).
• Parameters Describing Pulse Waves:
  1. Pulse Duration ($PD$)
  2. Spatial Pulse Length ($SPL$)
  3. Pulse Repetition Period ($PRP$)
  4. Pulse Repetition Frequency ($PRF$)
  5. Duty Factor ($DF$)

Pulse Duration (PD)

• Definition: Pulse duration is the time from the start of a pulse to the end of that pulse. It represents the "on" time of the transducer.
• Relationship to Image Quality: Pulse duration is inversely related to image quality.
• Units: Expressed in units of time, specifically $NS$ (microseconds).
• Typical Range: $0.1\,NS - 1.5\,NS$
• Determined By: Sound source only (cannot be changed by the sonographer).
• Mathematical Formulas:
  • $PD = n \times T$
    • Where $n$ is the number of cycles (typically $1-3$ cycles per pulse).
    • Where $T$ is the period.
  • $PD = \frac{n}{f_0}$
• Variable Relationships:
  • $PD \propto \text{period}$ (Directly related).
  • $PD \propto \text{number of cycles}$ (Directly related).
  • $PD \propto \frac{1}{\text{frequency}}$ (Inversely related).

Spatial Pulse Length (SPL)

• Definition: The distance from the beginning of the sound pulse to the end of that pulse.
• Importance: SPL determines the resolution of the image, specifically LAART (Longitudinal, Axial, Range, Radial, Transverse resolution).
• Units: Expressed in distance, specifically $mm$ (millimeters).
• Typical Range: $0.1\,mm - 2.5\,mm$
• Determined By:
  • Initially determined by the sound source.
  • Secondarily determined by the medium through which the sound travels.
• Mathematical Formula: $SPL = n \times \lambda$
  • Where $n$ is the number of cycles.
  • Where $\lambda$ is the wavelength.
• Resolution Relationship: A shorter SPL results in better resolution.
• Variable Relationships:
  • $SPL \propto \lambda$ (Directly related to wavelength).
  • $SPL \propto n$ (Directly related to the number of cycles).
  • $SPL \propto PD$ (Directly related to pulse duration).

Pulse Repetition Period (PRP)

• Definition: The time from the beginning of one pulse to the beginning of the next pulse. This duration includes exactly one pulse duration ("on" time) and one "off" time (listening time).
• Units: Expressed in units of time, specifically $ms$ (milliseconds).
• Typical Range: $0.07\,ms - 0.25\,ms$
• Determined By: Imaging depth.
• Impact of Depth:
  • As depth increases ($\uparrow$), the listening time increases ($\uparrow$), which subsequently increases ($\uparrow$) the PRP.
  • Imaging depth and PRP are directly related.
• Mathematical Formula: $PRP = \frac{1}{PRF}$
• Note on PD: Pulse duration ($PD$) does not directly affect the Pulse Repetition Period ($PRP$).
• Reciprocity: $PRP$ and $PRF$ are inversely related and are reciprocals.

Pulse Repetition Frequency (PRF)

• Definition: The number of pulses transmitted in each second. It is important to note this refers to pulses, not cycles.
• Units: Expressed in $kHz$.
• Typical Range: $4\,kHz - 15\,kHz$
• Determined By: Imaging depth.
• Relationship to Pulses: Fewer pulses per second occur at greater depths.
• Clinical Application (Doppler): In Doppler studies, the sonographer can change the $PRF$ directly using the velocity scale function without changing the depth.
• Example Calculation: If $PRF = 5\,kHz$, this means $5000$ pulses per second are created, resulting in $5000$ scan lines per second, which constitutes one frame.

Duty Factor (DF)

• Definition: The fraction or percentage of time that pulsed ultrasound is transmitted ("on").
• Units: Unitless; expressed as a decimal or percentage.
• Mathematical Formulas:
  • $DF = \frac{PD}{PRP}$
  • $DF = PD \times PRF$
  • $DF = \frac{PD}{PD + LT}$ (where $LT$ is the Listening Time).
• Variable Relationships:
  • $DF \propto PD$ (Directly related to pulse duration).
  • $DF \propto PRF$ (Directly related to pulse repetition frequency).
  • $DF \propto \frac{1}{PRP}$ (Inversely related to pulse repetition period).

Decibel (dB) Reference Values

• Negative Decibels (Attenuation/Reduction):
  • $0\,dB = 100\%$
  • $-3\,dB = 50\%$
  • $-6\,dB = 75\%$
  • $-9\,dB = 87.5\%$
  • $-10\,dB = 90\%$
• Positive Decibels (Gain/Increase):
  • $+3\,dB = \times 2$
  • $+6\,dB = \times 2 \times 2$ (or $\times 4$)
  • $+9\,dB = \times 2 \times 2 \times 2$ (or $\times 8$)

Relationship Between Depth and Pulse Parameters

• Shallow Imaging:
  • Decrease depth.
  • Decrease $PRP$ (affects the "off" time only).
  • Increase $PRF$.
  • Increase $DF$.
• Deep Imaging:
  • Increase depth.
  • Increase $PRP$ (affects the "off" time only).
  • Decrease $PRF$.
  • Decrease $DF$.