Technical University of Denmark Page 1 of 10 pages Written test, 12 December 2012 Course name: Introduction to medical imaging Course no. 31540 Aids allowed: None. Pocket calculator not allowed "Weighting": All problems weigh equally. Instruction: See answer sheet. The right answer is the one which is most correct Problem 1 The figure below shows a cross-sectional view of a phantom consisting of water and stone and four corresponding sketches of a possible CT image denoted (a) to (d).the CT image constitutes a nine by nine array of voxels, indicated by the dotted lines. There is no change in the phantom along the dimension perpendicular to the paper within the voxel. The grey scale bar is the same for all four sketches of the CT image. Which one of the sketches (a) to (d) shows the expected outcome? = Air = Water = Stone Cross-sectional view of phantom Sketches of corresponding CT images (a) (b) (c) (d) HU A. The CT image sketched in (a). B. The CT image sketched in (b). C. The CT image sketched in (c). D. The CT image sketched in (d).
Problem 2 Page 2 of 10 pages Consider an ultrasound scanning situation of an artery before and after balloon angioplasty (Danish: ballonudvidelse). Specifically, the figure below shows a sketch of the ultrasound measurement situation before the balloon is inserted (a) and the ultrasound measurement situation after the balloon has been expanded and then removed (b). The attenuation of ultrasound travelling in soft tissue is assumed to be 1 db/cm/mhz and the attenuation through blood is assumed to be 0.1 db/cm/mhz. The transducer frequency is 10 MHz. The thickness of the vessel above the blue marker is 0.5 cm and 1 cm in situation (a) and (b), respectively. What happens with the intensity of the forward propagating ultrasound pulse at the depth indicated by the blue marker? The blue marker indicates a region in the tissue located at the same depth, 2 cm, in both situations. Only attenuation is assumed to be of importance. Possible changes in the properties of the arterial wall from measurement situation (a) to (b) are to be ignored. Figure (c) and (d) is for your own purpose. Linear array transducer Soft tissue Artery Linear array transducer Soft tissue Artery (a) (b) Linear array transducer Linear array transducer (c) (d) = Blue marker (not a physical object) The drawing is not fully to scale A. The level has increased from -15.5 db to -11 db. B. The level has increased from -31 db to -22 db. C. The level has increased from -15.75 db to -11 db. D. The level has increased from -13.5 db to -22 db.
Problem 3 Page 3 of 10 pages What is FDG? A. Field Decay by Gradient in MRI. B. Fully Dynamic Gain correction in ultrasound. C. Focused Depth of Glass encapsulation, a special focusing technique for an X-ray tube. D. FluoroDeoxyGlucose. Problem 4 This problem concerns magnetic resonance imaging. Select the correct sentence below. A. Precession is a rotation of the nuclear spin axis. It is causing nuclear magnetism. B. Precession is a rotation of the nuclear spin axis. It is caused by nuclear magnetism. C. Spin is a rotation of the axis of nuclear precession. It is causing nuclear magnetism. D. Spin is a rotation of the axis of nuclear precession. It is caused by nuclear magnetism. Problem 5 Where is the MR signal coming from during normal imaging? A. Neutrons in hydrogen in mobile water molecules in the body. B. Protons in hydrogen in mobile water molecules in the body. C. Protons in oxygen in mobile water molecules in the body. D. Neutrons in oxygen in mobile water molecules in the body. Problem 6 How many types of ionizing radiation based on photons exist? A. Alpha, beta, gamma and X-ray radiation. B. Beta, gamma and X-ray radiation. C. Gamma and X-ray radiation. D. X-ray radiation.
Problem 7 Page 4 of 10 pages Which of the following options gives a T 1 -weighted magnetic resonance (MR) measurement? A. Measuring the absorption of radio waves reveals the T 1 -value. B. The MR-signal is T 1 -weighted, if it is recorded after repeated excitations, and if the "repetition time" is sufficiently short. C. T 1 -differences are reflected in measurements of the equilibrium magnetization. D. The decay rate of the MR signal recorded after excitation reflects the T 1 -value. Problem 8 Which of the following sentences about T 2 relaxation is correct? A. The T 2 parameter describes how fast the longitudinal magnetization is lost after excitation. B. The T 2 parameter describes how fast the transversal magnetization is lost after excitation. C. The T 2 parameter describes how fast the transversal magnetization recovers its full amplitude after excitation. D. The T 2 parameter describes how fast the longitudinal magnetization recovers its full magnitude after excitation. Problem 9 Which sentence below best characterizes magnetic resonance imaging? A. Projections of the magnetization density are measured using a rotating radio wave coil. Back projection is used to reconstruct images. B. The strong B0 field is applied in all directions in order to make the nuclear directions depend on position. Image reconstruction is performed by weighted averaging of measurements sensitive to different directions. C. Magnetic field gradients are applied to create linear relations between position and Larmor frequency. Frequency analysis is used for image reconstruction. D. The frequency of radio waves coming from the body after excitation equals the frequency of spatial phase roll patterns. Reconstruction is done by weighting phase roll patterns with the measured frequencies.
Problem 10 Page 5 of 10 pages This problem regards two different types of tissue: a red blood cell and a piece of the far wall of an artery. The far wall is the one farthest away from the transducer. When is angle-dependence present (local changes of the signal due to rotations of the target in all directions in the region of interest)? A. In ultrasound, the echo from a red blood cell does not exhibit angle-dependence, while the echo from a piece of the far wall of an artery does. Neither of these tissues exhibit angle-dependence in CT. B. In ultrasound, neither the echo from a red blood cell nor the echo from a piece of the far wall of an artery exhibits angle-dependence. Neither of these tissues exhibit angle-dependence in CT. C. In ultrasound, the echo from a red blood cell does not exhibit angle-dependence, while the echo from a piece of the far wall of an artery does. Both tissues exhibit angle-dependence in CT. D. In ultrasound, neither the echo from a red blood cell nor the echo from a piece of the far wall of an artery exhibits angle-dependence. Both tissues exhibit angle-dependence in CT. Problem 11 The figure below shows an ultrasound image of a spherical point target (diameter: 0.1 mm) located in water. The lateral width of the white dot on this image is 0.9 mm FWHM at the depth where the width is the largest. What is the best estimate of the lateral spatial resolution size - defined as the width of the point spread function (psf) - of this particular ultrasound imaging system? 21.5 6 22 0.1 mm 5 Depth (mm) 22.5 23 0.9 mm 4 3 2 23.5 18 19 20 Lateral (mm) V 1 A. The width of the psf, D FWHM, is 0.1 mm. B. The width of the psf, D FWHM, is a little less than 0.9 mm. C. The width of the psf, D FWHM, is more than 0.9 mm. D. The width of the psf, D FWHM, is 1.8 mm.
Problem 12 Page 6 of 10 pages What is one of the challenges in integrating a PET scanner with an MRI scanner? A. Some of the materials in a conventional PET scanner (detectors etc) are influenced by magnetic fields. B. The tracer used in PET is emitting ionizing radiation, which constitutes an electrical current which will cause artefacts in the MR image. C. The strong magnetic field of the MRI scanner creates currents in the body, which causes the ions of the ionizing radiation involved with PET to flow in a non-physiological manner. D. The magnetic gradient fields induce resonance in the nuclei which will change the energy of the registered annihilation radiation. Problem 13 In a PET/CT scanner, there are two types of radiation, one associated with PET and one with CT. How are these two types of radiation distinguished by the two scanners? A. By frequency discrimination (the frequency of the photons involved with CT is always lower than the photons involved with PET). B. By frequency discrimination (the frequency of the photons involved with CT is always higher than the photons involved with PET). C. PET uses a coincidence detector and as the X-ray always will be travelling in one direction, it will never be able to create a coincidence event. Likewise, the X-ray tube in the CT scanner uses such a high energy level that it will hinder the decay of the tracer. D. PET data is recorded when the X-ray tube is turned off, while CT data is recorded with the X-ray tube is turned on (the flux is much higher than that of the PET radiation). Problem 14 What does Rutherford s law of decay predict? If N 0 is the number of disintegrations at t = 0, then the number of disintegrations per unit of time is... A. N(t) = N 0 exp( ln2 t /T 1/2 ), where T 1/2 is the half life. B. N(t) = N 0 exp(-ln2 T 1/2 / t), where T 1/2 is the half life. C. N(t) = N 0 exp(-ln2 t 2 /T 1/2 ), where T 1/2 is the half life. D. N(t) = N 0 exp(-ln2 t /T 1/2 ), where T 1/2 is the half life.
Problem 15 Page 7 of 10 pages What is ionizing radiation? It is defined as... A. radiation consisting solely of ions of + and -. B. radiation of e - from the cathode to the anode in the X-ray tube. C. radioactive radiation, i.e., radiation that in itself creates radiation. D. electromagnetic radiation and particles both with kinetic energy high enough to create electronion-pairs. Problem 16 This problem concerns the length of the time window for coincidence registration in PET. Consider an incidence at a given place in the body at a given time. The distance between the two detectors that are located through a line also going through the point of annihilation is 1 m and the diameter of the body (assumed to be round) is 0.5 m. What is the best time window? The speed of light is 3 10 8 m/s (in tissue and air). A. 0.1 ns. B. 10 ns. C. 100 ns. D. 1 s. Problem 17 This problem regards so-called spiral CT scanning. The X-ray tube gantry (Danish: Røntgenrør og ophæng) is rotating constantly with one revolution per second. The table is translated with a speed of 1 mm/s. Every time the X-ray tube makes one revolution, one image can be constructed. Assuming that a subject 2 m high is to be scanned (full body scan), how many images will be recorded? A. 200 images. B. 1000 images. C. 2000 images. D. 3000 images.
Problem 18 Page 8 of 10 pages A 3D medical image is represented by a structure in MATLAB. Specifically, the image data is represented as: MyData.Images which has the size 200 222 18. Which set of axes corresponds to this? A. Sizes of the three axes are: MyData.Axes(1).Axis is 1 200, MyData.Axes(2).Axis is 1 222 and MyData.Axes(3).Axis is 1 3. Unit of Axis is meter. B. Sizes of the three axes are: MyData.Axes(1).Axis is 1 200, MyData.Axes(2).Axis is 1 222 and MyData.Axes(3).Axis is 1 3. Unit of Axis is in points. C. Sizes of the three axes are: MyData.Axes(1).Axis is 1 200, MyData.Axes(2).Axis is 1 222 and MyData.Axes(3).Axis is 1 18. Unit of Axis is meter. D. Sizes of the three axes are: MyData.Axes(1).Axis is 1 200, MyData.Axes(2).Axis is 1 200 and MyData.Axes(3).Axis is 1 200. Unit of Axis is meter. Problem 19 The drawing below shows a source element of an ultrasound transducer. The pressure waveform at two points - coloured green and red - is considered. If the elementary element shown in orange vibrates with the amplitude given by z(t), when will a pressure pulse be received at those two points? Transducer surface Arc of equal travel distance Source element d h (red point) (green point) z(t) 0 t A. At the same time, t = d/c. B. At different times. Green point: t = d/c. Red point: t = (d 2 +h 2 ) (1/2) /c. C. At the same time, t = 2d/c. D. At different times. Green point: t = 2d/c. Red point: t = 2(d 2 +h 2 ) (1/2) /c.
Problem 20 Page 9 of 10 pages This problem concerns the origin of the photons that interact with detectors used in medical imaging. A. Imaging with X-ray: photons originate from the electron shell. Imaging in nuclear medicine: photons originates from the nuclei. B. Imaging with X-ray: photons originates from the electron shell. Imaging in nuclear medicine: photons originates from the electron shell. C. Imaging with X-ray: photons originates from the nuclei. Imaging in nuclear medicine: photons originates from the nuclei. D. Imaging with X-ray: photons originates from the nuclei. Imaging in nuclear medicine: photons originates from the electron shell. Problem 21 An ultrasound system showing B-mode images can image down to a depth of 100 mm. Each image consists of 75 scan lines. The sound speed is assumed to be 1 500 m/s. With these two parameters, how large a frame-rate (number of images per second) can the system show? A. 1 Hz B. 50 Hz C. 100 Hz D. 10 000 Hz Problem 22 A flat object is placed in a conventional planar X-ray system. The distance from the focal point to the film is 1 m. The object is placed 10 cm above the film. How much smaller does the image appear on film when compared with the object? A. Distances on the film becomes about 11% smaller. B. The same size. C. Distances on the film becomes about 11% larger. D. Distances on the film becomes about 15% larger.
Problem 23 Page 10 of 10 pages Consider a (theoretical) phantom with two parallel layers of homogeneous material (liquid) placed on top of each other. The two layers have these characteristics: Layer one: Density 1 = 1000 kg/m 3 and speed of sound c 1 = 1500 m/s. Layer two: 2 = 1250 kg/m 3 and c 2 = 1200 m/s. These parameters do not vary inside a layer. The phantom is imaged with both ultrasound and CT. The ultrasound beam is perpendicular to the interface between the two layers and the transducer is pressed directly against medium 1. The CT imaging plane is perpendicular to the interface between the two layers. How will this phantom appear on ultrasound and CT? Since pocket calculators are not allowed, here are some calculations that might - or might not - be useful: 125 120 = 15000. 120/125 = 0.96. A. The ultrasound image will appear completely dark. The two layers will appear with different shades of gray (different Hounsfield units) on CT. B. The two layers will appear bright on ultrasound, but there will be a dark dotted line at the interface. The two layers will appear with different shades of gray (different Hounsfield units) on CT. C. The two layers will appear dark on ultrasound, but there will be a bright line at the interface. The two layers will appear with different shades of gray (different Hounsfield units) on CT. D. The two layers will appear dark on ultrasound, but there will be a bright line at the interface. The two layers will appear with different shades of gray (different Hounsfield units) on CT with a strong bright line at the interface. Problem 24 Calculate the frequency,, and energy, E, for monochromatic x-rays with = 0.02 nm when the speed of light is c 0 = 300 000 km/s and Planck s constant is h = 6.62 10-34 Js. 1 ev = 1.602 10-19 J. A. = c 0 / 1.5 10 19 Hz. E = h 62 kev B. = /c 0 1.5 10 19 Hz. E = 1/(h ) 62 kev C. = c 0 1.5 10 19 Hz. E = h/ 62 kev D. = c 0 1.5 10-19 Hz. E = h/ 62 kev