MRI Made Easy ® (for Beginners) Govind B Chavhan
Page numbers followed by f refer to figure and t refer to table.
coronal images of 44f
postcontrast axial image of 89f
vibe image of 112f
Abscess 147, 148f, 173
Acquired immunodeficiency syndrome 173
Air 98
Alanine 169
Alexander disease 172
Alzheimer's dementia 172
Alzheimer's disease 166f
American College of Radiology 79, 92, 93
Amino acids 169
Amyloidosis 208
Analog-to-digital converter 76
Aneurysm 83
Angiomyolipoma 44f
ascending 127
descending 127
Apparent diffusion coefficient 141144, 146148, 150, 157
Arachnoid cyst 147
Arch of aorta 206
Array spatial sensitivity encoding technique 45
Arrhythmogenic right ventricular dysplasia 202, 208f
Arterial spin labeling technique 152, 160, 161f, 214
Arthritis, enthesitis-related 190
Artificial sphincters 84
Atrial septal defect 206
Balanced steady state free precession 62, 212
Bandwidth 18
Bankart's lesion 192
Basal ganglion calcifications, bilateral 65f
Bile duct, cystic diseases of 183
Black blood imaging 126, 127f
Bladder imaging 118f
Blood 100
oxygen level-dependent imaging 174, 175
pool agent 91, 93
Body imaging 95
magnetic resonance imaging techniques 180, 193
Bone 100
marrow imaging 57
axial localizer image of 74f
contrast-enhanced T1-weighted sagittal image of 95f
diffusion weighted axial image of 103f
gradient hemo axial image of 65f, 107f
axial image of 73f, 103f
postcontrast axial image of 104f, 106f
sagittal image of 101f
axial image of 76f, 109f
sagittal image of 71f, 78f
tumor 95f, 170
magnetic resonance perfusion in 157
imaging 127, 128f
technique 203
Bronchial atresia 212f
Calcifications 98
Canavan's disease 168f, 172
Cardiac implantable electronic devices 84
Cardiac magnetic resonance 202
imaging 202
clinical applications of 206
Cardiac mass 210f
Cardiac motion, electrocardiogram gating for 69
Cardiomyopathy 207
hypertrophic 207
restrictive 207
Cardiovascular imaging 96
delayed gadolinium-enhanced magnetic resonance imaging of 201
sensitive sequences 65
Cell membrane 140f
Central nervous system 148
infection 94, 103
neoplasm 94
vasculitis 159
Cerebellopontine 64
angle lesions 105
artery, anterior 146
flow 156
volume 104f, 156, 158f, 160f
hemisphere 160f
Cerebrospinal fluid 14, 15, 39, 47, 59, 60, 64, 117, 117f, 124, 143, 147, 174, 178
flow study 176
Cerebrovascular reactivity 174, 178
Cervical spine
cerebrospinal fluid 72
medic axial image of 66f
Chemical shift
imaging 164
related artifacts 70
Cholangitis, primary sclerosing 184, 185f
Choledochal cyst 183f
Choledocholithiasis 184
Chondroblastoma 115f
Circle of Willis 126
Colloid 99, 99f
Common bile duct 183, 184
Common hepatic duct 183f
Computed tomography 213
Computer system 21
Congenital anomalies 183
Connective tissue disorders 208
lung perfusion 214
magnetic resonance angiography 48, 134, 135f
Coronary artery assessment 209
Corpus callosum 107f, 150f
Crohn's disease 186f
Cryostat 24
Dark-blood technique 203
Dental devices and materials 84
Diamagnetism 21
Diffuse axonal injury 107
Diffusion 139
kurtosis imaging 145
tensor imaging 139, 149, 150f
evaluation of 149
weighted imaging 139, 140, 146148
clinical applications of 145
Dixon method 44, 44f
Double spin-echo sequence 32
Down syndrome 172
Dowson's finger 106, 107f
Dual spin-echo sequence 32
Duodenal papilla 183f
Dynamic contrast-enhanced magnetic resonance lymphangiography 190
Dynamic susceptibility contrast 88, 179
magnetic resonance perfusion, technique of 154
perfusion 152
Echo planar imaging 31, 37, 40, 58, 154
Electrocardiograph-gated fast spin-echo magnetic resonance angiography 133
Electromagnet 23
Encephalopathy, hypoxic-ischemic 108
Endometrium 119f
Endoscopic retrograde cholangiography 182
Epidermoid 147
Epilepsy 105, 105f, 172
European Medicines Agency 92
Extracellular volume 198
Extrahepatic biliary tree 184f
Faraday cage 26
Fast field echo 40, 65, 67, 115, 116
Fast low angle shot 53, 55
Fast spin-echo 113, 180, 185, 187
sequence 32
Fat 98
quantification 197f, 198f
saturation 42f
signal fraction 196
suppression 41
techniques 42t
infiltration 53
liver 197
Femur, medial condyle of 60f
Ferromagnetism 22
Fever, rheumatic 207
direction map 150
tractography map 150
Fibrous tissue 100, 100f
Field of view 18, 20, 42, 69, 72, 80, 124, 131
Flight magnetic resonance angiography, time of 128, 130f
Flip angle 18
Fluid-attenuated inversion recovery 39, 51, 58, 61, 107f, 148
Foley catheters 82
Four-chamber view 205
Fourier decomposition 214
Fractional anisotropy 149
Free water protons 47
selective fat suppression 41
wrap 70
Functional magnetic resonance imaging 174
Gadolinium 88
based contrast agent 85, 88, 89, 90t, 91, 93, 115, 131, 135, 152, 154, 201, 210
injection 102
chelates 89
deposition 92
Gallbladder 183
Gamma-amino-butyric acid 165
Glenoid labrum ovoid mass 192
Global shimming 165
Glutamate 166f
Glutamine 166f
Glycosaminoglycan 200
field 7, 80
moment rephasing 47, 128
strength 26
Gradient-echo 30, 5355, 113, 115, 127, 182, 186, 208
sequence 13, 30, 34, 34f, 73
sequences, types of 34
Great arteries, transposition of 206
Half-Fourier acquisition single-shot turbo spin-echo 59
Heart disease
acquired 202
congenital 202, 206, 206f
Hematoma, subacute 148
Hemochromatosis 208
Hemorrhage 65f
Hemostatic clips 83
Hepatic encephalopathy 173
Hepatic fat quantification 196
Hepatic veins 208
Hepatobiliary agents 91
High-grade glioma 158f, 170f
Hip joint arthrogram 192
Horizontal long-axis 204
Human immunodeficiency virus 173
Hybrid chess and inversion recovery techniques 41
Hydrogen ions 97
Hyperpolarized gas ventilation 215f
Implantable cardioverter defibrillator 82
Infection 103
Inferior vena cava 133, 133f, 208
Inflow enhancement 128
Internal carotid artery 146
International electrotechnical commission 80
International Society of Magnetic Resonance in Medicine 92
Intrahepatic bile ducts 183f, 185f
Intravoxel incoherent motion model 143, 144
Inversion recovery sequence 31, 36, 36f
types of 37
overload imaging 195, 196f
oxide 93
Ischemic central nervous system diseases 94
Joint imaging 116
Key hole imaging 49
K-space 16, 16f
filling methods 17f
Langerhans cell histiocytosis 189
Left atrium 63, 205, 207, 208
Left renal pelvicalyceal system 62
Left ventricle 59, 63, 205, 208
Leigh's disease 172, 172f
Leukoencephalopathy, progressive multifocal 173
Ligamentum flavum, hypertrophy of 110f
acquisition 40
focal nodular hyperplasia 54f
Longitudinal magnetization 4, 6, 11, 14
Longitudinal relaxation time 9, 10f, 11, 15
Look-Locker inversion recovery sequence, modified 198
Low-grade glioma 158f
imaging sequences 211
magnetic resonance imaging
clinical applications of 215
techniques 211
parenchyma 213f
perfusion 214f
magnetic resonance imaging 214
ventilation magnetic resonance imaging 214
Lymphoma 148f, 173, 189
Macromolecules 140
Magnetic field 23f
homogeneity 25, 163
strength 22
Magnetic resonance 6, 21, 31, 32, 36, 63, 72, 83, 102, 103, 135, 154
angiography 49, 125f, 126, 134, 146
reformation of 129
types of 126
arthrogram 190
arthrography 190
cholangiopancreatography 94, 117, 125, 180, 181, 185
components 79
contrast enhancement, mechanism of 88
elastography 193
enterography 185
imaging 3, 9, 30, 41, 68, 79, 81f, 87, 94, 97, 113, 123, 126, 148, 162, 174, 175, 180, 188, 189, 193, 211, 213, 214
acquisition 30f
application of 211
artifacts 68
basics of 1
contrast media 87
formation 6f
perfusion 152
safety 79, 81
techniques 121
instrumentation 21
lymphangiography 190, 191f
perfusion 152
imaging methods 153t
signal 5
spectroscopic imaging 164, 171
spectroscopy 70, 162, 163, 170, 196
acquisition 165
clinical uses of 170
non-neurological applications of 173
urography 186, 187
Magnetic susceptibility
artifact 73
effect 75f
Magnetism 21
prepared rapid gradient echo 56
transfer 41, 47, 48f
diagram 48f
pulse 48
Main pancreatic duct 183f
Main pulmonary artery 127
Matrix 17
Maximum intensity projection 62, 135, 181, 185, 187
Mean transit time 156
Meconium 99
Melanin 99, 99f
Metabolic disorder 172, 172f
Metastatic diseases 189
Middle cerebral artery 146
Minimum intensity projection 176
Mitochondrial encephalopathy lactic acidosis 172
Mitral stenosis 207f
Molecules, crystalline lattice of 9
Moyamoya disease 159, 160f
Mucin 100
Multi echo data image combination 65, 66
Multifocal marrow edema 189f
Multivoxel spectroscopy technique 164
Muscle 97
tissue 97
Musculoskeletal neoplasms 114
Myelin 98
Myocardial perfusion 209
Myocardial perfusion study 209
Myocardium 63f
Myometrium 119f
N-acetylaspartate 162, 167, 168
Navigator technique 46
Neonatal hypoxia 172
Neoplasm 208
Neoplastic lesions 184
Nephrogenic systemic fibrosis 91, 93, 126
Net magnetization vector 9, 38, 39, 77
Neural foramen 66f
Neurofibromatosis 19f
Neuroimaging magnetic resonance imaging techniques 174
Non-contrast magnetic resonance angiography techniques 127
Non-gadolinium magnetic resonance contrast agents 93
Normal pressure hydrocephalus 177, 178f
Ocular implants 84
Optic glioma 19
Oral contrast agents 94
Orthopedic implants 84
Osteoarthritis 60f, 66f
chronic nonbacterial 189f
chronic recurrent multifocal 189, 189f
Osteophytes 66f
Otologic implants 84
Pacemakers 84
Pancreas divisum 183f
Pancreatic duct 62f, 183, 185f
Pancreatitis, chronic 184
Parallel imaging artifacts 78
Paramagnetic agents 87
Pediatric brain, magnetic resonance imaging in 108
Pelvic imaging 117
Penile implants 84
Pericardial disease 210
Pericardial masses 209
Pericarditis, constrictive 207, 208, 208f
flow quantification 132
magnetic resonance angiography 130
Positive relaxation agents 87
Positron emission tomography 149, 161
Postcardiac surgery 208
Posterior cerebral artery 103
Posterior circulation stroke 103f
Posterior cranial fossa 64f
Post-excitation refocused steady-state sequences 35
Pre-excitation refocused steady-state sequences 35
Prosthetic heart valves 84
Protein 99
Proton-density 9, 32, 51
fat fraction method 197, 198f
image 15, 15f
Pulmonary artery 59
Pulse sequences 30
Quadrature coil 27
Quench 25
Radiation necrosis 171f
Radiofrequency 3, 7, 24, 3134, 36, 38, 77, 123, 124, 127
coils 27
excitation 9
magnetic field 80
pulse 9, 30
Radiotherapy 208
Renal failure 208
Respiratory compensation techniques 46, 46f, 69
Right atrium 63, 205208
Right internal auditory canal 106
Right pulmonary artery stenosis 206f
Right ventricle 63, 205, 207, 208
Right ventricular outflow tract 205, 206f
Sacroiliitis 114f
Salvageable tissue, penumbra of 157
Sarcoidosis 208
Saturation band 49, 69
Scleroderma 208
Sclerosis, multiple 107f, 172
Secretin magnetic resonance cholangiopancreatography 182
Short tau inversion recovery 38, 39, 4143, 57, 58, 60, 114, 188, 189
images 112
sequence 37
Short-axis plane 204
Shoulder, magnetic resonance arthrogram of 191f
Signal intensity pattern 101, 111, 143t
Signal-to-noise ratio 18, 19, 19f, 20, 42, 45, 58f, 125, 161
Single-photon emission computed tomography 216
Single-shot fast spin-echo 57, 59, 63
sequence 33, 33f
Spherical volume, diameter of 25
Spine 95
imaging 108
T1-weighted axial image of 110f
T2-weighted sagittal image of 109f
Spin-echo 30
sequence 12f, 13, 30, 31, 31f
modifications of 31
Splenic parenchyma 196f
Steady-state free precession 35, 40, 113, 124, 127, 134, 180, 186
Stejskal-Tanner sequence 140f
Stimulated echo acquisition method 163
Stroke 145, 146f, 172
imaging 102
magnetic resonance perfusion in 155
Superior vena cava 59, 127, 208
Superparamagnetic iron oxides 88
Susceptibility-weighted imaging 175, 176, 176f
Swan-Ganz catheters 82
Synthetic magnetic resonance imaging 51
high resolution isotropic volume examination 40, 52, 100, 182, 186
three-dimensional gradient echo 52
fast spin-echo sequences 61
image 14
three-dimensional sequences 60
Tesla magnetic resonance imaging 123
Tetralogy of Fallot 206f
Thalassemia 196
Tibia, medial condyle of 60f
Time-resolved contrast-enhanced magnetic resonance angiography 136, 137
Tissue suppression 37
Toxoplasma 173
Transverse magnetization 5, 5f, 6, 11, 11f, 15, 27
formation of 10f
vector, precession of 6
Transverse relaxation time 10, 11, 11f, 12, 15
Trauma 106
Truncation artifact 70, 73f
Tuberculomas 95f
Tumefactive demyelinating lesion 157
Tumor vascularity 104f
Turbo spin-echo 53, 58, 59, 63
Ultrasonography 184
Urinary bladder 118f
Uterine imaging 119f
Valvular heart disease 206
Vascular access ports 85
Vasculitis 125f
Ventricular function 209
Ventricular septal defect 206
Vertebral arteries 130f
Vertical long-axis plane 204
Volume coil 28
Volumetric interpolated breath-hold examination 40, 53, 54, 56, 112
Water 98
excitation 45
selective excitation 45
White matter diseases 172
Whole-body magnetic resonance imaging 188
X-axis 7
Y-axis 7
Z-axis 4, 7
Zipper artifact 75, 76f
Chapter Notes

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Basic Principles1

Four basic steps are involved in acquiring a magnetic resonance (MR) image:
  1. Placing the patient in the magnet
  2. Sending radiofrequency (RF) pulse by a coil
  3. Receiving signals from the patient by a coil
  4. Transformation of signals into the image by complex processing in the computers.
Now, let us understand these steps at the molecular level. Present magnetic resonance imaging (MRI) is based on proton imaging. Proton is a positively charged particle in the nucleus of every atom. Since hydrogen ion (H+) has only one particle, i.e., proton, it is equivalent to a proton. Most of the signal on clinical MR images comes from water molecules that are mostly composed of hydrogen.
How do protons help in MRI?
Protons are positively charged and have rotatory movement called spin. Any moving charge generates current. Every current has a small magnetic field around it. So every spinning proton has a small magnetic field around it, also called magnetic dipole moment.
Normally, the protons in human body (outside the magnetic field) move randomly in any direction. When external magnetic field is applied, i.e., patient is placed in the magnet, these randomly moving protons align (i.e., their magnetic moment align) in the direction of external magnetic field. Some of them align parallel and others antiparallel to the external magnetic field. When a proton aligns along external magnetic field, not only it rotates around itself (called spin) but also its axis of rotation moves forming a “cone”. This movement of the axis of rotation of a proton is called as precession (Fig. 1).
The number of precessions of a proton per second is termed precession frequency. It is measured in Hertz (Hz). Precession frequency is directly proportional to strength of external magnetic field. Stronger the external magnetic field, higher is the precession frequency. This relationship is expressed by Larmor's equation:
f0 = γ B0
Where, f0 = precession frequency in Hz
B0 = strength of external magnetic field in Tesla
γ = Gyromagnetic ratio, which is specific to each nucleus4
zoom view
Fig. 1: Spin versus precession: Spin is rotation of a proton around its own axis while precession is rotation of the axis itself under the influence of external magnetic field such that it forms a “cone”.
Precession frequency of the hydrogen proton at 1, 1.5, and 3 Tesla is roughly 42, 64, and 128 MHz respectively.
Let us go one step further and understand what happens when protons align under the influence of external magnetic field. For the orientation in space consider X, Y, and Z coordinate system. External magnetic field is directed along the Z-axis. Conventionally, the Z-axis is the long axis of the patient as well as bore of the magnet. Protons align parallel and antiparallel to external magnetic field, i.e., along positive and negative sides of the Z-axis respectively. Protons that are diagonally opposite on negative and positive sides cancel each other's forces. However, there are always more protons spinning on the positive side of Z-axis as it takes less energy to be on positive side of Z-axis. So, after canceling each other's forces, there are a few protons on positive side that retain their forces. Forces of these protons add up together to form net magnetization represented by a vector along the Z-axis. This is called as longitudinal magnetization (Figs. 2A to C).
zoom view
Figs. 2A to C: Longitudinal magnetization: (A) More protons precess along positive side of Z-axis; (B) Protons that are diagonally opposite to each other cancel out each other's forces. A few protons with uncanceled forces remain along positive side; (C) Forces of these proton add up to form longitudinal magnetization, represented as a vector along positive side of Z-axis.
5Longitudinal magnetization thus formed along the external magnetic field can not be measured directly. It can be measured when it is tipped away from Z-axis and precesses at Larmor frequency.
As discussed in the previous paragraph when patient is placed in the magnet, longitudinal magnetization is formed along the Z-axis. The next step is to send RF pulses. The precessing protons pick up some energy from the RF pulse. Some of these protons go to higher energy level and start precessing antiparallel (along negative side of the Z-axis). The imbalance results in tilting of the magnetization into the transverse (X-Y) plane. This is called as transverse magnetization (Figs. 3A to C). In short, RF pulse tilts the magnetization into the transverse plane.
The precession frequency of protons should be same as RF pulse frequency for the exchange of energy to occur between protons and RF pulse. When RF pulse and protons have the same frequency protons can pick up some energy from the RF pulse. This phenomenon is called as “resonance”—the R of MRI.
Radiofrequency pulse not only causes protons to go to higher energy level but also makes them precess in phase or synchronously.
Transverse magnetization vector has a precession frequency. It constantly precesses at Larmor frequency in the transverse plane and induces electric current while doing so. The receiver RF coil receives this current as the MR signal (Fig. 4). The strength of the signal is proportional to the magnitude of the transverse magnetization. MR signals are transformed into MR image by computers using mathematical methods such as Fourier transformation.
zoom view
Figs. 3A to C: Transverse magnetization: (A) Longitudinal magnetization (LM) is along positive side of Z-axis; (B) 90-degree radiofrequency (RF) pulse creates imbalance of proton forces resulting in decrease in LM and increase in magnetization in transverse plane; (C) Magnetization vector is flipped in transverse plane, called as transverse magnetization.
zoom view
Fig. 4: MR signal. Magnetization tilted in the transverse plane precesses at Larmor frequency and induces current in the receiver coil while do so. This current in the receiver coil is the MR signal. The TM vector starts reducing in its magnitude immediately after its formation because of dephasing of protons. The LM starts gradually increasing in its magnitude. The net magnetization vector (NMV) formed by addition of these two (LM and TM) vectors gradually moves from transverse X-Y plane toward vertical Z-axis. As long as the NMV is away from the Z-axis, there is some component of the magnetization in the transverse plane inducing current in the receiver coil.(LM: longitudinal magnetization; MR: magnetic resonance; TM: transverse magnetization)
zoom view
Fig. 5: Major steps in magnetic resonance (MR) image formation.
Basic four steps of MR imaging include (Fig. 5):
  1. Patient is placed in the magnet: All randomly moving protons in patent's body align and precess along the external magnetic field. Longitudinal magnetization is formed along the Z-axis.
  2. Radiofrequency pulses sent: Precessing protons pick up energy from RF pulse to go to higher energy level and precess in phase with each other. This results in reduction in longitudinal magnetization and formation of transverse magnetization in X-Y plane.
  3. Precession of transverse magnetization vector: The transverse magnetization vector precession at Larmor frequency induces current in receiver RF coil. This is MR signal.
  4. Image formation: MR signal received by the coil is transformed into image by complex mathematical process such as Fourier transformation by computers.
Three more magnetic fields are superimposed on the main magnetic field along X, Y, and Z axes to localize from where in the body signals are coming. The strength of these magnetic fields varies from one end to other hence these fields are called “gradient fields” or simply “gradients”. The gradient fields are produced by coils called as gradient coils.
The three gradients are:
  1. Slice selection gradient
  2. Phase encoding gradient
  3. Frequency encoding (read out) gradient
Slice Selection Gradient
Slice selection gradient has gradually increasing magnetic field strength from one end to another (Fig. 6). It determines the slice position. Slice thickness is determined by the bandwidth of RF pulse. Bandwidth is the range of frequencies. Wider the bandwidth thicker is the slice.
Phase Encoding and Frequency Encoding Gradients
These gradients are used to localize the point in a slice from where the signal is coming. They are applied perpendicular to each other and perpendicular to the slice selection gradient (Fig. 7).
Typically, for transverse or axial sections following are axes and gradients applied even though X and Y axes can be varied:
  • Z-axis: Slice selection gradient
  • Y-axis: Frequency encoding gradient
  • X-axis: Phase encoding gradient
In a usual sequence, slice selection gradient is turned on at the time of RF pulse. Phase encoding gradient is turned on for a short time after slice selection gradient. Frequency encoding or readout gradient is turned on in the end at the time of signal reception.
zoom view
Fig. 6: Slice selection gradient. Magnetic field strength varies from one end to other. The magnetic field variation is in the range of few Gauss.(RF: radiofrequency)
zoom view
Fig. 7: Frequency and phase encoding gradients for a typical axial magnetic resonance (MR) image.
Information from all three axes is sent to computers to get the particular point in that slice from which the signal is coming.
Why proton only?
Other substances can also be utilized for MR imaging. The requirements are that their nuclei should have spin and should have odd number of protons within them. Hence theoretically 13C, 19F, 23Na, and 31P can be used for MR imaging.
Hydrogen atom has only one proton. Hence H+ ion is equivalent to a proton. Hydrogen ions are present in abundance in body water. H+ gives the best and most intense signal among all nuclei.