US Flashcards
How is a sonographic image formed?
Mechanical oscillations of a crystal are excited by electrical pulses (piezoelectric effect).
The oscillations are emitted as sound waves from the crystals through frequencies ranging from to 2 to 15 MHz.
The crystals are assembled to form a transducer from which sound waves propagate through the tissues to be reflected and returned as echoes to the transducer. These echoes are in reverse converted by the crystals into electrical pulses used to compute the ultrasound image
What is the relationship between acoustic density, intensity of the reflected sound and the transmitted sound ?
If the difference in acoustic density increases, intensity of the reflected sound increases and that of the transmitted sound decreases proportionately
What happens to the sound beam if the acoustic densities are vastly different ?
The sound beam is completely reflected and total acoustic shadowing results
How does a linear array transducer emit sound waves, what type of image does it produce, what frequency does it range between and what are the advantage (s) and disadvantage (s) ?
- emits sound waves parallel to each other
- produces a rectangular image
- primarily used with high frequencies 5 to 7.5 MHz - evaluates soft tissues, thyroid gland
- Advantage - Good near-field resolution
- Disadvantage - Large contact surface, which leads to artifacts when applied to a curved body surface due to air gaps between the skin and the transducer
How does a sector array transducer emit sound waves, what type of image does it produce, what frequency does it range between and what are the advantage (s) and disadvantage (s) ?
- emits sound waves in a fan-like distribution
- produces a fan-like image narrower near the transducer with increasing width the deeper the penetration
- Phased-array sector transducer (electronic movement of piezo elements) has frequencies of 2 to 3 MHz
- Advantage - better visualization of intercostal structures due to small transducer surface and the beam divergence to a 60 to 90 degree sector
- Disadvantage - poor-near field resolution, decreasing number of scan lines with depth (spatial resolution), handling difficulties
How does a curved or convex array transducer emit sound waves, what type of image does it produce, what frequency does it range between and what are the advantage (s) and disadvantage (s) ?
- offers a wide near and far zone
- Frequency from 2.5 MHz (obese patients) to 5 MHz (slim patients)
Advantage - better penetration of abdominal organs
Disadvantage- density of the scan lines decrease with increasing distance from the transducer
What is the principle behind relative distal acoustic enhancement ?
A physical phenomenon of increased echogenicity, seen as a bright band found where sound waves travel through homogeneous fluid.
Due to decreased reflection in fluid, the sound waves attenuate less and are therefore of higher amplitude posterior to the structure in comparison to adjacent sound waves
What is the concept of distal shadowing artifact ?
A zone of reduced echogenicity (Hypoechoic or anechoic) is found behind a strongly reflecting structure
eg. Calcified structures such as bone, air
What is the concept of edge shadowing ?
Occurs behind all round cavities that are tangentially hit by sound waves. It is caused by scattering and refraction of the sound waves.
What is the concept behind reverberation artifact ?
Instead of the echoes which originate from the acoustic interface returning to the transducer without further reflection, strongly reflecting boundaries are encountered which cause the sounds waves to reflect back and forth before they eventually return as echo to the transducer.
The delay in registering the echoes leads to the artifact.
What is the concept behind section thickness artifact ?
Occurs when the boundary between the wall of a fluid containing structure and the containing fluid is not perpendicular to the interrogating sound beam.
The echoes within the returning beam include echoes from both liquid and solid structures and are averaged by the processor which results in the boundary between solid tissue and fluid being seen as a low echogenic and in distinct structure which may mimic debris.
What is the concept behind arch artifact ?
Strongly reflecting interfaces cause scattered reflection of echoes, falsely displacing the acoustic interface laterally
eg. The duodenal wall projecting in the linen of the gallbladder ,
air-containing bowel loop seen within the urinary bladder
What is the concept behind mirror image artifact?
Primarily produced by the diaphragm and visceral pleura, causing intrahepatic structures to be seen on the pulmonary side of the diaphragm as an optical illusion
What are hyperplastic columns of Bertin?
An extension of renal cortical tissue which separates the renal pyramids. When unusually enlarged may be mistaken for a mass.
What are hyperplastic columns of Bertin?
An extension of renal cortical tissue which separates the renal pyramids. When unusually enlarged may be mistaken for a mass.
How soon is a GS detected on ultrasound ?
On TV a sac of 2 to 3 mm can be detected after the 14th day of conception, 3 days after the LMP or at the start of the 4th gestational week
and it should be detectable if the serum hcg exceeds 750 to 1000 U/I (international units), otherwise an ectopic pregnancy must be excluded
Cardiac activity is detected from the 6th week and should be 80-100 beats at this time
What are the different appearances of the endometrium throughout the menstrual cycle
The normal endometrium changes in appearance as well as in thickness throughout the menstrual cycle:
in the menstrual and early proliferative phase it is a thin, brightly echogenic stripe comprising of the basal layer; minimal fluid can be appreciated endovaginally within the endometrium in the menstrual phase
in the late proliferative phase it develops a trilaminar appearance: outer echogenic basal layer, middle hypoechoic functional layer, and an inner echogenic stripe at the central interface
in the secretory phase it is at its thickest, up to 16 mm 10, and becomes uniformly echogenic, as the functional layer becomes oedematous and isoechoic to the basal layer; there is through transmission and posterior acoustic enhancement noted
The postmenopausal endometrium should be smooth and homogeneous.
