Câu hỏi Siêu âm và chụp cộng hưởng

c khi phát tần số vô tuyến lên các mô cơ thể là gì?
SAR nghĩa là gì? Xác định các đơn vị được sử dụng để đo tham số này? Giới hạn đối với toàn bộ cơ thể là bao nhiêu và các cơ quan nào có thể liên quan đặc biệt đến các ảnh hưởng của tần số vô tuyến?
I. Questions of Ultrasound
Why is the low megahertz range used for ultrasonic imaging? Give, with reasons, the frequencies and transmission focal lengths you would choose to make ultrasound examinations of: (a) an eye, (b) a liver.
What factor(s) determine the reflection coefficient of an ultransound pulse at a tissue interface? What diffirences would you expect in the ultrasonic appearance of specularly and diffusely reflecting interfaces?
Why is it neccessary to use gel between the probe and the patient?
A source of pulsed ultrasound and a target are separated by normal soft tissue. Discuss the effect of each of the following on the amplitude of the ultrasonic pulse reflected back to the source: (a) the size and shape of the target, (b) the characteristic acoustic impedances of the target substance and intervening tissue, (c) the distance between source and target, (d) the range to which the transmission focus has been set, (e) the frequency of the ultrasound, (f) the acoustic power of the source.
What is the purpose of the backing layer, the impedance matching layer and the lens in a typical ultrasound transducer? What would be the consequences for image quality of leaving out each of these in turn?
Explain briefly how a focused beam of ultrasound can be obtained from the excitation of a number of transducer elements in a linear array probe.
Why is the active group of a linear array progressively enlarged during dynamic focusing in reception? How will the width of the effective receive beam vary with depth if the active group stops expanding when it contains about 20 elements?
Show that, in the scan plane, the beam of a single 1mm wide element of a 3 MHz linear array has a near field length of only about 0.5mm and then diverges at an angle of about ±300 to the axis. If a transmission or receive focus were required at a range of 15mm, would there be any point in having 20 elements in the active group?
Describe the different types of B mode real-time scanner, and list their advantages and disadvantages.
Describe the function of the TGC facility in an ultrasonic scanner. Describe two different forms of TGC controls and suggest applications for which each might be appropriate.
In B mode, which of the following controls affect the lateral resolution, which affect contrast resolution, which affect sensitivity and which affect temporal resolution? transmission focus; depth of field of view, overall gain; output power; TGC; write zoom; pre-processing;dynamic range; frame averaging.
Describle the feature seen in an ultrasound B-scan image that could be used to distinguish between an solid tumour an cyst.
Describle and explain a speckle pattern, as seen in a B-mode image. Which features of such a pattern are determined by the tissue and which by the machine?
Describle and explain two artefacts that produce image ‘echoes’ that do not correspond to any real target in the scan plane, and two artefacts that cause the image echoes of genuine tagets to be shown at incorrect positions.
Explain the Doppler effect and outline the principles of a continuous wave Doppler device.
Distinguish between the terms: Doppler shift, Doppler signal, Doppler frequency, Doppler power, Doppler spectrum, Doppler wave form.
Describle how a pulsed Doppler device isolates the Doppler signal from a particular depth interval. What limitation is associated with Doppler frequency shift measurement by pulsed Doppler?
Why does CFM mode involve a lower frame rates than B mode? Why is it possible for the same blood speed to be represented by different colours on the same CFM image? What effect does aliasing have on a CFM image? Can aliasing occur in power Doppler mode?
Explain what is meant by the thermal idex(TI) and the mechanical idex (MI), as displayed on the
II. Questions of Magnetic Resonance Imaging
What is meant by the term "resonance" in relation to MRI?
Descible what is meant by the Larmor frequency and how is it related to the applied magetic field.
What are the Larmor frequencies for water nuclei in magnetic field of 0.5 T, 1 T, 1.5 T?
The tesla (T) is a unit used to measure the magnetic field trength in MRI. What is the strength off the earth’s magnetic field?
What is meant by "net magnetization"?
Classically the spin population states are divided into "parallell" and "antiparallel": what is the approximate population difference between these two states at 1.5T?
What is the purpose of the radio frequency transmit and receive coils?
What determines the frequency of the rotating frame of reference?
What is meant by flip angle(a)?
Describle what is meant by free induction decay(FID) of the MR signal.
Describle what is meant by spin-lattice relaxation.How is this characterized mathematically?
Describle what is meant by spin-spin relaxation. How is this characterized mathmatically ?
At 0.5 T what are approximate T1 and T2 for fat and muscle ?
Describe the spin echo sequence ignoring the imaging process. How might the timing parameters be adjusted to reflect T1, T2 and proton density in the image?
What is the difference between a spin echo and a gradient echo sequence and how differences useful?
Describe how magnetic field gradients are used in the imaging process.
What is meant by frequency and phase encoding?
Describe how you would recognize motion and chemical shift artefacts. How are they related to the magnetic field gradients?
Describe the inversion recovery sequence. How might a modification of this sequence be used to remove the fat signal from the image?
What is the purpose of the Faraday cage and where might it be located?
If the Faraday cage were ineffective how might this manifest itself in a image?
What is the fringe field?
What is the maximum value in the fringe field that is generally regarded as safe for a person with a cardiac pacemaker to stand?
List the main contraindications for MRI?
What precautions should be taken for staff or patients who may be pregnant in relation to magnetic fields?
What is the biological effect of applying radio-frequencies to tissues?
What is meant by SAR? Define the units used to measure this parameter. What is the whole body limit and which organs might be of particular concern to radio-frequency effects?
Learning Objectives of Ultrasound (Characteristics, Production, and Control)
 Name and describe the basic physical characteristics of an ultrasound pulse that have an effect on it's imaging properties.
 Describe the basic function of a transducer and how it forms  an ultrasound pulse.
 State the range of ultrasound frequencies used for diagnostic procedures and give examples of clinical applications using the different frequencies.
 Explain how the operator changes the ultrasound frequency in a typical imaging system.
 Discuss the concept of ultrasound intensity, name the units, and state a typical value for a diagnostic imaging transducer.
 Identify the physical factors which determine the velocity of ultrasound.
 Discuss the physical factors that determine ultrasound wavelength and its significance in imaging.
 Describe the general relationship between wavelength and image quality.
 Describe the factors that determine the physical dimensions (length and width) of an ultrasound pulse.
 Describe the physical conditions in the body that produce ultrasound reflections or echoes.
 Describe the factors that determine the intensity of a reflected pulse.
 Identify anatomical sites that tend to give strong reflections.
 Briefly describe the physical process of ultrasound attenuation and loss of energy as a pulse moves through tissue.
 Identify the three physical factors that determine the total attenuation of an ultrasound pulse passing through a section of tissue.
 State the approximate ultrasound attenuation coefficient for:
          a.  water                b.  soft tissue                   c.  air                d.  bone
 Describe how ultrasound frequency relates to both image quality and tissue penetration.
 Identify the factors that determine ultrasound velocity and state the approximate velocity value in tissue
Explain the significance of velocity in ultrasound imaging and where it must be taken into consideration.
 Identify the specific physical quantity and units that are used to express the rate at which ultrasound energy is imparted to a patient's body.
 Explain the use of the unit decibel (dB) in ultrasound imaging.  State the ratio equivalence of 1, 3, and 6 dB's.
 Calculate the approximate attenuation for 2.5 MHz ultrasound passing through 5 cm of soft tissue.
 Describe the general effect of ultrasound attenuation on image formation.
 Describe and name the feature and controls of an ultrasound system that is used to compensate for tissue attenuation.
Learning Objectives of Ultrasound Vascular Imaging
  Describe the concept and use of an M-mode ultrasound.
 Describe the Doppler effect and how it is used in imaging.
 Explain the significance of beam angle and how it is accounted for in Doppler imaging.
 Identify the condition that can cause depth ambiguity with Doppler imaging.
 Describe the effect of aliasing in Doppler ultrasound imaging.
 Identify the condition that can produce aliasing.
 Explain the significance of the Nyguist limit and its relationship to the pulse-repetition rate.
 Sketch a spectrum display (for one instant in time) and show how it relates to:
 a.   average velocity
b.  flow characteristics within a vessel
 Describe the Doppler gate and how its affects the spectrum display.
 Explain how to determine the direction of blood flow in a color Doppler image.
 Describe the relationship between color in a Doppler display and direction of flow.