ELEC ENG 3BA3: Structure of Biological Materials. Solutions to Final Exam (2011)

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1 ELEC ENG 3BA3: Structure of Bioloical Materials Solutions to Final Exam (011) 1. Tendons and liaments: a. have fairly crystalline stress-strain relationships, b. contain a reater proportion of elastin than collaen, c. have very rubbery stress-strain relationships, or d. are manufactured by chondrocytes. (5 pts) The answer is a. have fairly crystalline stress-strain relationships. See slide 5 of Lecture #3.. Carbon materials have not been successful utilized in orthopaedic implants because: a. nobody has thouht of it yet, b. they all have very poor biocompatibility, c. they are much too heavy, or d. it is not yet possible to obtain suitable mechanical properties. (5 pts) The answer is d. it is not yet possible to obtain suitable mechanical properties. See slide 1 of Lecture #4. 3. The chemical acetylcholine (ACh) is: a. present in fast lycolytic muscle fibers but not in slow oxidative fibers, b. the chemical that causes myosin filaments to move alon actin filaments in myofibrils, c. the neurotransmitter used in the neuromuscular junction (i.e., the synapse between a motor neuron and a muscle fiber), or d. a constituent chemical of nylon. (5 pts) The answer is c. the neurotransmitter used in the neuromuscular junction (i.e., the synapse between a motor neuron and a muscle fiber). See slides 5 & 6 of Lecture #7. 4. Of these four blood vessel types, pulsatile blood flow is stronest in the: a. vena cava, b. aorta, c. capillaries, or d. arterioles. (5 pts) The answer is b. aorta. See slide 4 of Lecture #11. Dr. I. Bruce 5/11/01 1/9

2 5. In liposome delivery of a hydrophobic dru, the dru molecules would normally be: a. directly attached to the heads of the lipids on the outer surface of the liposome, b. attached to PEG strands on the outside of the liposome, c. in an aqueous solution in the very centre of the liposome sphere, or d. dissolved amonst the lipid tails between the two layers of the bilayer liposome. (5 pts) The answer is d. dissolved amonst the lipid tails between the two layers of the bilayer liposome. See slide 14 of Lecture # The term biosensor refers: a. to any sensor for measurin bioloical sinals, b. specifically to sensors that utilize a bioloical aent, c. only to sensors for measurin blood lucose levels, or d. only to sensors utilizin antibodies. (5 pts) The answer is b. specifically to sensors that utilize a bioloical aent. See slide of Lecture # In the Larmor equation for nuclear manetic resonance, ω o corresponds to the anular frequency at which a proton: a. flips in response to an RF burst, b. dephases due to spin-spin interactions, c. precesses (wobbles) around the axis of an external manetic field, or d. spins around its own axis. (5 pts) The answer is c. precesses (wobbles) around the axis of an external manetic field. See slide 5 of Lecture #0. 8. To compute the back-projection in tomoraphic imain, a widely used mathematical technique is: a. the Raylon equation, b. the radionuclide transform, c. the Radon transform, or d. Rayleih scatterin. (5 pts) The answer is c. the Radon transform. See slide 17 of Student Presentation #19. Dr. I. Bruce 5/11/01 /9

3 9. Discuss the bioethical complexities of choosin subjects for clinical trials of biomedical technoloies to treat life-threatenin diseases. ( pts) The main complexities arise from the possibly-competin oals of obtainin the best measurement of benefit and minimizin risk for the patients (non-maleficence). If the technoloy has some substantial risk of (i) not workin sufficiently to keep the patient alive or (ii) havin harmful sideeffects, then the only suitable subjects miht be those patients for whom all other treatment options have failed and are likely to die soon. owever, if such patients are chosen as subjects, then they may be so sick that (a) the full benefit of the technoloy is not clearly displayed or (b) they die anyway, and it may be difficult to determine whether the biomedical technoloy contributed to the death or not. ealthier subjects may better display the benefit of the technoloy, but the risks may be too reat, there may be other viable treatment options still available, or the impact of sideeffects of the treatment on quality of life may be more important for these patients. On the other hand, if the treatment has a hih probability of success and few risks, then choosin patients who are closer to dyin miht be a more ethical decision.. Compare and contrast the use of collimators in: i) x-ray CT, ii) SPECT and iii) PET imain. ( pts) In x-ray imain and nuclear medicine imain other than PET, collimators are used to determine the beam eometry (e.., parallel, fan, cone) for imae construction. Because x-ray imain is a transmission imain modality, the collimator is used in between the x- ray source and the object bein imaed. (Note that an anti-scatter rid can be used between the object bein imaed and the x-ray detector.) Because SPECT is an emission imain modality, the collimator is used in between the object bein imaed (which has a radioactive source within) and the detector. (Note that it is not possible to avoid the effects of scatter in this case.) In PET imain, pairs of photons emitted coincidently in opposite directions are detected to determine the source of the radioactive decay, i.e., it oriinates somewhere alon the line between the coincidently-activated detectors. Consequently, collimators are not necessary in PET scanners. 11. Explain the function of the 180 RF pulse in an MRI spin echo sequence. ( pts) The 180 RF pulse is used to counteract the effects of an inhomoeneous (i.e., imperfect) B 0 manetic field on T relaxation. Because of these inhomoeneities, dephasin of the proton manetic moments in the XY plane occurs faster than would be observed for a perfect B 0 field. This faster dephasin is referred to as T * relaxation. The 180 RF pulse is applied some time after the 90 RF pulse to flip the precession phase by 180 ; protons that have otten ahead of the mean phase of precession because of a slihtly stroner B 0 manitude at their location are now behind, and those that were behind because of a slihtly weaker B 0 manitude at their location are now ahead. Consequently, the dephasin due to B 0 inhomoeneities is undone in the period followin the 180 RF pulse, and the measured manetic moment in the XY plane starts rows aain to form an echo sinal. The echo sinal is maximum at the echo time (TE), which is twice the time period between the 90 and 180 RF pulses. The manitude of this maximal sinal will depend on T relaxation rather than T * relaxation. Dr. I. Bruce 5/11/01 3/9

4 1. J. and E. are practicin a judo throw called tsurikomi-oshi. Before throwin E., J. holds him static in the position illustrated below, with E. tipped forwards from vertical and E. s centre of mass (C.O.M.) 30 cm from the pivot point on J. s hips. J. is pullin with his riht hand on E. s sleeve with a force F R that is actin 45 cm from E. s C.O.M. at an anle of 46 from the line of E. s body. J. is also pushin on E. s collar with his left hand with a force F L that is actin alon the same line as F R but in the opposite direction. Assume that E. has a mass of 73 k and acceleration due to ravity is 9.8 m s, producin the body weiht F, and that the reaction forces of J. s hips on E. s les are actin directly throuh the pivot point. hat is the minimum difference between the force exerted by J. s left hand ( F L ) and his riht hand ( F R ) that will keep E. static in this position? ( pts) The complete free body diaram for the external forces applied to E is iven below. Note that there is a reaction force F hips actin throuh the pivot point, as stated, but the manitude and anle of this force are unknown. Similarly, there will be a round reaction force F from E. s feet actin at some point alon the line of his body, but location of this point and the manitude and anle of this force are unknown. Consequently, it is not possible to solve this problem from the balance of translational forces. owever, the hip reaction force acts throuh the pivot point and thus does not contribute any anular moment to the rotation of E. around the pivot point. In addition, as the difference between F is reduced, both E. s body the force exerted by J. s left hand ( ) L F and his riht hand ( ) weiht F and the round reaction force F will tend to cause E. to rotate forward around the Dr. I. Bruce 5/11/01 4/9 R

5 pivot point. If the difference F = FL FR is reduced enouh, then E. s feet will start to loose contact with the round and the round reaction force F vanishes. Thus, the minimum difference that will keep E. static in this position is when the anular moment due to the difference F is exactly balanced with the anular moment due to his body weiht F. E. s body mass of 73 k produces a downwards force F = m = = N. ith E. s body tipped from vertical and F actin 0.3 m from the pivot point, the manitude resultin moment is: ( ) M = r F sin θ = sin =80.40 N m. M of the The hand force difference F acts at anle of θ = 46 and a distance of r = = 0.75 m, resultin in a manitude M of the moment produced by the hands equal to: ( ) M = r Fsin θ = 0.75 Fsin 46 = F N m. In order for E. to remain static in this position, then the minimum value of M M = F = F = = N F can be found via: Note that if a force difference reater than N is applied by the hands, then E. will remain static because the additional force will be counteracted by the round reaction force F at the feet. Dr. I. Bruce 5/11/01 5/9

6 13. The Achilles tendon attaches the plantaris, astrocnemius (calf) and soleus muscles to the calcaneus (heel) bone. Both tendon and cortical bone have very low proton spin densities, so they both appear as dark areas in MR imaes in contrast to the surroundin tissue. owever, due to their somewhat different T1 and T relaxation times, iven in the table below, it miht be possible to pick up the interface between these tissues in T1-weihted or T-weihted imaes. Tissue T1 (ms) T (ms) Achilles tendon Cortical bone a. Assumin that the proton spin density is identical in the two tissues, determine the MR imae relative sinal manitudes that should be obtained for the Achilles tendon and cortical surface of the calcaneus bone for: i. a standard T1-weihted spin echo sequence (TE = 11 ms; TR = 460 ms), and ii. a standard T-weihted spin echo sequence (TE = 90 ms; TR = 400 ms). b. From the MR imae sinal manitudes predicted in part a., discuss the relative advantaes and disadvantaes of T1-weihtin and T-weihtin in imain the interface between the Achilles tendon and the cortical surface of the calcaneus bone. c. Discuss whether the standard TE and TR values for T1-weihtin and T-weihtin used in part a. are appropriate for imain these tissues, iven their T1 and T values. ( pts) a. Let the subscript t indicate the tendon and the subscript b the bone. There is no fast blood flow in tendon or cortical bone, so there is no modulatin effect of blood on the NMR sinal, i.e., f ( v ) = 1. The proton spin densities are identical in the two tissues, so Nt = Nb. For the T1-weihted sequence, the NMR sinal strenths for the tendon and done should be: and ( ) t t S = N f v e 1 e = N e 1 e = N 6.36 TE T TR T t,t1 t t t ( ) b b S = N f v e 1 e = N e 1 e = N 4.53 TE T TR T b,t1 b b b respectively. For the T-weihted sequence, the NMR sinal strenths for the tendon and bone should be, respectively: and St,T = Nt e 1 e = Nt.66, Sb,T = Nb e 1 e = Nb.93.,, Dr. I. Bruce 5/11/01 6/9

7 b. The contrast for the T1-weihtin will be: S S t,t1 b,t1 = 5 Nt 6.36 = 1.40, 5 N 4.53 b while for the T-weihtin it will be: S S t,t b,t = 33 Nt.66 = N.93 b Consequently, the T-weihtin ives the much reater contrast, but the absolute values are so miniscule that the backround noise in the sinal measurement and imae computations is extremely likely to swamp these sinals. Consequently, the contrast in the T1-weihtin miht be more feasible to measure, even thouh the ratio is smaller, because the absolute values are larer. owever, there could still be difficultly in pickin up a measureable sinal, because both N & N are very small compared to the surroundin tissue, as stated above. t b c. Given the T1 values of 490 and 80, the TR value of 460 ms in the T1-weihted sequence contributin to a fairly ood contrast based on the different T1 values (althouh a somewhat shorter TR value would ive better contrast), while the TR of 400 ms for the T-weihted sequence is lon enouh to minimize the effects of the different T1 relaxation of the two types of tissue. owever, the TE values of 11 ms and 0 ms are much too lon for the very short T values of 1. and 1.1 ms for these two types of tissues, ivin rise to very small sinals. Consequently, several new MRI pulse sequences have been developed to imae tissues like bone, liament and tendon with very short T values, includin ultrashort-te (UTE) imain, sinle-point imain (SPI), sinle-point imain with T1 enhancement (SPRITE), and fast spin echo (FSE) imain, but these sequences are not achievable on most standard clinical MRI scanners at the present time. Dr. I. Bruce 5/11/01 7/9

8 14. Consider the transmission of an ultrasound sinal throuh to the skin and soft tissue from an ultrasound transducer placed on the skin. In the first case, illustrated to the left in the fiure below, no el is used between the transducer and the skin. In the second case, illustrated to the riht in the fiure below, a el is used between the transducer and the skin. Assume that: i. the acoustic impedance of the transducer is.9 7 rayls, the acoustic impedance of the skin & soft tissue is rayls, & the acoustic impedance of the el is rayls; and ii. the attenuation of the acoustic sinal in the el is neliible. If the intensity of an ultrasound pulse produced by the transducer I o = 15 m/cm, what is the ultrasound pulse intensity, I, that is directly transmitted into the skin & soft tissue for the two cases: a. no el, and b. el? ( pts) a. In the case of no el, the intensity of the ultrasound transmitted from the transducer into the skin and soft tissue is: I = Io = I = 4.78 m cm. o b. In the case of el, the intensity of the ultrasound transmitted from the transducer into the el is: I = Io = I, o and the intensity of the ultrasound transmitted from the el into the skin and soft tissue is: I = I = I, Dr. I. Bruce 5/11/01 8/9

9 ivin a directly transmitted intensity of: I = I = I = 47.4 m cm, 0 which is almost double the intensity of the ultrasound transmitted without usin the el. Dr. I. Bruce 5/11/01 9/9