Aspects of Spectral Doppler Imaging

Due Date: Friday 23 October 2020, 11:55 pm Length: 2000 words Value: 20% Topic: Aspects of Spectral Doppler imaging INTRODUCTION The purpose of this assignment is to assess your understanding 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 some calculations that form part of questions 2 and 3.

1. The Spectral Doppler Display (650 words approximately)
Give an account of the mode of spectral Doppler imaging as follows: The overall spectral Doppler display incorporates two main components, a B-mode image and a Doppler spectrum. Obtain an image of a typical spectral Doppler display including the B-mode image and the Doppler spectrum (properly citing your source reference) and include it as a figure and then answer the following questions about it. Number your answers to correspond with the following question numbers. 1. Describe the main purpose of the spectral Doppler imaging mode. 2. Explain how the B-mode image is obtained and what its purpose is. 3. Name the scales on the vertical and horizontal axes of the Doppler spectrum including their units. 4. Draw and label the scan line on the B-mode image. Label the sample volume on the B-mode image and explain clearly what it is used for and how it works. 6. Explain the difference between the sample volume as indicated on the B-mode image and the range gate. Define the range gate in doing so. 7. Label the Doppler angle cursor and explain how it is used. 8. Explain the meaning of the Doppler spectrum and how it is used. 9. Find and include as a separate figure another image showing spectral broadening on the Doppler spectrum. Explain what spectral broadening is and give two possible causes of spectral broadening.

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2. 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 system measures the Doppler shift to be 1804.9 Hz and the blood flow velocity is determined to be 53 cm/s with a transducer frequency of 7 MHz. However, the sonographer has, unknowingly, set the Doppler angle cursor 3˚ too low. Calculate (i) the true blood flow velocity at the true Doppler angle of 71˚ for this case and (ii) the percentage by which the velocity value determined is in error? Describe some of the vascular blood flow issues that may arise when aligning the Doppler angle cursor on the monitor with the presumed blood flow direction, for example speed change, eddy flow, velocity profile etc. Explain how these arise and draw diagrams to support your explanations.

3. 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. The purpose of this part of the assignment is to assess your understanding of the mechanism of the steering and focusing of the beam and also your knowledge of the magnitudes of the time delays involved in firing the transducer elements. 1. Give a brief written description of the main principle behind electronic beam focusing and steering mentioning, in your description, (i) transducer elements, (ii) time delays between pulse emissions from individual elements and (iii) the effect on beam shape and lateral resolution. Use a carefully labelled diagram to support your description. 2. Demonstrate the magnitudes of typical time delays between pulse emissions from different transducer elements when focusing and steering the ultrasound beam by performing the following calculation. a. Consider a 600 element, linear transducer, which is 100 mm between the centre lines of the first and the six-hundredth elements. Determine the pitch of the elements, that is to say, the distance between the centre lines of successive elements. b. At a given time, a single beam is generated by an active group of 19 elements. This beam is steered 13 degrees off-axis to the right as measured at the centre of the group to a focal point at a depth of SON4000 S2 2020 Assignment_0.doc Page 3 of 6 7 cm. Figure 1 (below) is a schematic diagram of this single group showing elements 1 to 19 (not all elements are shown) and the focal point. (Note that this diagram is not to scale. Diagrams that are not to scale are often used in diagrammatic representations. In this case, the group of elements appears to be much larger than its actual size relative to the 7 cm depth dimension which appears much shorter than the actual depth, and the 13 degree angle appears much larger than 13 degrees.) c. State the order in which elements 1, 10 and 19 have to be fired. Then, calling the time at which the first element is fired, time zero, calculate the time at which the other two elements are subsequently fired. The two successive times will be given in ms, μs, or ns as appropriate. Hint: You can use knowledge of the element pitch, elementary trigonometry, and the theorem of Pythagoras, to determine the three path lengths from elements 1, 10 and 19 to the focal point and hence, determine the time delays between the firing times of the three elements. Note: Units (such as mm, cm, ms, µs etc) must be included. A figure without units is meaningless. 7 cm 1 3 ˚ 1 2 10 18 19 Focal point, O 13˚ Current element group using elements 1 to 19 (not all elements shown) Element 1 path to focal point Figure 1 A single active group of transducer elements with the beam steered 13˚ off axis to the right to the focal point shown. STRUCTURE AND LAYOUT General 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. Structure The assignment is to have a title, date, student name and number. 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. Introduction 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 Tables 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. Figures 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. DO NOT use figures taken from the unit Module notes. Search the literature and find your own figures. Conclusion 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. Style 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.) 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. REFERENCING While you may include references to the module notes and text books, 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 eight journal references, that is, references to the peer reviewed scientific journals. The following guidelines must be followed. References cited must include no less than eight 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 the module notes and to listed references, while they can, of course, be used, will be considered insufficient in themselves because they do not show evidence of wider reading in the scientific literature, an essential part of this unit. Reading of the scientific literature is also essential if future developments in the technology are to be followed and understood – one of the purposes of this course. 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.

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