This article explains the fundamentals of ultrasound imaging (also known as USG) and how it's used in clinical situations. It addresses the benefits of USG, and the various ultrasonic modes and probes, and explores how USG is used in various medical specialities.

Ultrasound imaging (USG)

Ultrasound imaging or Ultrasonography (USG) is an essential diagnostic tool in modern medicine and is one of the most frequently used imaging modalities in our country.

This guide aims to provide an overview of USG and its applications in general practice settings.

Basic principles of ultrasound Imaging

1. Ultrasound by definition is sound waves that are beyond the range of human hearing (>20,000Hz).

2. Diagnostic ultrasound machines typically use sound waves with frequencies in the range of 1 - 20,00,000Hz (1 - 20MHz).
3. A modern ultrasound machine is essentially a sophisticated computer which sends electrical signals to an ultrasound transducer, which are converted to short pulses of ultrasound waves, which penetrate the body and are reflected back at tissue interfaces.
4. The reflected sound wave is picked up by the same transducer and converted back to electrical signals which are sent to the computer and are then translated into images by the computer and displayed on a monitor in real-time.
5. The images and video clips can be archived for review and documentation.

Advantages of USG

1. Non-invasive
2. Relatively cheaper than other imaging modalities like CT and MRI making it widely available, a low running cost.
3. Cost-effective for patients.
4. No harmful radiation is involved making it safe for obstetric use and repeated imaging.
5. Real-time imaging - useful for guided biopsies, and aspiration.
6. Can be used for bedside imaging and in ICUs.
7. Doppler ultrasound, which is a clever application of ultrasound can visualise blood flow.

Disadvantages of USG

1. Interpretation is operator dependent and to some extent on the machine and available probes. Significant inter-observer variation in interpretation may be present.
2. Data acquired in a CT scan or MRI includes the complete anatomical information of the part being imaged which can be interpreted by any radiologist while this is not possible for most ultrasound examinations. Interpretation is real-time with limited scope to store and distribute the images.
3. Cannot be used for the visualisation of bones and gas-filled structures like lungs and bowels as ultrasound is completely reflected at such interfaces.
4. Degradation of image quality in obese patients.

Types of ultrasound

1. A-Mode (Amplitude mode): Records one-dimensional Amplitude of the returned echoes vs time which corresponds to depth. Mainly used in ocular USG.
2. B-Mode (Brightness mode): The most used mode. The transducer elements scan through a plane in the body resulting in a 2D image. Each pixel value of the image correlates to the voltage amplitude registered from the backscattered signal. The whole process is repeated many times in a second to give a real-time image.
3. M-Mode (Motion Mode): In motion mode, A-mode pulses are emitted in succession. The signal is converted to lines of bright pixels, whose brightness linearly correlates to backscattered voltage amplitudes. Each next line is plotted adjacent to the previous, resulting in an image that looks like a B-mode image. This can be used to determine the displacement of specific organ structures, most commonly the heart.
4. 3-D (3D / 4D): Three-dimensional imaging is done by combining B-mode images, using dedicated rotating or stationary probes. Real-time 3D is called 4D imaging.

Probes used in ultrasound

The attenuation and resolution of ultrasound are linearly proportional to the frequency. Higher frequency probes give excellent resolution but have limited penetration. Therefore a variety of probes are used for imaging depending on the organ imaged.

1. Convex Probe (1 - 5 MHz): Commonly used for imaging abdominal organs. It gives good penetration (up to 15 - 20cm) with reasonable resolution.
2. Linear Probe (8 - 12 Mhz): Used to image superficial organs like breast, scrotum, neck and for peripheral Doppler. It gives excellent resolution but has limited penetration (3 - 5cm).
3. Transvaginal /Transrectal Probe (~8MHz): This is an Endocavitory probe used for imaging the uterus and adnexa and prostate. This high-frequency probe gives excellent resolution as the probe can be placed in close proximity to the organ imaged. Gynecologic imaging should as far as possible be done using a TV probe.
4. Cardiac Probe: This is a low-frequency probe that has a small footprint that can fit between the ribs and is a phased array probe.

This is not an exhaustive list and many more probes are available depending on the applications.

Applications of USG in general practice

1. Abdominal USG: Abdominal USG is one of the most widely used imaging modalities in our country. It can be used to evaluate the Liver, Gall Bladder, Pancreas, Spleen, Kidneys, Urinary bladder, Prostate, Uterus and ovaries and bowel wall. Imaging of the Uterus and adnexa and prostate is better done using a TV probe. The appendix can usually be imaged, except in cases of the retrocaecal appendix.
2. Obstetric USG: Almost all of obstetric imaging is done using ultrasound with MRI used in limited cases as a problem-solving tool.

• In the early first trimester, USG is used to confirm a viable pregnancy, rule out ectopic and detect twin or higher order pregnancy and the chorionicity and amnionicity. Dating is most accurate in the first trimester.
• In the late first trimester, USG is used to assess risk for Trisomies (NT scan), Pre-eclampsia, Miscarriage at 11-24 weeks, Stillbirth, Fetal growth restriction, Fetal Macrosomia, Preterm birth etc.
• In the Second trimester ultrasound is vital to detect fetal anomalies and the position of the placenta and liquor quantity.
• In the third trimester ultrasound is used to assess fetal growth, fetal lie, liquor quantity and placenta and aid the obstetrician to manage the pregnancy.

3. Neck USG: Ultrasound can be used to assess the Thyroid gland, submandibular gland, parotid gland, neck lymph nodes and neck vessels. USG can be used to guide thyroid and lymph node FNACs.
4. Breast USG: Ultrasound can be used to assess a variety of pathologies of the breast especially in young females where mammography is less desirable due to denser breast and radiation concerns. It can be used to characterise breast swellings and abscesses. Ultrasound is the modality of choice to differentiate cysts from solid lesions. It is also used as a guide to needle biopsy breast lesions.
5. Scrotal and Penile USG: USG is very useful in the acute scrotum to differentiate epididymal-orchitis from torsion. USG can also be used to evaluate scrotal masses, varicocele, and trauma. USG is an excellent modality to evaluate penile lesions including trauma, priapism, erectile dysfunction or suspected Peyronie's disease. However penile evaluation for erectile dysfunction requires intra-penile injection of Papaverine.
6. Musculoskeletal ultrasound: Ultrasound can be used for evaluating joints like the Shoulder, Knee, Ankle, Elbow and Wrist in adults for ligament tears and joint effusions. USG is very useful to diagnose CDH in children. Ultrasound is also very useful for foreign body localisation in soft tissues and evaluating soft tissue masses.
7. Doppler evaluation of lower and upper limb vessels and Neck vessels: Doppler study utilises the Doppler effect (change in frequency of sound due to relative motion of source and observer) to gain information about the velocity of blood flow in blood vessels, which can be displayed as colour and power doppler and spectral doppler by the machine. This is used to evaluate Venous thrombosis, especially of the lower limb, arterial obstruction and varicose veins.
8. USG of eyes: A scan is used to accurately calculate the length of the eye to determine the power of IOL after cataract surgery. B-scan of the eye is useful to evaluate a variety of disorders of the eye such as retinal or vitreous detachment, intraocular haemorrhages and foreign bodies.
9. Echocardiography: is an essential tool in cardiology for the evaluation of heart valve function such as stenosis or insufficiency, hypertrophy or dilatation of the main chambers and the estimation of Ejection Fraction EF.
10. Endoscopic Ultrasound (EUS): The ultrasound probe can be mounted on an endoscope and is a problem-solving tool that gives high-resolution imaging of the digestive tract and adjacent organs, especially the pancreas. Trans-oesophageal Echocardiography (TEE) can better evaluate the aorta, pulmonary artery, valves of the heart, both atria and atrial septum. TEE is very sensitive for locating a blood clot inside the left atrium.

Disclaimer- The views and opinions expressed in this article are those of the author and do not necessarily reflect the official policy or position of M3 India.

About the author of this article: Dr Ashoka GK (UGC) is a D.M.R.D, M.M.S.T from I.I.T – Kharagpur, working as a Radiologist at Chirayu Scan Centre, Puttur, Karnataka.