{
"id": "mri-physical-principles",
"modality": "MRI",
"category": "Image Production: Physical Principles",
"title": "MRI Physical Principles: Spins, Relaxation and Signal",
"minutes": 9,
"slides": [
{
"id": 1,
"diagram": "title",
"heading": "MRI Physical Principles",
"bullets": [
"Part of Image Production: 106 of 200 scored (53%)",
"Hydrogen, Larmor, resonance, T1, T2, T2*",
"Localization, k-space, hardware, artifacts"
],
"narration": "Welcome to MRI Physical Principles. This lesson is part of Image Production, the biggest category on the MRI boards at 106 of the 200 scored questions, which is 53 percent. Physical principles are the foundation for everything else in that category. We'll cover why we image hydrogen, the Larmor equation, resonance, T1 and T2 relaxation, T2 star, how gradients locate the signal and fill k-space, the hardware that makes it all work, and which way the classic artifacts run. Let's start with the proton.",
"svg": ""
},
{
"id": 2,
"diagram": "net-magnetization",
"heading": "Hydrogen and net magnetization",
"bullets": [
"Hydrogen: abundant in water and fat, strong moment",
"Slightly more protons align parallel (low energy)",
"That small excess forms net magnetization, M0",
"M0 and SNR grow with field strength"
],
"narration": "We image hydrogen because it is everywhere in water and fat and has a strong magnetic moment. Put a patient in the main field, B zero, and the protons line up either parallel, which is low energy, or antiparallel, which is high energy. Slightly more line up parallel. That tiny excess adds up to the net magnetization vector, called M zero, pointing along the main field. A stronger field makes a bigger excess, so M zero and the signal to noise ratio grow with field strength, roughly doubling from 1.5 to 3 tesla.",
"svg": ""
},
{
"id": 3,
"diagram": "larmor",
"heading": "The Larmor equation",
"bullets": [
"Frequency = gyromagnetic ratio x B0",
"Hydrogen: 42.58 MHz per tesla",
"1.5 T = 63.86 MHz; 3 T = 127.7 MHz",
"Frequency is directly proportional to field"
],
"narration": "The Larmor equation is the most tested formula in MRI physics. Precessional frequency equals the gyromagnetic ratio times the field strength. For hydrogen, the gyromagnetic ratio is 42.58 megahertz per tesla. So at 1.5 tesla, protons precess at 63.86 megahertz, and at 3 tesla, at about 127.7 megahertz. The trap answer puts 63.86 at 3 tesla, so watch for it. The relationship is directly proportional: double the field and you double the frequency. That is also why gradients can encode position by frequency.",
"svg": ""
},
{
"id": 4,
"diagram": "resonance",
"heading": "Resonance and the RF pulse",
"bullets": [
"RF at the Larmor frequency causes resonance",
"90 degree pulse tips M into the transverse plane",
"Protons precess in phase: phase coherence",
"180 degree pulse inverts or refocuses",
"FID: signal right after the pulse, decays by T2*"
],
"narration": "Resonance happens when we send a radiofrequency pulse at exactly the Larmor frequency, so the protons absorb its energy. A 90 degree pulse tips the net magnetization fully into the transverse plane. The pulse also brings the protons into phase, called phase coherence, and that spinning in-phase magnetization induces signal in the receive coil. A 180 degree pulse either inverts the magnetization or refocuses it to form a spin echo. The signal you get right after the pulse is the free induction decay, or F I D, and it fades by T2 star.",
"svg": ""
},
{
"id": 5,
"diagram": "t1-recovery",
"heading": "T1: spin-lattice recovery",
"bullets": [
"T1 is spin-lattice, longitudinal recovery",
"T1 = time to regrow to 63% of Mz",
"Fat has short T1: bright on T1 images",
"Water and CSF have long T1: dark on T1",
"T1 lengthens at higher field strength"
],
"narration": "After the pulse, two things happen at once. The first is T1 recovery, also called spin-lattice relaxation, as protons hand energy back to their surroundings and longitudinal magnetization regrows. The T1 time is when it has recovered to 63 percent of its maximum. Fat has a short T1, so it recovers fast and is bright on T1-weighted images. Water and cerebrospinal fluid have a long T1, so they are dark. And T1 values get longer at higher field strength, which is why inversion times and repetition times often grow at 3 tesla.",
"svg": ""
},
{
"id": 6,
"diagram": "t2-decay",
"heading": "T2 and T2* decay",
"bullets": [
"T2 is spin-spin, transverse decay",
"T2 = time to fall to 37% of Mxy",
"T2* adds field inhomogeneity, so it decays faster",
"Always T2* shorter than T2; T2 no longer than T1",
"Spin echo's 180 pulse removes T2* effects"
],
"narration": "The second process is T2 decay, also called spin-spin relaxation. Protons interact, drift out of phase, and transverse magnetization fades. The T2 time is when it has dropped to 37 percent of its starting value. Remember, T1 is 63 percent up and T2 is 37 percent left. T2 star adds field inhomogeneity and susceptibility, so it decays faster. T2 star is always shorter than T2, and T2 is never longer than T1. In a spin echo, the 180 degree pulse cancels those fixed inhomogeneities, so the contrast is true T2.",
"svg": ""
},
{
"id": 7,
"diagram": "k-space",
"heading": "Localization and k-space",
"bullets": [
"Gradients make frequency vary with position",
"Thinner slice: steeper gradient or narrower transmit BW",
"Phase encoding steps directly set 2D scan time",
"Frequency encoding happens during echo readout",
"K-space center = contrast; periphery = detail"
],
"narration": "Gradients add a small, sloped field so frequency varies with position. To select a slice, the radiofrequency pulse is sent while a gradient is on, and a thinner slice comes from a steeper gradient or a narrower transmit bandwidth. Phase encoding happens before readout, one line per repetition time in conventional spin echo, so the number of phase steps directly sets 2D scan time. Frequency encoding happens while the echo is read. All of it fills k-space, the raw data. The center of k-space holds contrast and signal, and the edges hold fine detail.",
"svg": ""
},
{
"id": 8,
"diagram": "magnet-hardware",
"heading": "Magnets, shims, coils",
"bullets": [
"Superconducting: niobium-titanium in liquid helium",
"Shimming improves homogeneity, measured in ppm",
"Surface coils: high SNR that falls with depth",
"Slew rate = gradient amplitude / rise time",
"Faraday's law: signal induces current in the coil"
],
"narration": "Clinical magnets are mostly superconducting, with niobium-titanium wire bathed in liquid helium, and the field is always on. Inside it sit the shim coils, which improve field uniformity, measured in parts per million, critical for fat saturation and echo planar imaging. Then come the x, y and z gradient coils, and the radiofrequency body coil. Surface coils give high signal to noise close to the coil that drops off with depth. Gradient slew rate equals amplitude divided by rise time. And Faraday's law explains reception: the changing magnetization induces a current in the receive coil.",
"svg": ""
},
{
"id": 9,
"diagram": "artifact-directions",
"heading": "Which way artifacts run",
"bullets": [
"Aliasing, motion ghosts, zipper: phase direction",
"Fix aliasing: bigger FOV or phase oversampling",
"Chemical shift: frequency direction (phase in EPI)",
"Fat-water shift about 3.5 ppm, about 220 Hz at 1.5 T",
"Susceptibility worst on gradient echo and EPI"
],
"narration": "Artifacts come straight from these physics. Aliasing, or wrap, motion ghosts, and the zipper from radiofrequency leaks all run in the phase direction. Fix aliasing with a larger field of view, phase oversampling, or swapping phase and frequency. Chemical shift runs in the frequency direction, except in echo planar imaging, where it shows in phase. Fat and water precess about 3.5 parts per million apart, about 220 hertz at 1.5 tesla, and the shift worsens with higher field and narrower bandwidth. Susceptibility artifact is worst on gradient echo and echo planar sequences, because there is no 180 degree refocusing pulse.",
"svg": ""
},
{
"id": 10,
"diagram": "summary",
"heading": "Lock these in",
"bullets": [
"Hydrogen Larmor constant: 42.58 MHz per tesla",
"63.86 MHz at 1.5 T; 127.7 MHz at 3 T",
"T1 = 63% recovery; T2 = decay to 37%",
"T2* is shorter than T2, and T2 is no longer than T1",
"K-space center = contrast; edges = detail"
],
"narration": "Let's lock it in. The Larmor constant for hydrogen is 42.58 megahertz per tesla. That gives 63.86 megahertz at 1.5 tesla and about 127.7 at 3 tesla. T1 is the time to 63 percent recovery, and T2 is the time to decay to 37 percent. T2 star is always shorter than T2, and T2 is never longer than T1. And the center of k-space sets contrast while the edges set detail. Now hit Practice this chapter and test yourself.",
"svg": ""
}
]
}