Lesson 6_Image Optimization PDF

Ultrasound Physics and Instrumentation MRD535 Image Optimization By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Explain the principles of ultrasonography including ultrasound p...

Ultrasound Physics and Instrumentation MRD535 Image Optimization By Dr Leong Sook Sam Learning objectives Describe the principle, physics, instrumentations, accessories and image recording in ultrasonography (PLO1, C2) Explain the principles of ultrasonography including ultrasound physics Contents 1. Image optimization Output power Gain Total gain compensation (TGC) Frequency Focal zone Depth Zoom Sector width Dynamic range Frame rate 2. Image orientation Brightness Control 3 brightness controls: Output power Gain control Total gain compensation (TGC) Transmission/ Output Power Transmission power describe the energy per unit time (mW/cm2). Increasing the transmit power will increase the amplitude of the voltage pulses across the crystals and determines the strength of the crystal vibration. Transmission/ Output Power (cont) Output power increases or decreases the brightness levels by changing the strength of the sound pulse send to the body. The stronger pulse means that all the returning echoes are stronger, and all the echoes of the image are increased in brightness uniformly. Depending on output power, ultrasound exert mechanical and thermal effects on the tissue. Gain Gain adjusts the displayed amplitude of the returning signals. Gain adjust the brightness level of the entire ultrasound image by amplifying the strength of the electrical signals in the receiver (that have already returned back to the machine) Gain does not change the strength of the pulse send into the body. It has no effect on output Time Gain Compensation (TGC) TGC boosts the signals in the receiver at different depth in the image. TGC is important due to beam attenuation, greater for deeper interfaces. TGC corrects for attenuation (loss of strength of a sound wave as it penetrates further into the tissue) TGC does not change the strength of the pulse sent into the body. Time Gain Compensation (TGC) (cont) ALARA (AS LOW AS REASONABLY ACHIEVABLE) When considering how to improve an image and adjusting controls, consider first the impact to the patient: 1st: adjust TGC 2nd: Gain 3rd: Frequency 4th: Focal zones Last: output power Frequency For wide-bandwidth transducer, operator can choose the higher frequency for detail resolution, or lower frequency for better penetration (refer lesson 3_bandwidth and Q factor) Frequency (cont) The transducer frequency is calculated from the ratio of speed and wavelength. f = c/ λ (refer to lesson 1_Frequency) Higher frequencies have a shorter wavelength, and the wavelength is reciprocally proportional to the resolution. Operator can choose between having higher frequencies (RES), general (GEN) and penetration (PEN) modes (frame rate reduce).  (a): Low frequency (b): High frequency Examination of the gallbladder and hepatic hilum using a linear transducer in a slim person: adequate depth penetration at lowest possible frequency (a) and loss of depth penetration using a high frequency (b). Focal zone The focal zone or plane is the part of the US beam where its diameter is focused and narrowest (refer to lesson 3_Focusing). Proximal to this zone, the beam diameter is wider. It increases again when travelling past this zone. A thicker layer width will decrease image resolution. Since the layer thickness is lowest in the focus area, it should always be at the level of interest or start just above the structure to be displayed.  b a c US devices offer one focal plane a, b. Newer machines allow two or more c. An increasing number of foci will lowers the frame rate.  A - trackball B - caliper C - body mark D - auto optimization E – still image F – freeze G – color power angiography H- Pulsed wave I – overall gain J- color K- dual window L- zoom M- depth N- focal zone O – cini loop P – time gain compensation Depth The depth determines how "deeply" into the body one wishes to image. This setting controls the vertical depth of the field of view. Increasing the depth allows deeper tissue interrogation. It influences the size of the examination window. Depth (cont) Increased depth increases the scan time since the machine will need to wait longer to hear returning echoes from deeper structures; both transmit and receiving time are increased. This in turn increases the time per frame which decreases the frame rate. Using a lower frequency probe may allow us to see deeper structures. The layer thickness rises with increasing depth penetration, which further deteriorates the resolution. Ultrasound examination of the pancreas with inadequate (a) and adequate (b) selection of depth penetration. The field of interest (pancreas) is in the middle to the lower third of the adequate image (b). Zoom Zoom can help to enlarge an image section on the monitor. Zoom has no influence on depth penetration. The image is displayed larger within a region of interest (box). As the scan line density may be concentrated on a smaller region, image resolution is also higher. A magnification of a frozen image does not gain any advantages. Sector Width The imaging sector is the area that is interrogated and displayed on the ultrasound machine. Reducing the sectoral width will improve lateral resolution by narrowing the acoustic window. Decreasing the scan size is the same as using fewer scanning lines per image and thus improves the frame rate. Keep sector width at a minimum. Dynamic Range Dynamic range defines the echo strengths shown on the monitor (Specifically it is the range - minimal to maximal of amplitudes that the system is capable of displaying). Adjusting the dynamic range will not change the amplitudes of the returned signal, only of how that returned signal is displayed. Increasing the dynamic range allows operator to see very low and very high amplitudes on the display and thus a wide range of grayscale. Dynamic Range (cont) A high dynamic range offers more information about the echo patterns, appears brighter and softer and is therefore preferable for representing organ parenchyma. A low dynamic range image results in a more “black-and-white” like shape and thus, higher contrast. Dynamic range is available on live or frozen images. Low dynamic range creates images that Very high dynamic range creates images have less of “shades” of gray on the that appear blurry or grainy borders range of different combinations of since there are no absolute white or white and black. black. Frame Rate How many times / second the image is updated Determines temporal resolution potential, important when assessing moving interfaces, e.g.: cardiac. Frame rate is influenced by several settings: Imaging frequency Depth Sector width Focus Image Recording Options Thermal paper Coated with a heat-sensitive material When the paper is exposed to heat, it reacts with the thermal printing head of the ultrasound machine to generate a clear and sharp image. Do not require ink to generate images. Visualize image durable for up to 7-10 years under proper storage. Present excellent resistance to moisture and humidity. Resistant to fading and discoloration over time. Image Recording Options (cont) Film CD Picture archiving and communication system (PACS) is a medical imaging technology used primarily in healthcare organizations to securely store and digitally transmit electronic images and clinically-relevant reports. PACS replacing the roles of conventional radiological film. Thank you