- USG is performed with the pulse-echo technique. By the virtue of piezoelectric effect (AIMS 04, PGI 98) in the ultrasound probe (or transducer), electric energy is converted to sound energy that is transmitted into patient’s tissues. The US transducer (probe) then becomes a receiver, detecting echo of sound energy reflected (Al 94) from tissue.
Probe or Transducer
- The probe or transducer is any device that converts one form of energy to another. In case of USG, the tranducer converts electric energy to sound energy and vice versa. The ultrasound transducer uses the principle or property of piezoelectricity (AIIMS 04, PG1 98, TN 95, NEET) which occurs naturally in some materials whereby an applied electric field produces a change in linear dimensions.
Quartz (Maharashtra 01)
- is a naturally occuring peizoelectric material having the unique ability to respond to the action of an electric field by changing shape and to the change in polarity of voltage applied by generating small potentials and thus producing an USG image. Currently, Lead Zirconate titanate (PZT) (Bihar 05) is the most widely used material in the ultrasound tranducer/probes.
Frequency & Velocity
- The ultrasound machine emits high-frequency sound waves, ranging from 2-15 MHz (PGI01, Maharashtra 06) Whose frequencies are considerably above the upper limit of human ear’s audible range, i.e., greater than 20000 Hz (Bihar 641) .Speed of these sound waves in the body is 1540 m/s Uhaskhand 03) (in comparison to air, where velocity of sound wave is 330m/s).
- The sound beam is not transmitted with 100% efficiency through tissues. Factors that cause the beam to be attenuated include reflection, scattering and absorption. In imaging, reflection is the critical factor (Al 94), because it allows us to create images by collecting the echoes that return to the transducer.
- Acoustic impedence refers to the reflection or transmission characteristic of a tissue. Because the velocity of sound within each tissue and the tissue’s density determine the percentage of beam reflected or transmitted as it passes from one tissue to another, acoustic impedence is the product of tissue density and sound velocity in the tis
Acoustic impedence = Tissue density x Sound velocity
- It is the impedence difference between tissues that counts, i.e., reflections themselves result from differences in acoustic impedance between tissues, i..e., when a sound wave encounters an acoustic interface or the boundry between two media of different acoustic impedence, the sound waves may be reflected.
- The amplitude of returning echo is proportional to the difference in acoustic impedence between two tissues as the sound beam passes across an interface from one to the other. There are only small differences in acoustic impedence among the body’s soft tissues. This is ideal for imaging purpose because only a small percentage of sound beam is reflected at such interfaces, whereas the majority is transmitted and available for imaging deeper structures.
Air (gas) 0.0004
- Fat → 1.38
- Water → 1-54
- Brain → 1.58
- Blood → 1.61
- Kidney → 1-62
- Liver → 1.65
- Muscle → 1.70
- Lens → 1.84
- Bone → 7.8
- Air has a very low and bone has a very high acoustic impedence. This means when the sound beam encounters a ‘soft tissue- bone’ or ‘Soft tissue-gas’ interface, nearly all sound is reflected (as reflection is proportional to the difference in acoustic impedence between two tissues) and little is available for imaging deeper structures. This effect represents a high acoustic impedence mismatch.
- To avoid this, ‘acoustic window’ must be used that avoids bone or gas during imaging, for example, the liver is imaged through the window of the intercostal spaces, pelvic organs are examined through the urine-filled bladder which displaces the gas filled bowel out of the pelvis. For the same reason, an acousting coupling gel is used between the transducer and skin for all ultrasound examination to eliminate interposed air.
- Regions distal to highly attenuating structures, such as bone or gas, appear dark on image because of shadowing -> As most of the sound beams are reflected by highly attenuating structures, little is available for imaging of deeper structures. It is referred to as Acoustic shadowing. Acoustic shadowing is produced by bone, calculus, and air (gas) (Maharashtra 03).
- On the other hand, structures deep to low attenuating structures, such as liquid, appear bright -> Acoustic enhancement. Acoustic enhancement is produced by cyst.
- Two types of tissue reflectors are sources of echoes on ultrasound : ?
- Specular reflectors (interface or specular echoes): – Specular echoes originate from interface at right angles to the beam. These echoes produce bounderies between structures much as they would appear in a gross anatomic cross section. For example, specular echoes may be seen at organ boudries or at the wall of vessels 4 Specular echoes are produced when the ultrasound wave encounters interface of two adjacent tissues with different acoustic properties.
- Non specular (scatter) reflectors : – Nonspecular echoes are produced by tissue components that scatter ultrasound wave.
- Specular echoes 4 Interface of adajacent tissues with different acoustic properties
- Nonspecular echoes From tissue material itself, i.e., parenchymal tissue.
- A mixture of specular and scattered echoes forms ultrasound images, but the complete mechanism of echo production within tissues is not yet fully understood.
- Bile 00“mi“,urine
- Renal medulla
- Renal cortex
- Storage fat
- Renal sinus
- Structural fat, vessel walls
- Bone, gas, organ boundaries
- Disease process changes the echogenicity of the tissues. The image echogenicity of a tissue is described relative to surrounding tissue or structures : –
- Areas with high echo intensity are referred to as echogenic/hyperechoic/echo rich.
- Areas with low echo intensity are referred to as hypoechoic/echo poor.
- Areas with no echo are referred to as anechoidecho free.
- The Doppler effect results from an apparent shift in sound frequencyou 13) as sound wave are reflected from moving targets, usually blood cells. If motion is toward the transducer, the frequency of returning echo is higher that of the transmitted sound. If the motion is away from the transducer, the echoes have a lower frequency than the transmitted sound. The difference between the transmitted and received frequencies is known as the Doppler shift. Colour doppler : – The newest array Doppler units are capable of colour coding blood velocity measurement over a wide area of the field and superimposing this information on two dimension gray-scale image. Signals from moving RBCs are displayed in colour as a function of their motion toward or away from transducer. The amount of colour saturation (color intensity) indicates the relative velocity of the cells (PGI 98‘ &ism 98).For example yellows, oranges and reds may represent flow directed toward the transducer, with yellow-white representing the highest measured velocity. Flow away from the tranducer may be represented as shades of blues and greens, with the highest velocities displayed as green-white.
- Easily available → Operator dependent
- Portable → Limited depth penetration
- Non-invasive → Limited anatomical access cannot penetrate
- No ionizing radiation→ through bone or air)
- Real time imaging→ Patient factors : – Liver is under rib cage, obesity
- Deleterious effect on small organism by acoustic cavitation w”)
Real – Time sonographic sector scanning of neonatal cranium
- A commercially available wide field of view real-time mechanical sector scanner can be used to image the neonatal cranium. Because of small tranducer head size, the open anterior fontanelle can function as acoustic window (PG’ 9 By thus avoiding bone, higher frequency transducers may be used to improve image resolution. Infants may be scanned quickly without sedation in their isolettes in the neonatal ICU. Sterility of the infant environment is maintained by placing the transducer in a surgical glove using this technique, detailed normal anatomy can be seen such as vascular structures, caudate nucleus, thalamus, third ventricle, cavum septum pellucidum, and the thalamocaudate notch. Angled coronal and sagittal sonographic anatomy is correlated with neonatal cadaver brain sections sliced in similar planes centered on the anterior fontanelle.
New imaging techniques in USG
- Harmonic imaging(DNB 16, DPG” :—This technique does not use the fundamental frequency of the transmitted sound waves, but their integer multiple, so called harmonics (e.g., 7.0 MHz for a fundamental 3.5 MHz).
- Second hormonic imaging : – This technique uses only doubled frequency of the fundamental signal for imaging.
- Phase inversion technique : – This is a recently introduced broadband technique that enables the dynamic optimization of hormonic multiples of the transmitted frequency with a broader bandwith.
- Contrast enhancement : – The echogenicity of blood and tissue can be enhanced with tiny micro-bubbles with a diameter of 3-5 pm that pass through the capillaries and change the impedence within the bloodstream.
- Spatial compounding: – This is another technique to suppress artifacts. The “real time compound imaging” does not scan the grid lines individually, but at different angles with real time computation of the image.
- To detect a 4 mm nodule in the pancreas, the investigation of your choice would be Endoscopic USG.
- USG examination of an 8 weeks pregnant female shows a gestational sac with absent fetal parts. The diagnosis is Blighted Ovum.
- Basanti, a 28yrs aged female with a history of 6 weeks of amenorrhea presents with pain in abdomen; USG shows fluid in pouch of douglas. Aspiration yields dark colour blood that fails to clot. Most probable diagnosis is Ruptured Ectopic Pregnancy.
- Young lady presents with acute abdominal pain and history of 1 1/2 months amenorrhoea. On USG examination there is collection of fluid in the pouch of douglas and empty gestational sac. Diagnosis is Ectopic Pregnancy.
- USG sign of fetal death:Heart beat absent; and Spalding sign.
- Contrast used in USG is Sonavist.
- USG is sensitive in Gall Stone,Blunt abdominal trauma.
- Piezoelectric crystals are made use of that is safe from radiation also,in USG.
- Midline hypoechoic mass with dilated lateral ventricles (on USG), bruit on auscultation, hydrocephalus and high output cardiac failure in neonates/ infants is diagnostic of vein of galen malformation.
- Ultrasonography is the investigation of choice to confirm diagnose of hypertrophic pyloric stenosis with accuracy > 95% (approching almost 100%). USG visualizes thickened and elongated pyloric canal. USG criteria for diagnosis include >16 mm pyloric length and >4mm pyloric muscle wall thickness.
- Thickened gall bladder wall in USG seen in? Acute cholecystitis ,Mucosal thickening, Cholesterosis.
- Features of cholecystitis on USG : Thick fibrosed gallbladder wall , Stone impacted at neck of gall bladder , Perigallbladder halo.
- Focal and diffuse thickening of gall bladder wall with high amplitude reflections and ‘comet tail’ artifacts on USG suggest the diagnosis of Adenomyomatosis.
- Ectopic pregnacny, characteristic finding in USG is absence of gestational sac in uterus.
- Most accurate assessment of gestational age by USG is done by Crown Rump Length.
- 20 yr old man with progressive proptosis which increases on bending forward. It is compressible and has no bruit or thrill. USG showed “hyperintense” mass with shadowing of mass. The diagnosis is Orbital Varix.
- Ultrasound is the first radiological investigation done to evaluate obstructive or any jaundice. It is both sensitive and specific for diagnosing gallbladder stones and biliary tract dilatation..
- USG is very sensitive for ascites.USG can detect as little as 100 ml of peritoneal fluid.
- Peritoneal deposits could also be detected on USG, but with difficulty. CECT and MR are the preferred investigation for peritoneal deposits in that order..
Ultrasound findings of pyelonephritis include the following:
- Renal enlargement
- Compression of the renal collecting system
- Decreased echogenicity (secondary to edema) or increased echogenicity (potentially from hemorrhage)
- Loss of corticomedullary differentiation
- Poorly marginated mass(es)
- Gas within the renal parenchyma
- Focal or diffuse absence of color Doppler perfusion corresponding to the swollen inflamed areas.
- Pseudo kidney is thickened bowel loop on USG,which is seen in Intussception.
- A 3 year old girl presents with recurrent UTI. On USG shows hydronephrosis with filling defect and negative shadow of bladder with no ectopic orifice.Diagnosis is Ureterocele.
- Investigation of choice for blunt trauma abdomen in unstable patient is USG.
- Investigation of choice for screening of proximal internal carotid artery stenosis is Doppler Flow USG.
- Gold standard investigation for diagnosing cystic echinococcus is USG.
- Initial investigation for an amoebic liver abscess is USG.
- Investigation of choice for acute cholecystitis is USG.
- Investigation of choice for gall bladder stone is is USG.
Common anomalies that can be seen in USG are:
- Cranial anomalies-anencephaly, hydrocephalus.choroid plaexus cysts
- Spinal anomalies-Spina bifida occulta and Spina bifida aperta
- Fetal heart
- Fetal abdomen and abdominal wall-Esophageal atresia, omphalocoele, gastroschisis
- Hydrops fetalis.
- Best time for diagnosing fetal abnormalities by USG is 13-19 weeks of pregnancy.
- Embryonic structure, identified earliest on USG, for confirmation of pregnancy is Gestational Sac.
- Best time to do USG in pregnancy, if it is to be done once in entire pregnancy, would be 18-22 weeks.
- Non invasive method for locating an expelled Cu-T is USG.
- Snow storm appearance of USG is seen in H.Mole.
- Initial IOC for intussesception is USG.
- Transvaginal USG can detect fetal cardiac activity in 5 weeks.
- Best parameter by USG to assess fetal maturity is Biparietal Diameter at 12 weeks.
- Antenatal diagnosis of hydrocephalus is done by USG.
- Acoustic shadow on USG is due to Reflection.
- The earliest congenital malformation that can he detected on USG is Anencephaly.
- Lactating women with painful breast, 1st investigation to be done should be USG.
- USG feature: String of pearls/ Necklace appearance is seen in PCOD.
- Investigation of choice for acute appendicitis in children is USG.
- First trimester USG finding in Down syndrome is Nuchal Translucency.
- Investigation of choice for varicose veins:Duplex Ultrasound.