The clinical importance of diagnostic modalities: Ultrasound Imaging

3. The clinical importance of diagnostic modalities: Ultrasound

Author: Attila Kollár

 

3.1. Aim of the chapter

• Introduction to medical ultrasound, its types and new techniques
• Review the physical principles of ultrasound examination
• Become familiar with the clinical indication (its place in the diagnostic algorithm) and usage (examinable organs) of ultrasound.
• Become acquainted with ultrasound terminology
• Know the advantages and disadvantages of the technique

 

3.2. Physical and technical bases

3.2.1. The physical characteristics of ultrasound:

Mechanical waves above 20 kHz are called ultrasound, which is unperceivable for the human ear.
Ultrasound is produced by small piezo-crystals which are made of lead-zirconate-titanate. These tiny ceramic plates can convert electric signals (the alternating electric current) to mechanical vibrations (Fig. 1). The frequency of the transducer is determined by the thickness of the piezoelectric ceramic plates. In the fraction of a second the ceramic plates can switch between being transmitters and receivers. In receiver function the reflected ultrasound from the investigated area causes vibration in the piezo-electric crystals, from which electrical impulses can be collected.
The ultrasound images are echo-images (visualizing the sound reflections from inside the body) assembled by high-performance PC (they can be displayed almost real time - 14 to 25 frames per sec, with minimal delay).

3.2.2. The propagation of ultrasound

3.2.2.1. The velocity of the ultrasound

For propagation ultrasound needs medium. The velocity of mechanical vibration in the medium is constant, in biological tissues this is around 1540 m/sec, which varies in different fluids or tissues:
Water (20 C degree) - 1480 cm/s
Water (36 C degree) - 1530 cm/s
Brain - 1540 cm/s
Fat - 1450 cm/s
Bone - 2500-4700 cm/s

3.2.2.2. The frequency and wavelength of the ultrasound

The wavelength (λ) can be calculated from the frequency and velocity of the ultrasound: λ=c/f,
for example by 5 MHz frequency the wavelength is: λ= 0,3 mm.
Along the longitudinal propagation of the ultrasound compression and decompression can be observed in the medium, which depends of the density of the medium.

3.2.2.3. The propagation of ultrasound through interfaces

In image the objects are not "exactly there", where displayed by the ultrasound because of refraction of sound waves. This should be reckoned specially by US-guided intervention.

3.2.3. The energy content of the ultrasound, safety concerns

The important physical parameter is energy per unit of area, W/cm2. The intensity is typically below 100 mW / cm2 in case of medical diagnostic applications.
According to our current knowledge, the amount of energy imparted during an average, 10-12 min. examination is not harmful for the human body. However, in case of a longer examination a few tenth of degrees of increase in the temperature can be measured in the affected area. This explains that (especially in the first Trimester) Doppler test during pregnancy can only be done for a limited time.

3.2.4 Ultrasound Imaging

A-mode (Amplitude mode)
On the horizontal axis the depth of the reflected echoes is represented while the vertical axis represents the amplitude of the echoes. It is used in ophthalmology for distance-measurement (Fig. 2).

 
M-mode (Motion mode)
It displays the movement of reflectors in the path of a stationary ultrasound beam along the time as horizontal axis. The desired plane can be selected on B mode image (Fig. 3). It is mainly used in echocardiography.

 
B-mode (Brightness mode)
Ultrasound waves generated by piezo-electric crystals in a transducer (i.e.: 256 element array) are reflected from various interfaces within the body. The reflected echoes with different amplitudes can be displayed on a monitor as pixels with different brightness. This mode requires quick data acquisition and processing, which also makes real time imaging (25-40 frame/sec) possible.

 

3.2.5. The types of echogenicity

Behavior of ultrasound at interfaces:
Air or calcified structures are high attenuating interfaces with high amplitude reflections. As most of the ultrasound is reflected the structures located behind such interfaces cannot be visualized.
According to amount of reflected and transmitted ultrasound the visualized structures can be described by their echogenicity as:
Cystic: 1. echo free
Solid: 2. echo-poor
3. echo-rich
4. echo dens

Nowadays US diagnostic is based on the real time B-mode US imaging. In the structures, examined with US, the above mentioned four different types of echo intensity are - many different ways - mixed.

3.2.6. The ultrasound images Resolution

Depth (axial) and sideway (lateral) Resolution
The better the resolution in the appropriate directions are, the better picture, that is rich in details, will be the result of the Examination with the same device. The axial resolution will be better by the transducer with higher frequency. Improving the lateral resolution by adequate depth zone(s) is in need for focused US beam. The usage of the dynamically focused beam allows almost identical lateral resolution along almost the entire depth of Examination.

3.2.7. Doppler Method by US (spectral Doppler)

The Doppler technique is based on the reflection of sound waves from particles moving (approaching, receding) with different velocity. (Doppler shift.)

Spectral Doppler:
In the simplest cw (continuous wave) Doppler instrument there is one source and one receiver. With this technique in theory the velocity measurement has no known limits.
The pulsed Doppler technique uses a sampling gate (the width is variable) along the US-beam from where the velocity information is acquired (arteries - fig. 5, venous - fig. 6). Based on the measured curve with the velocity from the chosen vessel section we can quantitatively characterize the flow in time relation.

 

3.2.8. Color Doppler US

In the chosen sampling area (color box) the flow will be encoded by the computer that the particles moving toward transducer will appear in red, and the particles moving away from the transducer will appear in blue. Other tint will be assigned to variable velocity of the flow. Therefore, (Fig. 7) at stenosis or at major curves various colors are visible. Beside the color Doppler, for quantitative measurement of territorial flow spectral Doppler can be used. (The smaller the sampling gate the less "noisy" Doppler curve will be.)

 

3.2.9. Power Doppler US

This technique displays the presence of flow within the sampling box, but it does not measure velocity nor can it appoint the direction of the flow. However it is 7-8 times more sensitive compared to the Color Doppler. This method (Fig. 8) is suitable to detect the small flow with variable velocity regions.

 

3.2.10. Three-dimensional (3D) and four-dimensional (4D) US

Conventional 2D US examination is done in one selected plane. However, by 3D US the visualization of a three-dimensional volume is possible (Fig. 9).
The rapid development of 3D US technique in the last 8-10 years made it possible, that 3D US images created by special transducers can do almost real time imaging (4D).

 

3.3. Contrast enhanced US procedures

The gas bubbles as ultrasound contrast agents have been used in1968, but radiology only uses it more widely since the mid 90's. Initially the cardiac Doppler examinations used the contrast agents to increase sensitivity of ultrasound. Doppler studies can still detect flow in vessels with few mm of diameter, but with intravenous administration of 2-3 ml of ultrasound contrast agents capillary-level flow detection is possible. At low mechanical index, the contrast material gives a strong, well-separable sign.
In Hungary there is only one approved and clinically used contrast agent, the Sonvue (life-time of about 5min after iv. administration, and consists of Sulfur hexafluoride gas bubbles and phospholipid).
The use of contrast material by US-methods is becoming increasingly in various Organs imaging (Fig. 10, 11).

 

3.4. Tissue Harmonic Imaging - THI

In THI during a 2D ultrasound examination not only the echoes of the fundamental US wave is used for formation of the image but also the harmonic frequencies (multiples of fundamental frequency, i.e.: fundamental frequency is 5 MHz, harmonic is 10 MHz) generated within tissues. There are tissue (THI) and contrast-enhanced harmonic imaging (Contrast Harmonic Imaging - CHI).
Harmonic waves are produced by the tissues after insonation of fundamental ultrasound, because at the period of compression US wave velocity is greater therefore the original sinus wave distorts. Upon reception by canceling the fundamental frequencies and using the harmonics better image quality is possible with less noise. This method is primarily used to produce more detailed assessment of the structure of parenchymal organs, and to visualize lesions with sharper contours (Fig. 12) THI and CHI techniques require the use of broadband transducers.

 

3.5. Endocavital, endoscopic ultrasound methods

Besides over skin surface applied ultrasound techniques - phased array, with divers convex (3.5-6 MHz) and linear (5-10 MHz) (Fig. 13). Ultrasound transducers - due to ongoing technical development the different endocavital and laparoscopic ultrasound methods are becoming more important. By these transducers the ultrasound image resolution is dramatically improved by applying very high, 10-14 MHz frequency:
Endoscopic US - oesophagus, stomach, duodenum, endobronchial, endonasal
Intraductalis US - bileducts, Wirsung-duct
Transrectal US - rectum, prostata, perirectal space (Fig. 14)
Transvaginal US - vagina, uterus, ovariums
Laparoscopic US - abdominal, pelvic, mediastinal region
The non-palpable differences (er. smaller metastases in the liver parenchyma) can be imagined with special intraoperative transducers used on the surface of the parenchymal organs.

For example on the usage of the endoscopic ultrasound we can mention, that in gastric cancer it is an important imaging method in the accurate evaluation of the propagation in the wall, as well as in detection of the pathological lymph nodes around the stomach, and its sensitivity and specificity can be considered identical with MDCT. In the assessment of distant metastases, of course MDCT, MRI and PET-CT imaging techniques are the adequate methods.
In the case of the tumors located in the pancreatic head, with the help of endoscopic ultrasound in the height of the duodenum, the propagation of the lesion, the inner structure and vascularisation can be very well classified. Furthermore with special needle we can do ultrasound guided biopsy.

3.6. The role of ultrasound imaging in oncology

It is very important and non-invasive examination method in all cases of tumor types; they can be examined and visualized.
The method compared with CT and MRI is significantly examiner dependent procedure, therefore, for example in oncologic imaging - where the reproducibility, comparability and regular monitoring is very important - it can not comply with the same degree despite the considerable technical development.
All parenchymal organs and superficial soft tissue can be examined well with help of the conventional 2D imaging techniques, but the air, the bones and chalky structures are impenetrable obstacle for the US, as they fully reflect it. In the assessment of the intra-abdominal organs the image quality of ultrasound can be disturbed and worsened by significant obesity and postoperative status (bandages, drainage).
In visualizing and morphological assessment with US a gel pad might help by the very superficially located tumor suspicious lesions in the subcutaneous layer, and lymph nodes located in the superficial regions.
With the use of Color-Doppler and Power-Doppler procedures we can obtain valuable information regarding the vascularisation of the tumors (Fig. 15). In some tumors (such as hepatocellular carcinoma, FNH, adenoma) with vascularisation analysis using Doppler spectra important information can be obtained, that can help in the differential diagnostic assessment.

 

3.7. Sonoelastography

During sonoelastographic examination the selected region is gently compressed with the transducer. The soft tissues in the region will be compressed more, the harder will be compressed less. Then these results are color coded and superimposed on the B-picture.
Due to inflammatory or neoplastic processes tissue structures in the body become harder and more inflexible. The rate of this change can be measured in the modulus of elasticity. The tissues are lengthened by compression with the transducer due to the elasticity, both in axial and in lateral dimension.
With proper use of autocorrelation software these alterations can be quantitatively assessed. The hardest structures will be displayed with blue, and the softest tissues with red color other values will be represented by shades on the spectrum between these two colors. (Fig. 16). More and more research is published in the topic of sonoelestography, especially in the field of breast, thyroid and pancreatic cancer studies.

 

Summary:

• The physical principles of medical ultrasound examination were reviewed
• US has a key role in medical imaging as it is a noninvasive and non-ionizing technique
• Proper application of ultrasound terminology was introduced
• Important fields where US is used:
  • Endocavital US in oncology
  • Vascular US
  • Assessment of vascularization
• Advantages of contrast enhanced emanations were presented

 
Translated by Csaba Korom

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