Focusing:

Mechanical - performed by placing an acoustic lens on the surface of the transducer or using a transducer with a concave face.

© Atlantic Interactive

External Focusing - a lens is used on the transducer face to bend the soundwaves to a focal point.

Internal Focusing - the piezioelectric element is formed in a curved element that focus the beam.

Electronic - using a process called phased array, multiple elements are fired sequentially to focus the beam .

Beam Focusing

Array transducers have the ability to be dynamically focused by stimulating each element as shown in the diagram below. The individual wave fronts add up to a tight beam of ultrasound energy.

  
© Atlantic Interactive


Beam Steering:

Array transducers have the ability to be steered as well as focused. Like focusing, the beam is directed by sequentially stimulating each element as shown. This feature creates the sector scan by rapidly steering the beam from left to right to give the two dimensional cross sectional image.

     
© Atlantic Interactive

	steering the beam electronically through the scan plane

Resolution:

Lateral resolution - the ability to resolve objects side by side. Lateral resolution is proportionally affected by the frequency, the higher the frequency the greater the lateral resolution. Higher frequency transducers are used in fetal and pediatric echocardiography because the lateral resolution displays the smaller structures in those patients and there is less need for depth penetration. Lower frequencies are used for adults where structures are larger and the need for greater depth penetration is important.

Axial Resolution - Axial resolution is the ability to resolve objects that lie one above the other. Axial resolution is inversely proportional to the frequency of the transducer depending on the size of the patient. The higher the frequency the lower the axial resolution is in large patients. This state results from the rapid absorption of the ultrasound energy with lower penetration. Lower frequencies are utilized to increase depth of penetration.

Depth of Penetration - Higher frequencies are attenuated by tissue more than lower frequencies. This means that the higher the frequency the greater the resolution but the lower the depth of penetration. User lower frequencies for adults and higher frequencies for children. The advent of harmonic imaging allows the use of a lower frequency pulse to be picked up and sampled at a higher frequency (the second harmonic) where the low frequency allows greater penetration and high frequency provides better resolution.

		2.0mHz		5.0mHz
Axial		Decrease		Increase
Lateral		Decrease		Increase
Penetration		Increase		Decrease

Basic Components of the Imaging System:

Transducer - the probe housing the elements, backing material, electrodes, matching layer and protective face that both sends and receives the sound waves.

Transmitter - the component that creates the impulses sent to the transducer to generate sound energy. Also called the pulser.

Receiver - the component that receives the current generated in the transducer from the returning sound energy.

Amplifier - the component that amplifies the returning signals and prepares them to be displayed on the CRT.

CPU - the central processing unit that