ARTIFACTS Flashcards by Unknown Unknown

is used to describe
any part of an image that does not accurately represent
the anatomic structures present within the subject being
evaluated.

artifact

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may cause structures to appear in an image that are not present
anatomically or a structure that is present anatomically
may be missing from the image.
- may also
show structures as present but incorrect in location, size,
or brightness.
-the potential to interfere with image interpretation.

ultrasonography (US), artifacts

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The creation of a US image is based on the physical
properties of ?

ultrasound pulse formation,
-the propagation of sound in matter,
-the interaction of sound with reflective interfaces, and
-echo detection and processing

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Ultrasound display equipment relies on physical assumptions to assign the ? and ? of each received echo.

location and intensity

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the depth of an object is directly related to the

-amount of time for an ultrasound pulse to return to the transducer as an echo,
-the speed of sound in human tissue is constant,
-the sound beam and its echo travel in a straight path, and
-the acoustic energy in an ultrasound field is uniformly attenuated.

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Artifacts thus arise secondary to errors inherent to
the ?

-ultrasound beam characteristics,
-the presence of multiple echo paths,
-velocity errors, and
-attenuation errors

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The ultrasound beam exits the
transducer as a complex three-dimensional bow-tie
shape with additional off-axis low-energy beams,
which are referred to as ? and ?

side lobes and grating lobes

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The ultrasound beam exits the transducer as a complex ? shape

three-dimensional bow-tie

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side lobes or grating lobes
A strong reflector located outside of the main
ultrasound beam may generate echoes that are
detectable by the transducer. These echoes will be
falsely displayed as having originated from within
the main beam. This form of artifact is more likely
to be recognized when the misplaced echoes
overlap an expected ?

anechoic structure

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grating lobes and side lobes are froms of

off-axis energy

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? can be identified by understanding the shape of the ultrasound beam.
-may be recognized when a structure that should be anechoic such as the bladder contains peripheral echoes.
-overlapping of the object of interest
-the focal zone is improperly set too shallow

Beam width artifact

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beam width artifact is recognized during scanning, image quality may be improved by

adjusting the focal zone to the level of interest and by placing the transducer at the center of the object of interest

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are multiple beams of low amplitude ultrasound energy that project radially from the main beam axis.
-generated from the radial expansion of piezoelectric crystals and is seen primarily in linear-array transducers.
-Strong reflectors present in the path of these low-energy, off-axis beams may create echoes detectable by the transducer.
-to be recognized as extraneous echoes present
within an expected anechoic structure such as the bladder.
-dublicates the structure

Side lobes

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is generated from the radial expansion of piezoelectric crystals and is seen primarily in linear-array transducers.

Side lobe energy

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Side lobe energy is generated from the radial expansion of piezoelectric crystals and is seen primarily in ? transducers

linear-array transducers

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ARTIFACTS ASSOCIATED WITH ULTRASOUND BEAM CHARACTERISTICS

-beam width
-side lobes

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ARTIFACTS ASSOCIATED WITH MULTIPLE ECHOS

-reverberation
-comet tail
-ring
-mirror image

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ARTIFACTS ASSOCIATED WITH VELOCITY ERRORS

-speed displacemnt
-refraction

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ARTIFACTS ASSOCIATED WITH ATTENUATION ERRORS

Shadowing

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-At imaging, this is seen as multiple equidistantly spaced linear reflections
-In the presence of two parallel highly reflective surfaces, the echoes generated from a primary ultrasound beam -may be repeatedly reflected back and forth before returning to the transducer for detection

Reverberation Artifact

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? is a form of reverberation.
In this artifact, the two reflective interfaces and thus sequential echoes are closely spaced.
On the display, the sequential echoes may be so close together that individual signals are not perceivable.
-In addition, the later echoes may have decreased amplitude secondary to attenuation; this decreased amplitude is displayed as decreased width.
-The result is an artifact caused by the principle of reverberation but with a triangular, tapered shape

Comet tail artifact

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-has been thought to be a variant of comet tail artifact.
-This assumption was based on the often similar appearance of the two artifacts.
-the transmitted ultrasound energy causes resonant vibrations within fluid trapped between a tetrahedron of air bubbles.
-These vibrations create a continuous sound wave
that is transmitted back to the transducer
This phenomenon is displayed as a line or series of parallel bands extending posterior to a gas collection.
-image shows bright reflector with an echogenic line extending posteriorly

ring-down artifact

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are also generated by the false assumption that an echo returns to the transducer after a single reflection.
-In this scenario, the primary beam encounters a highly reflective interface. -The reflected echoes then encounter the “back side” of a structure and are reflected back toward the reflective interface before being reflected to the transducer for detection.
-The display shows a duplicated structure equidistant from but deep to the strongly reflective interface

Mirror image artifacts

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The speed of sound within a material is dependent on its ? and ? properties.

density and elastic

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US image processing assumes a constant speed of sound in human tissue of ?

1540 m/sec.

density of air

1.2 kg/m^3

density of fat

924 kg/m^3

density of bone

1912 kg/m^3

denisty of soft tissue

1050 kg/m^3

speed of bone

4080 m/sec

speed of soft tissue

1540 m/sec

speed of fat

1450 m/sec

speed of air

330 m/sec

When sound travels through material with a velocity significantly slower than the assumed , the ? returning echo will take longer to return to the transducer.

1540 m/sec

The ? assumes that the length of time for a single round trip of an echo is related only to the distance traveled by the echo. The echoes are thus displayed deeper on the image than they really are

image processor

echo is related only to the distance traveled by the echo. The echoes are thus displayed deeper on the image than they really are. -sound beam travels slower, longer than expected. -in clinical imaging, it is often recognized when the ultrasound beam encounters an area of focal fat THIS IS REFERRRED TO AS

speed displacement artifact

A change in velocity of the ultrasound beam as it travels through two adjacent tissues with different density and elastic properties may produce a ? -nonperpendicular incident ultrasound energy encounters an interface between two materials with different speeds of sound. When this occurs, the incident ultrasound beam changes direction.

refraction artifact

The degree of this change in direction is dependent on both the angle of the incident ultrasound beam and the difference in velocity between the two media. This relationship is described by ?

Snell’s law

The degree of this change in ? is dependent on both the angle of the incident ultrasound beam and the difference in ? between the two media. This relationship is described by Snell’s law:

direction velocity

Snell's law equation

sinOr/ sin Oi = c2 /c1 or n1sin01 = n2sin02

The ultrasound display assumes that the beam travels in a straight line and thus misplaces the returning echoes to the side of their true location In clinical imaging, this artifact may be recognized in pelvic structures deep to the junction of the rectus muscles and midline fat. ? may cause structures to appear wider than they actually are or may cause an apparent duplication of structures.

Refraction artifact

As an ultrasound beam travels through the body, its energy becomes attenuated secondary to absorption and scatter. An echo that travels a greater distance in the body will be attenuated more than an echo of similar energy that travels a shorter path. Ultrasound processing incorporates “?” of echoes that take longer to return to the transducer.

compensation amplification

In this process, the echoes that return later are amplified more than earlier returning echoes. This serves to make the image appear more uniform in the deep field. “? refers to a user-adjustable form of compensation. The attenuation coefficient expresses the loss of ultrasound intensity per distance traveled and varies in different mediums.

Time gain compensation”

attenuation coefficient of water

0.0002 dB/cm

attenuation coefficient of soft tissue

0.3-0.8 dB/cm

attenuation coefficient of fat

0.5-1.8 dB/cm

attenuation coefficient of bone

13-26 dB/cm

attenuation coefficient of air

40 dB/cm

when the ultrasound beam encounters a strongly attenuating or highly reflective structure, the amplitude of the beam distal to this structure is diminished The echoes returning from structures beyond the highly attenuating structure will also be diminished. In clinical imaging, this phenomenon is recognized as a dark or hypoechoic band known as a ?” deep to a highly attenuating structure

shadowing

? is also dependent on the frequency of the ultrasound. it increases with increase in frequency

Attenuation

. In ? , the relationship between attenuation and frequency is linear. In ? attenuation increases as the square of the frequency.

soft tissues bone and water,

– a technique that evaluates blood flow in the umbilical cord, fetus or placenta – may be part of this exam.

Doppler ultrasound study

Obstetrical ultrasound is a useful clinical test to:

-establish the presence of a living embryo/fetus -estimate the age of the pregnancy -diagnose congenital abnormalities of the fetus -evaluate the position of the fetus -evaluate the position of the placenta -determine if there are multiple pregnancies -determine the amount of amniotic fluid around the baby -check for opening or shortening of the cervix -assess fetal growth -assess fetal well-being

Some physicians also use ? to image the fetus and determine if it is developing normally.

3-D ultrasound

A gestational sac should be visible at ? weeks and ? days,

4 weeks and 4 days,

, a yolk sac at ? weeks

5 weeks,

Human pregnancy lasts an average of ? days from the first day of the last menstrual period in patients with regular ?-day menstrual cycles. Both clinical and ultrasonographic gestational age are expressed using this standard.

280 days 28-day menstrual cycles

The use of ? has greatly enhanced our ability to detect pregnancy early. The earliest ultrasonic evidence of pregnancy is the finding of a ? with an echogenic border.

endovaginal ultrasound fluid-filled gestational sac

a fetal pole with cardiac activity at ? weeks.

6 weeks

Using a ?, the gestational sac and fetal pole can be found earlier than with the abdominal approach. The ? eliminates the need for a full bladder before examination and significantly improves resolution in obese patients.

vaginal probe endovaginal transducer

The ?) of the fetus is an accurate predictor of gestational age. -has been promoted as the most accurate method of dating a pregnancy, but experience has shown that the average of the biparietal diameter (BPD), femur length, and abdominal circumference obtained before 20 weeks' gestation is comparable in accuracy.

crown-rump length (CRL

A practical approximation is that fetal CRL in centimeters plus ? equals gestational age in weeks. A 9-week fetus, for example, would have a CRL of ? cm.

6.5 2.5 cm

After the first trimester, CRL determination is not practical because of fetal posturing and size. From 14 weeks onward, the ? is one of several useful estimators of fetal gestational age. -the largest transverse measurement of the fetal skull and usually is measured from an ? scan plane at the level of the fetal thalami and the septum pellucidum cavum -measured from the outside of the proximal skull table to the inside of the distal skull table.

occipitofrontal scan plane (BPD) Biparietal Diameter

BPD the largest transverse measurement of the fetal skull and usually is measured from an occipitofrontal scan plane at the level of the fetal ? and the ?

thalami septum pellucidum cavum

the occipitofrontal (OF), used often to measure ?; and the ? which shows the posterior fossa anatomy

(BPD) biparietal diameter and suboccipitobregmatic (SB),

The fetal head should have an ? shape with clear ?. The measurement is determined, by convention, from the outer edge of the proximal fetal parietal bone to the inner edge of the distal parietal bone The gestational age is estimated by comparing the observed BPD to tables relating BPD to gestational age.

oval shape midline margination

Because of increasing biologic variation with advancing gestational age, the precision of estimated gestational age from BPD is greatest early in the ? trimester.

second trimester

Between ?and ? weeks, the BPD correlates with gestational age within 1 week in ?% of cases. Late in the third trimester, the measured BPD may predict gestational age within only ?-? weeks.

14 and 20 weeks, 95% 3–4 weeks.

In approximately 5% of cases, BPD cannot be obtained because of fetal position; in a fetus in ? or ? presentation, the proper plane of the fetal skull cannot be visualized. Deferring the study for a short time may allow the fetus to assume a more favorable position. If an appropriate BPD cannot be obtained, an alternative fetal dimension is chosen.

occipitoanterior or occipitoposterior

In approximately ?% of cases, BPD cannot be obtained because of ?;

5% fetal position

The ? is measured in the same plane as the BPD and is a measurement of the longitudinal axis, by convention taken from outer skull tables on each side. This measurement can be averaged with the outer-to-outer skull measurement in the transverse plane to provide a basis for estimating head circumference.

occipitofrontal diameter

The ?is the ratio of the transverse to longitudinal diameters measured in a similar fashion;

cephalic index

a normal cephalic index is

0.78 ± 0.05

fetal head is either more oval (?) or more round (?) than average.

dolichocephalic brachycephalic

A ? , for instance, often is seen in breech presentations and may introduce error into the use of BPD to estimate gestational age.

low cephalic index

Measurement of the fetal limb bones also may be used to determine gestational age. The ? length usually is chosen because of its relative positional stability. The measurement is obtained by aligning the transducer with the lower end of the ? and rotating toward the ? aspect of the fetus. At first, only part of the femur may be seen; slight rotation and angulation of the transducer allows the entire length to be imaged. When properly visualized, the femur should have clear distal margins. Often, the bone casts a shadow to help identify this landmark

femur fetal spine ventral

The ultrasound images typically do not include ? in early gestation; therefore, ?-?% of the palpable bone is not measured. Because the reference tables were derived in this same fashion, accuracy is preserved.

epiphyses 8–15%

As is the case with BPD, the reliability of femur length in prediction of gestational age is best in early pregnancy. In the ? trimester, however, femur length shows less variation than the BPD. Femur length may be difficult to obtain in a breech presentation, and measurement of the fetal ? provides a good alternative. it usually can be identified arising from the lateral upper fetal chest.

third humerus

? or ?, also is useful in predicting gestational age.7 This measurement is obtained in a transverse plane, perpendicular to the fetal spine, at the level of the stomach and umbilical vein

fetal abdominal circumference, or mean abdominal diameter

fetal abdominal circumference, or mean abdominal diameter, also is useful in predicting gestational age.? This measurement is obtained in a ?plane, perpendicular to the ? at the level of the ? and ?

age.7 transverse plan fetal spine, stomach and umbilical vein

The fetal image in this traverse plane is nearly circular; the ? is the average of the anteroposterior and transverse diameters, measured from the outer diameter to outer diameter. -can be obtained by multiplying the mean abdominal diameter by π (approximately 3.14), or by directly measuring the circumference using a ? on the ultrasound image, which is provided on most current ultrasound systems.

abdominal circumference perimeter system

The relation between ultrasonographically derived fetal dimensions and gestational age is purely empiric. The older a normal fetus is, the larger its dimensions are. The reproducible ultrasonographic measurement of any fetal dimension in a normal reference population of fetuses of known gestational age allows the construction of a ? between that dimension and age.

regression relation

A basic ultrasound examination should contain a ?(3)

-fetal survey, -an evaluation of fetal biometry, and an -anatomic screening examination.

The fetal survey includes a confirmation of ?

-fetal number, -viability, -position, -assessment of amniotic fluid volume, and -location of the placenta.

In assessing fetal biometry, the applicable standard fetal measurements already discussed, including ?(4) , should be taken.

-CRL, -BPD, -abdominal circumference, and -femur length

The estimation of ? is a clinically useful parameter computed from the fetal biometric measurements.

fetal weight

Indications for ultrasound in pregnancy

-Unsure last menstrual period -Vaginal bleeding during pregnancy -Uterine size not equal to that expected for dates -Use of ovulation-inducing drugs in early pregnancy -Obstetric complications in a prior pregnancy: prior cesarean section, preterm delivery -Screen for fetal anomaly in selected high-risk pregnancies: elevated maternal serum α-fetoprotein level, prior fetal malformation, certain drug exposure in early pregnancy, maternal diabetes -Antepartum fetal assessment in high-risk pregnancy: postdate fetus -Following fetal growth in certain high-risk situations: twins, suspected intrauterine growth restriction

-Screen for fetal anomaly in selected high-risk pregnancies: elevated maternal serum ? level

α-fetoprotein level,

ARTIFACTS Flashcards

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