Kurt Grey


Practicing accurate measurement of areas -

The relation between the size of the lesion,

the transducer size and the treatment time


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Total treatment time  = local treatment time  ·  A / ERA


Thus, the setting of the total treatment time (T) requires

  •   The size of the transducer  (ERA)

  •   The size of the treated area (A)

  •   Your choice of local treatment time (lt)

The size of the transducer  ≈  ERA = Effective Radiation Area = the cross section of the beam.


Often these concepts are mixed up, when they should be used accurately.  ”The size of the transducer” is really larger than the beam itself.  It certainly is the beam itself, which is of interest in determining the exact dose. You can often read the ”ERA” on the apparatus or the transducer itself, e.g. ERA = 5cm².


Otherwise, you can measure the diameter of the metal plate of the transducer:

Place a transparent ruler across the metal plate that is in contact with the patient and read the measurement in millimetres.

Radius = diameter/2.

Plate Area =  3,1416 · radius² .

ERA approximates to 70% of the plate area.

A number of products on the market in the 1990's were used to compare ERA and plate area and the mean ratio found was 70% .


The size of the treated area

 The size of the treated skin area has often been measured by eye, or by comparing the size of the transducer to the size of lesion. Both of these two methods have been demonstrated lacking accuracy and reproducibility, 22% and -48%, respectively and with large variance (c.f. paper on Area Measurement). More reliable results can be achieved by using a tape measure to read the length and width of the treated area (1% and small SD). Equally accurate and rather convenient is a plate with ellipses of known area.  You can practise this type of measurement on the test areas.

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Plate with ellipses


The plate with ellipses is a tool designed for measuring surface areas. The purpose is to add accuracy to the calculation of treatment time. The majority of the regions I have treated with ultrasound presented shapes, which had some resemblance to ellipses. I therefore constructed the plates with ellipses as a measuring tool that would cover most of my needs.


The drawing is meant to be copied to a transparent plastic sheet (e.g. overhead film). After drawing the palpated lesion on the skin surface, put the transparent ellipses onto the skin, and compare the lesion to the area of the ellipses, that come closest to your treated area.

The ellipses on the sample plate are printed on a scale of 1:1.

This method works best on shapes that have a high degree of symmetry. They need not be ellipses. Shapes close to circular or rectangular forms for example, can be measured with good accuracy, if care is taken to ensure that equal parts of the measured shape fall outside and inside the matching ellipse (fig 1 and 2). The measurement of more irregular shapes by this method will be less accurate (fig 3 and 4), and another method should be preferred. The tape measure is a more versatile method, but often requires several measurements, and some calculation since it must be read in MILLIMETRES to provide sufficient accuracy.

The plates are available here (if you have installed Acrobat Reader). :


Plates with ellipses


You may print directly to for example a transparent inkjet-sheet. Just click on the printer icon in Acrobat Reader.

Please, pay attention to any distortion of the measuring tool during the process of printing and copying.

Check out the control measures (10 x 10 cm) on the printed sheet.

Xerox copying changes the dimension of the plate, sometimes up to 5% on both axes. Copying a copy of a copy of a ...  makes measuring unpredictable. You should prefer to use the original.


 Fig 1. Circular lesion outline with matching ellipse.  Fig 2. The hatched areas equalize one another.


 During  palpation the lesion should be drawn on the skin (the circle in fig.1). The line should be resistant to the coupling agent you use. I prefer a waterproof permanent ink-pen together with coupling gel or oil, which allows the line to be visible during the entire treatment and acts as a guideline for the transducer. The transducer "house" is often larger than the metal plate. The plate is usually larger than the transverse section of the beam. In my equipment the difference is 8 mm. An extra guideline 8 mm outside of the palpation trace can prevent the course of the transducer from being too narrow or inaccurate. This guideline area should not be measured, since it is not exposed to ultrasound.

When you have finished the palpation and the drawing, put the plate with ellipses over the drawing on the skin and move it around, until the best fit between the drawing on the skin and one of the ellipses has been found.

The scale should be read in as much detail as possible to achieve the best accuracy. The scale shows the area (A) in square centimetres.

Fig. 1 demonstrates how to apply this method to areas, that are not elliptical, if only they present some symmetry. The hatched areas in fig.2 equalize one another (loss=gain, vertical versus horizontal pattern)



To protect the plate during use and to allow cleaning or disinfection, put the plate inside a plastic cover.

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Some will prefer a TAPE measure to determine the size of an area. An ordinary flexible measuring tape will bend nicely around the surface curve, and is read with suitable accuracy (centimetres with one decimal e.g. 3.1 cm rather than 3 cm). It works better with irregular areas, if you don’t mind the extra calculations.



Start with a palpation and drawing on the skin of the outline of the lesion. A permanent ink pen that is resistant to the coupling medium is preferred. (See the pear shape in fig.3). 


Fig 3. Draw the outline of the lesion on the skin. Mark the length(l) and width(b). Area = l x b


Estimate the length (L) and width (B) and mark them on the skin. Read the measure as accurately as possible (centimetres with one decimal) and multiply to reach the lesion area. (Area = L x B).

Be accurate: 3.1cm x 1.7cm = 5.27cm² instead of 3 x 2 = 6cm².

The error in this example is 13%.

More irregular shapes require a subdivision in two or more rectangles (fig. 4).Text Box:  


Fig 4. Subdivision of complex lesion area.



The plate with ellipses has an accuracy of -0.3% (SD=6.2%). The accuracy for tape measure is  -0.5% (SD=9.8%). Tape measure is more versatile, but the two methods work well together.


Test areas:  

Challenge and improve your skills with these test areas:

Open the links to the test areas (pdf- files), and print them on two sheets of paper, and then try out your favourite measuring method.

The drawings were made by CAD software with extreme accuracy. The true results are printed upside down.

Please note that printing, scanning and Xerox copying may change the shape and size of the test areas by zooming and skewing the original!

Please check the control measures (5 x 10 cm) on the printed sheets, before using them to determine your measuring skills!


Test areas 1

Test areas 2



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