Length: 2000 words
Topic: Aspects of Spectral Doppler imaging
Due date: 15 April 2019
The purpose of this assignment is to assess your understanding of aspects of spectral Doppler imaging. In particular, two important aspects of it will be considered, namely,
A. The measurement of blood flow velocity in spectral Doppler imaging and the way in which the blood flow direction is specified and handled. The definition of the Doppler angle and the reasons for limitations to its value are considered.
B. Electronic beam focusing and steering and the magnitude of time delays between the firing of successive elements required to achieve this.
To provide you with as much information as possible about our expectations in the writing of formal submissions, instructions and guidance are given below about content, structure and layout and referencing. This will help you in the preparation of your submissions.
In the questions below, an approximate word length is given for guidance only. You will notice that the suggested word lengths in questions 1, 2 and 3 do not add up to 2000, the nominal word length of this assignment. This is to allow for the fact that there are come calculations that form part of questions 2 and 3.
- The Spectral Doppler Display (650 words approximately)
Give an account of the mode of spectral Doppler imaging including:
A general description of the spectral Doppler imaging mode and how it works.
A description of the key components of the spectral Doppler display using a well-labelled image of a typical display (properly citing its source reference) showing:
The B-mode image and the Doppler spectrum
The scales: define the horizontal and vertical scales of the spectral Doppler display and their units
The scan line
The sample volume and range gate: explain the difference between these two
The Doppler angle cursor: explain how it is used
Spectral broadening: explain what this means, what it looks like in the spectral Doppler display (sketch it or show an image from the literature), and what causes it.
- Blood Flow Velocity Measurement (600 words approximately)
Draw a labelled diagram showing the patient’s surface, a linear transducer, a scan line approximately perpendicular to the patient’s surface and a blood vessel at an angle to the surface. On this diagram, show the Doppler angle. Give a written definition of the Doppler angle based on this diagram.
The Doppler angle is normally chosen so that it lies between about 30˚ and 60˚. Consider a scenario in which the sonographer has set a Doppler angle of 68˚ (that is, above the normal permissible range). In this case the measured Doppler shift is 1120.5 Hz and the blood flow velocity is determined to be 49 cm/s with a transducer frequency of 5 MHz. However, the sonographer has, unknowingly, set the Doppler angle cursor 3˚ too low.
Calculate the true blood flow velocity at the true Doppler angle of 71˚ for this case and the percentage by which the velocity value determined is in error?
- Beam Focusing and Steering (600 words approximately)
The ultrasound beam may be focused to a point electronically and also steered to provide a beam angle which is not perpendicular to the patient’s surface. Both focusing and steering of the beam is achieved by setting appropriate time delays between the emission of pulses from the various transducer elements.
- Using a labelled diagram, give a brief written description of the main principle behind electronic beam focusing mentioning (i) transducer elements, (ii) time delays between pulse emissions from individual elements and (iii) the effect on beam shape and lateral resolution.
- Demonstrate typical magnitudes of time delays between pulse emissions from successive transducer elements when focusing and steering the ultrasound beam by performing the following calculation.
a. Consider a 500 element linear transducer, 100 mm wide. Determine the spacing of the elements.
b. At a given time, a single beam is generated by a group of 20 elements, and this beam is steered at an angle of 13 degrees to the left as measured at the centre of the group. Figure 1 (below) is a schematic diagram of this single group showing element 1 on the left and elements 18, 19 and 20 on the right (not all elements are shown).
c. If the beam is steered and focused at the point shown in the diagram, calculate the time delay that would be required between the firing of elements 19 and 20.
Hint: You can use trigonometry, knowledge of the element size and spacing, and the theorem of Pythagoras to determine dimensions A and B and hence the path lengths LA and LB. Once these are known, you can use the pulse flight times to the focus to determine the time delay required between the firing of elements 19 and 20.
Note: Units (such as mm, cm, ms, µs etc) must be included. A figure without units is meaningless.
Fig. 1 A single group of 20 transducer elements (not all shown) with beam steered 13˚ off axis. (Not to scale.) Elements 18, 19 and 20 are on the right
STRUCTURE AND LAYOUT
Assignments are to be written in a professional, formally structured, yet straightforward style. They are also to be written in such a way as to demonstrate clearly your understanding of the material. As would be expected when reporting formally to a professional body, a high standard of presentation is required. Do not use sketches or colloquial language.
It will be divided into standard sections, with section headings: Introduction, appropriate intermediate sections to break up the subject matter into logical components, Conclusion and References.
The introduction will contain some history and background to the subject matter in question citing several references from the standard literature and highlighting any difficulties or complications involved. References should include a number of journal references from the professional literature and perhaps some text book references. You are not required to go into excessive detail on the contents of such journal articles, particularly if it is highly technical and mathematical, but you should be able to summarise the key results and make an informed comment on them in a few sentences.
Tables and Figures
Data must be included in the form of properly labelled tables each with a suitable caption. Tables are always numbered and labelled as Table 1, Table 2 etc. For tables, the table number and caption go on top of the table.
Images, diagrams, graphs and charts must be included as properly numbered and captioned figures. All diagrams, plots, graphs, images and so on are labelled and numbered as figures and nothing else. You do not have Graph 1, Image 2 etc. For figures, the figure number and caption go below the figure. Units must be included. Full understanding cannot be demonstrated without showing units.
Your conclusion should include key numerical results along with a summary of the contents of the report. Mention any technical difficulties or assumptions involved in the report.
Marks will be lost for incorrect spelling and grammar. Do not use dot points or note form. Use grammatically correct and complete sentences. You can use properly enumerated complete sentences occasionally and where appropriate.
Do not use direct quotation. In arts, direct quotation is essential, in science, it is virtually never used. (Perhaps an insightful statement by Einstein or someone of that calibre might, on rare occasions, justify a brief direct quotation but not otherwise.)
Numeracy and quantification
Because this is a physics assignment, you are expected to be numerate. A physicist will not just say “… the spatial resolution is significantly higher with this method” or even worse, “… the spatial resolution is unparalleled”!! He would always say, where possible, “… the achievable spatial resolution is xxx mm at yyy MHz in zzz mode” or similar. You will be expected to quantify your text by quoting typical numerical values often through this assignment to demonstrate sound knowledge.
This exercise is designed to bring about wider reading on the topic and to assess your ability to study and assess the existing state of knowledge in the given field, therefore, your submission must include reference to a minimum of six journal references, that is, references to the peer reviewed scientific journals. The following guidelines must be followed. References cited must include no less than six peer reviewed ‘journal’ references, that is, they must be taken from some of the hundreds of professional journals available using the library databases. Relevant articles may be found by searching databases like Medline, Inspec and Compendex. Medline has a clinical bias while Inspec and Compendex have more of an emphasis on physics.
In citing references, websites, such as may be obtained by a Google search for example, are not considered sound in academic work and are little used in scientific literature because they are not subject to the stringent peer review process of journal articles. Websites may be written by anybody including totally unqualified people and may be, and often are, full of errors. If a Google search locates a reference to a professional journal, the proper journal reference must be given, not the website.
References to web sites are not entirely prohibited, and may be used very sparingly. This may be necessary, for example, if referring to unpublished manufacturer’s data. If used, the date on which the site was accessed must be stated using the correct format.
Be wary of manufacturers’ websites. They contain language such as ‘superlative performance in any clinical circumstance’, ‘enhanced performance is attained through …’, ‘exceptional imaging performance over a variety of clinical applications’, state-of-the-art’ and many other phrases such as these. These phrases are for marketing purposes and are totally meaningless. They do not explain specifically in what respect performance is enhanced, they provide no quantitative data to back up the statements and they are never negative. Do not use phrases such as these in your reports.
The Vancouver system (numeric) must be used.
Do not rely on ‘EndNote’ to format your reference list. Without setting up the appropriate referencing style, its reference list formatting will almost certainly be incorrect.