Ultrasound and Lithotripsy

This week in IDIS my interest was mainly drawn to the ultrasound lecture I received. Ultrasound is a topic that interests me hugely; whilst on clinical placement I spent one day in an ultrasound department witnessing a variety of procedures, the subject area was also briefly touched upon within the radiographic science module last year. During the lecture there was a reasonable amount of recapping, which made me realize that I had perhaps forgotten some basic concepts relating to how ultrasound works. There were also branches of ultrasound which I have little understanding of such as lithotripsy and high intensity focused ultrasound (HIFU). With this in mind I shall be blogging on what I have relearnt about ultrasound, and what I have found out about lithotripsy.

I have learnt that ultrasound seems to be one of the most widely used modalities within medical imaging, I believe this to be due to its high sensitivity in cardiology, obstetrics, gynecology and abdominal imaging. Comparing ultrasound to other modalities such as CT or even conventional X-ray imaging its resolution may not be as high (see image below); looking at various sources 0.5mm seems to be the maximum resolution ultrasound is capable of (Allan, et al, 2011). Ultrasound is still extremely useful, as it does not expose the patient to any ionizing radiation making it the favored modality for obstetrics. Ultrasound is also mainly non invasive (although invasive intravascular ultrasound can be used) and is being increasingly used in a non diagnostic context; guiding interventional procedures.

(Delius, 2000)

Ultrasound is still part of the electromagnetic spectrum and works by sending longitudinal sound waves through a medium, these waves vary in strength from 2-20MHz. Technically any sound wave above 20KHz is ultrasound however waves this strength would not be useful for medical imaging. The sound waves are generated from a transducer probe; the transducer uses the piezoelectric effect to create the waves. There are a variety of different transducers available, each one being suited to visualizing different organs or aspects of the body (see image below).  Piezoelectric literally means ‘pressure electricity’ effect and is caused by applying electric current to one or more quartz crystals which change shape and vibrate rapidly, therefore producing sound waves that travel outwards (Ali, M. (2008).

(Cardiovascular Sales, 2012)

As well as being used for sending out waves the transducer is also used for detecting waves, which are reflected back from the area of interest, this is known as ‘pulse echo’ and was first developed as sonar. The speed at which the waves travel vary depending on the medium they are propagating though, this is highlighted in the table below:

Material	Velocity ( m/s)
Air	330
Water	1497
Metal	3000 – 6000
Fat	1440
Blood	1570
Soft tissue	1540

(Edelman, 2006)

The table demonstrates how sound waves needs a medium to travel through in order to work most efficiently; air will block the sound wave as the particles are too spaced out for the wave to travel through. This demonstrates the importance of making a good connection with the patient by using acoustic gel on the end of the transducer to minimize the air gap, which will attenuate/ block the wave.

The image is constructed using the data acquired from the reflected pulsed wave (once sent out the wave can be reflected, refracted or absorbed). Once sent out the wave is partly reflected from a boundary between two tissue structures and partially transmitted (Haiying Huang, et al, 2011). The strength of the reflection depends on the difference in impedance of the two tissues. Different structures reflect the wave back at different strengths the computer can therefore interoperate these differences in strengths and construct an image from them. Interestingly different wave lengths (low frequency) should be used depending on the examination, for example a longer wave length travels deeper and further inside of the patient but does not produce such high resolution images; where as a shorter wavelength (high frequency) is less penetrative but produces I much higher resolution image.

Understanding ultrasound I then looked into extracorporeal lithotripsy as I saw them as similar. Lithotripsy uses sound waves much like ultrasound however the lithotripter creates the waves differently to an ultrasound transducer. The waves produced are also higher intensity, therefore used therapeutically to oblate an objects such as a renal stone, rather than to image a specific piece of anatomy (El-Nahas, et al ,2012). The shock wave sent out from the lithotripter is quite different to ultrasound as it is caused by a rapid pressure increase in the transmission medium rather than the piezoelectric effect.

Although the lithotripsy and ultrasound are very different in what they do, they often work along side each other. Ultrasound is used to locate renal stones within the kidneys or ureters; once the stone is located the lithotripter can then start directing high frequency shockwaves of approximately 10MPa towards the stone to oblate it (Fleischer, et al, 2005). The longitudinal waves are essentially pockets of energy, which travel, when they hit the stone they begin to crack it. Although the waves generated by the lithotripter damage the stone they cause very little damage to the tissues surrounding it. This method is therefore very effective in breaking down renal stones and is minimally invasive for the patient, it is therefore understandable that in the last 20 years around 70% of renal stones are treated with lithotripsy, this study is a little out of date however it has compiled data from a long time period, which i feel increases reliability, but it may not be so valid in 2012 (Delius, 2000). I am unable to find a more recent source of reference for these figures.

Understanding what lithotripsy is and a little about how it works will benefit me when working clinically, as last time I was on clinical placement I encountered many patients with renal stones. This has filled a gap in my knowledge as seeing a renal stone on an x-ray or CT scan is interesting but now I know more about the patient pathway and how this is treated.

Reference list

Ali, M. (2008) Texas Instruments. Signal Processing Overview of Ultrasound Systems for Medical Imaging[online]. 1 (3), pp.12-14.

Allan, P.A., Baxter, GM., Weston, M.J. (2011) Clinical Ultrasound. 3^rd ed. Oxford: Elsevier.

Cardiovascular Sales. (2012) Cardiovascularsales.com. Available from: http://www.cvsales.com/cvs/PartsProbes/MindrayProbes.aspx [Accessed 08 November 2012].

Delius, M. (2000) Lithotripsy. Ultrasound in Medicine & Biology [online]. 26, Supplement 1 (0), pp.S55-S58.

Edelman, S,K. (2006) Understanding Ultrasound Physics. 2nd. Arizona: Arizona Health Science Center.

El-Nahas, A., Ibrahim, H.M., Youssef, R.F. and Sheir, K.Z. (2012) Flexible ureterorenoscopy versus extracorporeal shock wave lithotripsy for treatment of lower pole stones of 10-20 mm. BJU International[online]. 110 (6), pp.898-902.

Fleischer, A.C. and Andreotti, R.F. (2005) Color Doppler sonography in obstetrics and gynecology. Expert Review of Medical Devices [online]. 2 (5), pp.605-611.

Haiying Huang and Paramo, D. (2011) Broadband electrical impedance matching for piezoelectric ultrasound transducers. Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on [online]. 58 (12), pp.2699-2707.