Pages 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 12481 result(s) returned

1.3.2 Describing positions along a line

To take a definite case, consider a car moving along a straight horizontal road. The car can be modelled as a particle by supposing the particle to be located at, say, the midpoint of the car. It is clearly convenient to measure the progress of the car with respect to the road, and for this purpose you might use the set of uniformly spaced red-topped posts along the right-hand side of the road (see Figure 5). The posts provide a way of assigning a unique position coordinate to the car
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1.3.1 Simplification and modelling

Everyday experience teaches us that unconfined objects are free to move in three independent directions. I can move my hand up or down, left or right, backwards or forwards. By combining movements in these three directions I can, at least in principle, move my hand to any point in space. The fact that there are just three independent directions, and that these suffice to reach any point, shows that the space in which my hand moves is three-dimensional.

The motion of a large objec
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Acknowledgements

Video Materials

This extract is taken from S809 © 2005 The Open University.

All written material contained within this unit originated at the Open University.


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7 Conclusion

Activity 15

Perhaps you are asking yourself why there are so many different imaging modalities. Is there not one that will do everything that is required? The answer, at the moment, is ‘No’. With most of the imaging techniques, we cons
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6.4 Taking the image

Activity 12

Now watch this video clip of a patients lungs being imaged, called a VQ (ventilation quotient) scan. What are the two different types of acquisitions used called? What radioactive substance is used for each acquisition, and why
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6.3.2 Crystal

Almost all modern gamma cameras use (thallium-doped) sodium iodide (NaI) as the scintillation crystal. A gamma photon interacts with the crystal to produce many photons of visible light.

Sodium iodide is hygroscopic so cannot be left exposed to the air. The front surface is coated with a low atomic number metal that allows the gamma photons to pass through. The rear surface is covered with a transparent coating so that the visible photons can pass through to the photomultiplier tubes.
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6.3.1 Collimator

Without a collimator, gamma rays from all directions would be collected by the crystal and no useful image could be obtained. Gamma rays cannot be focused by a lens but a collimator consisting of a series of holes in a lead plate can be used to select the direction of the rays falling on the crystal. Most collimators in use today are parallel hole collimators. A parallel hole collimator is shown schematically in Author(s): The Open University

6.2 Producing the radioactive substance (elution)

In the radiopharmacy Tc-99m is produced in a generator.

Mo-99, a product of the fission of uranium, is isolated from a nuclear reactor and absorbed on to an alumina column in the generator. When a saline solution is passed over the column, ion exchange results in the production of sodium pertechnetate. This can then be chemically manipulated to form a variety of compounds. The removal of the technetium by the passage of saline is known as elution.

Conveniently, the optimum interva
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6.1 Introduction

Radionuclide imaging is a very valuable way of examining the function of an organ, as opposed to the more structural images obtained by other methods such as X-ray and CT.

The basic principles of radionuclide imaging are as follows:

  • a radioactive substance, usually combined with a biologically active compound, is injected into the patient;

  • this targets a particular organ or tissue type;

  • the radiation emitted i
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5 Ultrasound

Ultrasound imaging uses acoustic waves, rather than ionizing radiation, to form an image. The principle is rather like radar; a pulse of ultrasound (1–15 MHz) is sent out from the transducer and reflected from tissue boundaries. Measurement of the time taken for the pulse to return allows the distance to the reflecting boundary to be calculated.

The important parameter determining the amount of reflection is known as the acoustic impedance (Z) of the tissue and is the product o
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2.2.4 Film cassette and grid

As the X-rays pass through the patient some of them will be scattered and will therefore not follow the expected line through the patient. If these reach the detector they will blur the image. Some of the scattered radiation can be removed by a grid, usually oscillating, placed between the patient and the detector.

Analogue imaging systems use either film alone (rarely) or a combination of a film and fluorescent material (phosphor). The phosphor fluoresces and produces visible light whi
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2.2.3 Couch

The couch or patient trolley must be radio-translucent (i.e. it allows through most of the X-rays). Nonetheless there is some interaction between the couch and the X-rays and this can be a cause of scattered radiation.


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Learning outcomes

By the end of this unit you should be able to:

  • explain the underlying scientific principles of the major medical imaging techniques;

  • explain the mode of operation of the major medical imaging techniques;

  • understand the advantages and disadvantages of the major imaging techniques.


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Introduction

This course is about the scientific basis of medical imaging. Imaging techniques have long been part of the physician's diagnostic repertoire. Their use has been developed from simple X-ray measurements in the Accident and Emergency Unit to a range of subtle investigations of both the structure and functional status of targeted organs, including the brain. Medical imaging is at the heart of contemporary medical practice.

This unit is an adapted extract from the Open University course
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Acknowledgements

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The following appears in Introduction to Astrobiology (Planetary Science Book 2: ISBN 0-521-54621-4) which is published in ass
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4 Unit summary

  • Many of the large icy bodies in the outer Solar System are internally differentiated. Thanks largely to tidal heating, some, especially Europa, are likely to have an ocean sandwiched between the icy exterior and the rocky core. Others may have had such an ocean in the past.

  • Wherever water rests on warm rock, water must percolate into it and become heated. This will cause hydrothermal convection to begin. Hot, chemical-rich water will emerge
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2.4 How thick is Europa's ice?

You learned in Section 1.4 that geophysical data show the ‘icy’ outer part of Europa to be about 100 km thick, but that the information is inadequate to distinguish between the extreme possibilities of solid ice all the way down to the bedrock and a floating sheet of ice supported above a liquid ocean (Author(s): The Open University

2.3.4 More surface disruption

Now let's examine some detailed images of the region of Europa's northern hemisphere that was indicated on Figure 17 (see Section 2.3.2). A medium resolution image is shown in Figure 21, and higher resolution images from within this area are shown in Author(s): The Open University

2.3.2 The crater Pwyll

You might also have noted that there are no obvious impact craters visible in Figure 16 (see Section 2.3.1). In fact there are a few. One is a bright spot, 15 km in diameter, surrounded by a dark halo of ejecta that occurs 10 mm from the top edge and 65 mm from the left-hand edge of the figure. Another is a s
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2.2 Ice and salt

As noted in Section 1.5, Europa's near-infrared reflectance spectrum was used as long ago as the 1950s to demonstrate that its surface is mostly water-ice. More recently, spectroscopic observations by the Hubble Space Telescope and Galileo have revealed some regions where the ice appears to be salty (see below) and ha
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