Axial and Lateral Resolution - Ultrasound Physics

Axial and Lateral Resolution What does”Resolution” mean? The ability to accurately image There are many ways to describe accuracy and many types of resolution  Axial Resolution Describes one measure of detail resolution in an image Measures the ability of a machine to dis...

Axial and Lateral Resolution What does”Resolution” mean? The ability to accurately image There are many ways to describe accuracy and many types of resolution  Axial Resolution Describes one measure of detail resolution in an image Measures the ability of a machine to display two structures that are very close together when the structures are parallel to the main axis of the sound beam.  Axial Resolution Measures the systems ability to display structures that are parallel to the beam (positioned one in front of the other)  Axial resolution deals with the minimum distance that two structures can be apart and still be seen as two distinct structures on an ultrasound image. How structures are in the body What we see on screen 1 1 1 OR 2 2 2 Separate structures One structure Good Axial Not Good Resolution Axial Resolution  Axial resolution is measured in mm. It is determined by the spatial pulse length. Shorter pulses improve axial resolution Synonyms (know for SPI): LAARD longitudinal resolution Axial resolution range resolution radial resolution depth resolution Can it be changed? No because spatial pulse length is fixed. ( a pulse is a pulse is a pulse) Typical values: 0.1 – 1 mm. Lower values = shorter pulses and improved image accuracy  AXIAL RESOLUTION Minimum reflector separation required along the direction of the sound travels to produce separate echoes Increasing frequency improves axial resolution  Lower values of axial resolution (smaller distances) indicate a shorter pulse. Shorter pulses create more accurate images so image quality is better with lower numbers.  Axial Resolution  Numerical Value Lower numerical value indicate shorter pulse Lower numerical value = better image quality Axial Resolution(mm) = SPL(mm) 2 Axial Resolution (mm) = wavelength(mm) x #cycles in pulse 2 In Soft tissue Axial Resolution(mm) = 0.77 x # cycles in pulse frequency Why do some transducers have better axial resolution than others? Axial resolution is determined by the pulse length. Shorter pulses yield improved axial resolution. A short pulse is created in two ways: Less ringing Higher frequency (shorter wavelength) Less Ringing  A pulse is short if there are few cycles in the pulse.  Transducers create pulses that are only 2-3 cycles.  One way to reduce ringing is to dampen the crystal after it has been excited by an electrical signal (this keeps it from ringing for a long time)  transducers are designed w/ backing material to have few cycles per pulse, so that the numerical LARRD resolution is low and the image accuracy is superior High Frequency  A pulse is short if each cycle in the pulse has a short wavelength.  Shorter wavelengths are characteristic of high frequencies.  Pulses from high frequency transducers have superior axial resolution. Clinical Compromise High Frequencies improve image detail (resolution Low frequencies provide deeper penetration Always choose the transducer that is appropriate for your area of interest and imaging depth Excellent Axial Resolution is associated with: 1.Shorter spatial pulse length 2.Shorter pulse duration 3.Higher frequencies 4.Few cycles per pulse (less ringing) 5.Lower numerical values (smaller distances)  Lateral Resolution AKA: Lateral Angular Transverse Azimuthal Units: distance (mm, cm) Determined by: width of the sound beam, varies with depth (why?) Lateral Resolution(mm) = Beam Diameter(mm) The MINIMUM distance that two structures can be apart and still be seen as two structures when they are perpendicular to the sound beam  Lateral Resolution Resolution of structures that are perpendicular to the beam Better Lateral Resolution - lateral resolution is best at the focus because the beam is narrowest lateral resolution = beam diameter  Axial vs Lateral Resolution Axial resolution is better than lateral resolution because ultrasound pulses are shorter than they are wide (but they are both important!) Higher frequencies improve both axial & lateral resolution: Axial is improved because of the shorter pulses associated with high frequency sound Improved in the entire image Lateral is improved because in the far field high frequencies diverge less than low frequencies Improved in the far field only because of less divergence  AXIAL RESOLUTION LATERAL RESOLUTION Orientation of Front to back/parallel to Side by side, perpendicular structures the sound beam to the sound beam Mnemonic LARRD LATA Determined by Pulse length Beam width Does it Same at all depths Changes with depth as change? beam diameter changes In the near Short pulses Small diameter crystals field best with: In the far field Short pulses Large diameter and high best with: frequency crystals (less divergence)  Estimating Lateral Resolution To estimate lateral resolution, you have to know precisely what your structures that you are scanning look like. Phantom and a tiny bb You would measure a structure that is a point reflector – meaning it’s a single point. (like a bb) The structure would appear sonographically as a wide bar. To estimate lateral resolution, you would measure the bar. The width of the bar is the width of the sound beam and thus your lateral resolution. Focusing Focusing CONCENTRATES the energy in a sound beam. How we can create an “ideal” beam Focusing Focusing Alters the beam in three ways: Narrower “waist” in the beam beam diameter in the near field and the focal zone narrows Shallower focus – The focus is moved closer to the transducer so the near zone length is reduced (focal depth is reduced) Beam diameter beyond the focal zone widens (diverges), lateral resolution will be degraded in the far field Size of the focal zone is reduced Focusing Three methods of focusing Lens-external focusing (fixed) Internal focusing - Curved Crystal (fixed) Phased array focusing - Electronic Focusing (adjustable) Internal and external focusing can be used with single element transducers. Phased array focusing is for transducers with multiple elements (arrays) Types of Focusing: Fixed Focusing the focal depth is fixed and cannot be adjusted determined by the properties of the transducer Also called conventional or mechanical Includes internal and external techniques Has the poorest lateral resolution – focal depth can’t be changed or adjusted Techniques of fixed focusing: External focusing Lens is placed in front of PZT Similar to focusing light waves with a lens Larger arc in the lens creates more focusing Internal Focusing PZT is curved Larger curve = more focusing Most common form of fixed focusing  Electronic focusing (phased array transducers) generally have better lateral resolution because the focus is adjustable by the sonographer The system’s electronics focus the sound beam Can be adjusted by the sonographer Only useful with mutli-element transducers Phased array technology provides dynamic, variable focusing or multi-focusing.  Method of Name Type Focusing Lens External Fixed/conventional/ mechanical Curved active Internal Fixed/conventional/ element mechanical Electronic Phased Adjustable array Focusing and Intensity