DK2985_C000 1..28 - AlSharqia Echo Club

50 Transesophageal EchocardiographyPulse(A) (B) (C)LARALVRVImage generationFigure 3.14 Image generation. (A–C) Each line in the image is created by a pulse. Summation of all the pulses creates the final twodimensionalimage (LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle).echocardiography is the vector array, in which phasing isapplied to a linear sequenced array to generate focusedpulses in different directions.(scan line). It is affected by the spatial pulse length, thatis, the length that a pulse takes up, equal to the productof wavelength by the number of cycles in the pulse.V. THE IMAGE QUALITYResolution is a characteristic that describes the quality ofthe image. It allows echoes to be distinguished in termsof space, intensity, and time. These are known as detail,contrast, and temporal resolution, respectively (Fig. 3.15).A. Resolution1. Detail ResolutionDetail or linear resolution refers to the ability of theimaging system to represent two closely related butspatially distinct objects (ultrasound reflectors) as separate(or resolved) nonoverlapping echoes. As resolution is theminimum reflector separation required to discriminatebetween two objects, the smaller the value of resolution,the better the imaging system. There are two aspects todetail resolution: axial and lateral.AxialAxial resolution is the ability to separate two objectsspatially along the direction of the ultrasound beamAxial resolution ¼ 1 spatial pulse length (3:3)2Therefore, axial resolution is improved by decreasingboth the wavelength and the number of cycles per pulse.The latter is determined by the damping material in thetransducer and cannot be currently modified by the user.On the other hand, wavelength can be reduced by increasingthe transducer imaging frequency, an operation whichis easily accessible to the imaging system operator(Fig. 3.16). However, the drawback in increasing frequencyfor better resolution is higher attenuation withimaging depth and consequently a reduction in penetrationcapability.LateralLateral resolution represents the ability to discriminatebetween two objects lying perpendicular to the ultrasoundbeam direction (scan line). It is determined by the beamwidth: if the spatial separation between two objects isgreater than the beam width, two different echoes can begenerated and resolved as the beam is scanned acrossTable 3.1Transducer ArraysTransducer array Scanning or steering Focusing Image displayMechanical sector Motor drive Curved lens or element SectorLinear array Electronic sequencing Electronic phasing RectangularPhased linear array Electronic sequencing Electronic phasing ParallelogramConvex array Electronic sequencing Intrinsic property due toSectorits configurationPhased convex array Electronic sequencing Electronic phasing SectorAnnular array Motor drive Electronic phasing SectorVector arrayElectronic sequencing andelectronic phasingElectronic phasingSector