Magnetic Resonance Imaging Hariqbal Singh, Varsha Rangankar, Abhijit Pawar
INDEX
Page numbers followed by f refer to figure and t refer to table
A
Abdomen 13
Abdominal wall fibromatosis, anterior 14
Acoustic schwannoma 358
Acromio-clavicular
degeneration 206
joint 206, 207f
Adamantinoma 188
Adenoma, adrenal 38
Adrenogenital syndrome 38
Adrenoleukodystrophy 324
Air, hypointense signal of 112f
Alcoholism, chronic 379
Alexander's disease 328
Annular tear 103
Aplasia 372
Aqueductal stenosis, congenital 280
Arachnoid cyst 341
Astrocytoma, cerebellar 361
Atrophy, cerebellar 379, 380f
Avascular necrosis 175, 220
B
Baker's cyst 257, 258f
Basal ganglia hyperintensities 377
Basilar invagination 92
Basi-occiput hypoplasia 92
Bicornuate uterus 46, 47f
Bile duct calculus 32
Bone 159
cyst 167
infarct 177
Bow tie sign, absence of 240
Brain 279
abscess 307
Broad ligament fibroid 50
Brodie's abscess 181
Bucket handle tear 240
C
Café-au-lait spots 169, 274f
Canavan's disease 326
Carbon monoxide poisoning, acute 377
Carcinoma, ovarian 55, 57
Carolis disease 22
Cell death, central zone of 177
Center line artifacts 396
Cerebral
vein thrombosis, internal 377
venous thrombosis 371
Cerebrospinal fluid 343
Cervical vertebra, compression fracture of 97
Cervix, carcinoma 62
Chamberlain's line 92
Chiari malformation 88, 92, 282, 286
Choledochal cyst 22
Cholelithiasis 26
Cholesterol stone 26
Cirrhosis 18
Clay Shoveler's fracture 95
Cleido-cranial dysplasias 92
Collateral ligaments 231
Complete tear tendoachilles 259
Compression fracture 99
Computed tomography 287
scanning 86
topogram 171f
Condylus tertius 92
Conns syndrome 38
Cord hematoma 97
Corpus callosum 282, 335f, 384f
Craniopharyngioma 356
Craniovertebral junction lesions 90
Crosstalk artifacts 398
Cruciate ligament 162, 228
Cushing's syndrome 38
Cystic encephalomalacia 321
Cysts, ovarian 54
D
Dacryocystocele 264
Dandy-Walker malformation 291
Dermoid
cysts 52, 267
spinal 145
Diastematomyelia 82
Disc
bulge 105
disease, degenerative 111
herniation and migration 109
lesions 103
Dorsal intercalary segmental carpal instability 160
Double
anterior horn sign 240
posterior cruciate ligament sign 240
Douglas, pouch of 63
Dural ectasia 140
Dysembryoplastic neuroepithelial tumor 367
Dysraphism, spinal 80
E
Echinococcus multilocularis 270
Echo planar imaging technique 4
Edema 97
Ehlers-Danlos syndrome 92
Ejaculatory duct cyst 70
Empty sella 369
Endometrial carcinoma 61
Epidermoid 343
Ewing's sarcoma 198
Extrapontine myelinolysis 377
F
Familial adenomatous polyposis 22
Fibrous dysplasia 169
Flipped meniscus sign 240
Fluid attenuated inversion recovery 10
Focal cortical dysplasia 285
Fracture
avulsion 163
scaphoid 160
Friedreich ataxia 379
Fukuyama congenital muscular dystrophy 298
G
Gallstone
composition of 26
type of 26
Germinal matrix hemorrhage 375
Gibbs phenomenon 395
Glenohumeral ligament, bony humeral avulsion of 210
Glenoid labrum tear 210
Glioblastoma multiforme 352
Glioma 348
Global hypoxia 322
Globus pallidus 386f
Grave's ophthalmopathy 266
Greater tuberosity, avulsion of 204
H
Haglund's syndrome 259
Hemangioma 186
Hemochromatosis 16
Hemorrhagic renal cyst 42
Hepatic metastasis 24
Hepatocellular carcinoma 20
Hepatocerebral degeneration, acquired 385
Heterotopia 284
Hill-Sachs lesion 202
Hip joint 218
Huntington disease 377
Hyperdense cyst 42
Hyperintense cystic structure 71f
Hyperparathyroidism 92
Hypoplasia 372
Hysterosalpingography 46
I
Infection 301
hematogeneous spread of 226
spinal 126
Infectious organisms, direct implantation of 226
Infective arthritis 226
Inflammatory bowel disease 22
Intercondylar notch sign 240
Intraspinal lipoma 88
Ischemia 377
Ischemic
encephalopathy, hypoxic 319
injury, zone of 177
J
Japanese encephalitis 312
Joints 201
Joubert's syndrome 291, 293
Juxta-articular osteoporosis 217
K
Kayser-Fleischer rings 387
Kienbock's disease 173
Kissing contusions, classification of 241
Klatskin tumor 22
Klippel Feil syndrome 92
Knee joint 228
Kupffer cells 19, 401
L
Lacunar infarct 316
Laminectomy 124
Legg-Calv Perthes disease 177
Leigh disease 377
Leukomalacia 334
Ligamentum flavum
hypertrophy 120
thickening of 121f
Lipomeningocele 76
Liposarcoma 252
Lumbar disc desiccation with protrusion 107
Lymphoma 377
M
Macrodystrophia lipomatosa 250
Magnetic resonance
angiography 401, 402
cholangiography 30
cholangiopancreatography 34
imaging 24, 163, 177, 212, 240, 291, 400
venography 372
Marchiafava-Bignami
disease 383
syndrome 383
Marfan's syndrome 92
Masses, ovarian 54
McCune Albright's syndrome 169
McGregor's line 92, 94
McRae's line 92
Median nerve, fibrolipomatous hamartomas of 255
Medulloblastoma 363
Meningioma 149, 339
Meningocele 76
Meningomyelocele 86
Meniscal cysts 243
Mercedes-Benz sign 26
Metachromatic leukodystrophy 330
Methylmalonic acidemia 377
Mitochondrial diseases 377
Modic classification 113t
Monostotic fibrous dysplasia 169
MR angiography 10
Mucopolysaccharidosis 332
Müllerian duct cyst 64, 70
Muscle-eye-brain disease 298
Muscular dystrophies 298
Myelomeningocele 76, 88
N
Neoplasm 143, 339
Neural tube defect 80
Neurocysticercosis 302t, 305
Neurofibroma 155
Neuromuscular scoliosis 79
O
Olivopontocerebellar atrophy 379
Open lip schizencephaly 296
Opisthion-Basion line 286
Orbit 263
Orbital
hematoma 269
hydatid cyst 270
rhabdomyosarcoma 276
OS odontoideum 90
Osteochondroma 110, 183
Osteochondromatosis 217
Osteogenesis imperfecta 92
Osteoid osteoma 218
Osteomalacia 92
Osteomyelitis 181
Osteoporotic compression fractures 116
Osteosarcoma 196
P
Paget's disease 92
Pancreas, pseudocyst of 34
Pancreatic cysts 36
Parameniscal cyst 243
Pedunculated fibroids 48
Pelvis 45
Perisylvian infarct 317
Phemister's triad 217
Pigment stone 26
Pigmented villonodular synovitis 217, 245
Pineal epidermoid cyst 346
Pituitary macroadenoma 354
Polyostotic fibrous dysplasia 169
Pontine glioma 350
Pott's
disease 126
spine 126, 127
Preisers disease 175
Prostate, carcinoma 68
Psoas muscles 127
R
Rasmussen encephalitis 309
Renal angiomyolipoma 40
Rheumatoid arthritis 92
Rosenmuller, valve of 264
Rotator cuff tears 212
S
Sacrococcygeal teratoma 157, 194
Sacroiliitis 136
Scheuermann's disease 110
Schmorl's nodes 110, 111f
Sclerosing cholangitis 22
Scoliosis 76, 77
Segond fracture 162
Septo-optic dysplasia 296
Shoulder joint 202
Simple ovarian cyst 55f
Spina bifida 76
cystica 76
occulta 76
Spinal canal stenosis 122
Spinal cord
contusion 102
ependymomas 151
lipoma 147
Spine, degenerative 103
Spinoglenoid cyst 208, 209
Spinous process fracture 95
Spondylolisthesis 84, 142
Stress fracture 165
Stroke 314
Structural scoliosis 79
Sturge-Weber syndrome 337
Subcutaneous lipoma 248
Subdural hemorrhage 381
Subependymal heterotopias 284
Superparamagnetic iron oxide 400
T
Taenia solium 305
Tallus chondroblastoma 192
Temporal choroidal fissure cyst 294
Tibial plasmacytoma 190
Torsion, ovarian 59
Transverse myelitis 138
Trauma, spinal 95
Triple PCL sign 240
Tuberculoma 301, 302t
Tuberculous
arthritis 215
infection 217
Tuberous sclerosis 336
Tumors, neurogenic 153, 261
U
Undescended testis 72
Urinary bladder, carcinoma 66
Uterine fibroids 48
V
Vacuum phenomenon 111
Vermian hypoplasia 291
Vertebral hemangioma 143
W
Walker-Warburg syndrome 298
White matter disease 324
Wilson disease 377, 387
X
X-linked adrenoleukodystrophy 324
Z
Zebra artifacts 396
Zipper artifacts 396
×
Chapter Notes

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History and PhysicsCHAPTER 1

Hariqbal Singh,
Parvez Sheik
2
Magnetic resonance imaging (MRI) is the most important diagnostic imaging discovery in medicine since the discovery of X-ray in 1895 by Wilhelm Conrad Röntgen. The first MRI was commercially available in 1980. Since then its importance in field of medicine continues to grow at a tremendous space and is now established beyond doubt. Before beginning a study, it is important to know a brief history of MRI.
Sir Joseph Larmor (1857–1942) developed the equation that the angular frequency of precession of the nuclear spins being proportional to the strength of the magnetic field referred as Larmor relationship. In the 1930's, Isidor Isaac Rabi of Columbia University succeeded in detecting and measuring single states of rotation of atoms and molecules, and in determining the magnetic and mechanical moments of the nuclei.
Working independently, Felix Bloch of Stanford University and Edward Purcell of Harvard University made the first successful nuclear magnetic resonance experiment to study chemical compounds in 1946, thus magnetic resonance phenomenon was discovered. They developed instruments, which could measure the magnetic resonance in bulk material such as liquids and solids. In 1946 they came up with the idea to use magnets to take pictures of a living being and called it magnetic resonance. Both Felix Bloch and Edward Purcell were awarded Nobel Prize in 1952.
In 1971, Raymond Damadian a physician and scientist of State University of New York demonstrated that there are different T1 relaxation times between normal and abnormal tissues of the same type, as well as between different types of normal tissues on his nuclear magnetic resonance (NMR) device. In the same year he proved that magnetic resonance could be 3used to help detect diseases by the different nuclear magnetic relaxation times between tissues and tumors thus motivating scientists to consider magnetic resonance for the detection of disease.
In 1973, Paul Christian Lauterbur (6 May 1929–27 March 2007) of State University of New York described a new imaging technique that he termed zeugmatography. By utilizing gradients in the magnetic field, this technique was able to produce a two-dimensional (2D) image. Magnetic resonance imaging was first demonstrated on small test tube samples. He used a back projection technique similar to that used in CT.
In 1975, Richard Ernst introduced 2D NMR using phase and frequency encoding, and Fourier-Transformier instead of Paul Lauterbur's back-projection, he timely switched magnetic field gradients. This basic reconstruction method is the basis of current MRI techniques.
On 3 July 1977, Raymond Damadian performed the first MRI examination on a human being on the machine which he named “Indomitable”. It lasted 4 hours and 45 minutes to complete. This machine is now in the Smithsonian Institution. Indomitable represents a milestone in the history of medical imaging. Its story is a timeless one of a driven inventor who perseveres through every obstacle only to find that others are racing along similar paths, which in this case led to today's ubiquitous magnetic resonance imaging (MRI) machines.
Peter Mansfield further developed the utilization of gradients in the magnetic field and the mathematical analysis of these signals for a more useful imaging technique. In 1977 the first images taken of a cross section through a finger were presented by Peter Mansfield and Andrew Maudsley. Peter 4Mansfield also could present the first image through the abdomen. In the same year Peter Mansfield developed the echo planar imaging (EPI) technique. This technique developed in later years to produce images at video rates (30 ms/image). Paul Lauterbur and Peter Mansfield were awarded with the Nobel Prize in Medicine in 2003.
Raymond Damadian in 1978 founded the FONAR Corporation, which manufactured the first commercial MRI scanner in 1980. As late as 1982, there were a handful of MRI scanners in the world. Today there are a million or even more, and images can be created in seconds what used to take hours. Current MRI scanners produce highly detailed two-dimensional (2D) and three-dimensional (3D) images. The technique was initially called nuclear magnetic resonance imaging (NMR or NMRI) but because of the negative connotations associated with the word nuclear it is called as magnetic resonance imaging.
 
 
Physical Principle
Magnetic resonance imaging (MRI) is based on the principle of electromagnetic character of atomic nuclei which was first described by physicist Felix Bloch and Edward Purcell in 1946. They received a Nobel prize for this in 1952. However it was long after this that nuclear magnetic resonance was used for imaging. In 1973, Lauterbur showed that images of human body could be acquired by placing a magnetic field around it. First human images were published by Damadian et al. in 1977. Since then use of MRI for medical imaging has seen an exponential growth and now it is a mainstay in the field of medical diagnostics.5
Electromagnetism is at the core of MRI physics. When current is passed through a wire, a magnetic field is created around it. Similarly, in a nucleus with odd number of protons or neutrons, the electrons rotating around the nucleus produce a field around them. This gives a “charge” to the nucleus, also called as the spinning charge or “the spin”. Thus these nuclei behave as tiny magnets. Hydrogen proton is the most favorable nucleus for MRI as it is widely available in the water molecules present in the body.
When these nuclei are placed in an external magnetic field (B0), they either align along the magnetic field or against it. When the number of nuclei along the magnetic field is more as compared to those against the field, a net magnetization is created in the direction of the field.
In order to generate a signal from these spinning nuclei they have to be tipped out of alignment with B0 (i.e. out of the longitudinal plane and towards the transverse plane). The signal generated by each rotating nucleus is much stronger if the nuclei precess in unison with each other at 90° to the main magnetic field. For this a second magnetic field is introduced and it is referred to as B1. This B1 should be applied perpendicular to B0, and it has to be at the resonant frequency. Radiofrequency (RF) coils are used to transmit B1. If sufficient RF pulse is applied the spins are flipped into the transverse plane. This is the 90° RF pulse and it generates the strongest signal. However as this is a high energy state, the signal starts decaying quickly and is called free induction decay (FID). This decay or relaxation is of two types:
T1 relaxation is the relaxation in the longitudinal plane due to the spins returning to the normal equilibrium state and 6aligning with the main magnetic field. In T2 relaxation there is dephasing in the transverse plane (90° plane). Each individual proton precesses at slightly different speed. After a while, the signal from protons in transverse plane degenerates as protons start precessing out of phase with each other. This is T2 relaxation.
In human tissue T1 is usually 10 times longer than T2 which means that T2 decay occurs before T1 recovery (Table 1). In actual practice the T2 dephasing time is much quicker than the ‘natural’ T2 due to inhomogenities in the magnetic field B0. This reduced T2 is called T2*. T2* is decay of transverse magnetization because of spin-spin relaxation and inhomogeneity of the magnetic field. In GRE T2* sequence is the main determinants of image contrast and forms the basis of MR applications including perfusion imaging, and functional imaging and susceptibility-weighted imaging. A low flip angle, long echo time and long repetition time can make the GRE sequence T2*.
Table 1   Signal intensity of various tissues at T1, T2 and proton density imaging
Tissue
T1
T2
Proton density
Fat
Bright
Bright (less than T1)
Bright
Water
Dark
Bright
Intermediate bright
Cerebral gray matter
Gray
Gray
Gray
Cerebral white matter
White
Dark
Dark
TR values
TR < 500
TR > 1500
TR > 1500
TE values
TE 50–100
TE > 80
TE < 50
7
Bleed, calcium and iron deposition in various tissues can be depicted on GRE with T2*.
T1W and T2W images result by manipulating the manner and frequency in which RF pulses are applied (Time to Repetition), and by changing time to start signal acquisition after RF has been applied (Time to Echo), T1-weighted or T2-weighted images can be obtained (Table 2).
 
Pulse Sequences
  • Partial saturation (PS): It is also known as gradient echo or field echo and it uses a 90° RF pulse.
  • Spin echo (SE): A 90° pulse is followed by 180° refocusing RF pulse.
  • Inversion recovery (IR): A 180° pulse is followed by a 90° pulse.
In a typical image acquisition the basic unit of each sequence (i.e. the 90°–180°-signal detection) is repeated hundreds of times. By altering the time to echo (TE) or time to repetition (TR), i.e. the time between successive 90° pulses, the signal contrast can be altered or weighted. For example if a long TE is used, inherent differences in T2 times of tissues will become apparent.
Table 2   Time to echo and time to repetition for MR sequences
Time to echo TE
Time to repetition TR
T1 weighted or T1WI
Short TE
Short TR
T2 weighted or T2WI
Long TE
Long TR
Proton density weighted or PDW
Short TE
Long TR
8
Tissues with a long T2 (e.g. water) will take longer to decay and their signal will be greater (or appear brighter in the image) than the signal from tissue with a short T2 (e.g. fat). In a similar manner TR governs T1 contrast. Tissue with a long TR (water) will take a long time to recover back to the equilibrium magnetization value, therefore a short TR interval will make this tissue appear dark compared to tissue with a short T1 (fat). When TE and TR are chosen to minimize both these weightings, the signal contrast is only derived from the number or density of spins in a given tissue. This image is said to be proton density weighted (PDW).
Air is black in all sequences because of very few protons and cortical bone is always black due to no mobility of protons.
Each volume element in the body has a different resonant frequency which depends on the protons present within it. This produces a signal which is specific to the resonant frequency of that volume element. This signal is analyzed by the computers using a mathematical technique called as Fourier analysis.
Magnet forms the main component of the MRI, it is of two types:
  1. Permanent or resistive magnets used in low field scanners and are usually referred to as open MRI.
  2. Superconducting magnet are used in all scanners above 1.0 Tesla. It is wound from an alloy (usually Nb-Ti) that has zero electrical resistance below a critical temperature. To maintain this temperature the magnet is enclosed and cooled by a cryogen containing liquid helium which has to be topped-up on a regular basis.
RF coils are needed to transmit and/or receive the MR signal. The RF coil should cover only the volume of interest. This gives an optimal signal-to-noise ratio (SNR). To achieve this there are 9various types of RF coils with trade-offs in terms of coverage and sensitivity. Head coil being smaller in size provides better SNR. Body coil is integrated into the scanner bore and is not seen by the patient. Both these coils act as transceivers, i.e. they transmit and receive. Surface coils are used for imaging anatomy near to the coil. They are simple loop designs and have excellent SNR close to the coil but the sensitivity drops off rapidly with distance from the coil. These are only used as receivers, the body coil acting as the transmitter. Quadrature or circularly-polarized coils comprise of two coils 90° apart to improve SNR by a factor of 2½.
Advanced applications include diffusion imaging, perfusion imaging, functional MRI, spectroscopy, interventional MRI.
Possible adverse effects of MRI can be due to static magnetic field, gradients, RF heating, noise and claustrophobia.
Caution needs to be exercised while selecting patients for MRI. Patients with pacemakers, metallic implants, aneurysm clips should be excluded. Metallic objects should not be taken near the magnet as they can be injurious to the patient, personnel and equipment.
 
Special Sequences
 
Short Tau Inversion Recovery (STIR) Sequence
It is heavily T2 weighted imaging, as a result the fluid and edema return high signal intensity and it annuls out the signal from fat. The resultant images show the areas of pathology clearly. The sequence is useful in musculoskeletal imaging as it annuls the signal from normal bone marrow.10
 
Fluid Attenuated Inversion Recovery (FLAIR)
This is an inversion-recovery pulse sequence that suppresses or annuls out the signal from water. The sequence is useful to show subtle lesions in the brain and spinal cord as it annuls the signal from CSF. It is useful to bring out the periventricular hyperintense lesions, e.g. in multiple sclerosis.
 
Gradient Echo Sequence
This sequence reduces the scan times. This is achieved by giving a shorter RF pulse leading to a lesser amount of disruption to the magnetic vectors. The sequence is useful in identifying calcification and blood degradation products.
 
Diffusion-weighted Imaging
‘Diffusion’ portrays the movement of molecules due to random motion. It enables to distinguish between rapid diffusion of protons (unrestricted diffusion) and slow diffusion of protons (restricted diffusion). GRE pulse sequence has been devised to image the diffusion of water through tissues. It is a sensitive way of detecting acute brain infarcts, where diffusion is reduced or restricted.
 
MR Angiography
The most common MR angiographic techniques are time-of-flight imaging and phase contrast. In these sequences, multiple RF 11pulses are applied with short TRs saturate the spins in stationary tissues. This results in suppression of the signal from stationary tissues in the imaging slab. In-flowing blood is unaffected by the repetitive RF pulses, as a result, as it enters the imaging slab, its signal is not suppressed and appears hyperintense compared with that of stationary tissue. Time-of-flight imaging may be 2D, with section-by-section acquisition, or 3D, with acquisition of a larger volume. MRA can also be performed with intravenous gadolinium when in the vascular phase of enhancement.