T1 and T2 Relaxation in Tissues

Intrinsic Tissue Characteristics

• Tissues have T1 & T2 Relaxations, and Proton Density.

T1 Relaxation

• How long it takes for 63% of the NMV to recover/align with B₀ (Spin Lattice/ T1 Recovery)
• The NMV is knocked down from being aligned with B₀ into the transverse plane (Z) with an RF pulse
• Since they don't want to stay in the T.P., they stand up and realign themselves with B₀
• This process is called T1'ing
• Tissues release energy (heat) with the excitation pulse in T1
• Fat has a short T1 and T2
• Water has a long T1 and T2
• T1 Relaxation Curve: REGROWTH process

T2 Relaxation

• Time for NMV in the transverse plane to decay to 37% of its original value
• Also known as Spin-Spin Relaxation, Spin-Spin Interaction, TRUE T2, or Loss of Phase Coherence
• T2 Relaxation Curve: DECAY process, so the signal is generated less

Contrast

• Contrast is the distance between the curves in a singular picture
• As the lines start getting closer, the contrast decreases.

Proton Density (PD)

• A tissue characteristic
• Also named Hydrogen Density or Spin Density
• Some tissues have a high proton density, like water
• Some tissues have a low proton density, like fat
• A PD curve does NOT exist
• The body sends water to an injury

Image Contrast/Weightings

• The 3 major weightings are T1, T2, & PD
• TR controls T1 contrast
• TE controls T2 contrast

Time of Repetition (TR)

• Time in milliseconds from the first 90-degree RF pulse to the next 90 RF pulse that excites the same slice
• TR questions are usually aimed at T1 answers

Long TRs vs. Short TRs

• Long TRs mean both Long & Short relaxing tissue (CSF or EDEMA) have been knocked down, and given time to stand up
• Both will give a TE signal, so both will be bright
• MRI signals are mixed of both long and short relaxing protons, depending on the TR length
• Short TRs allow ONLY short relaxing tissues to be knocked down and contribute signal @TE.
• Short T1 tissues (Fat, Gad, Proteins) will be bright, long relaxing tissues (Water, CSF, Urine) will be dark
• Tissue needs to first T1 relax before going to the next TR
• TR controls T1 contrast
• If TR shortens, T1 contrast increases, and SNR decreases
• If TR increases, T1 contrast lessens, and SNR increases
• Short TRs in a Spin Echo sequence will create more T1 weighted images
• Long TRs in a Spin Echo sequence will create less T1 weighted images

Time of Echo (TE)

• Time from the center of the excitation pulse to the center of the echo
• TE is when the coil turns on to receive a signal from vectors in the X/Y plane
• As TE lengthens, there is more Long T2 signal and less Short T2 signal.
• Short TEs in a Spin Echo sequence will create less T2 weighted images
• Long TEs in a Spin Echo sequence will create more T2 weighted images
• Another name for TE is 2TAU
• If there is more fat in the X/Y plane @TE, fat is bright
• If there is more water in the X/Y plane @TE, water is bright

T2 Relaxation Specifics

• T2 Relaxation involves getting knocked down by RF and fanning out over time, also known as Loss of Phase Coherence
• The longer the TE, the more spreading out occurs and the brighter fluids will be
• The longer the TE, this allows fat to get out of the way, allowing water to have majority of signal at TE
• First, they're all together IN PHASE, there's no T2 yet
• Then protons start to DEPHASE

Echo Train Length (ETL)

• More than one 180 refocusing Pulse for each slice per TR
• Results in multiple echoes for each slice and multiple lines of K-Space filled per TR, which speeds up scan time

Example

• Slice #1 gets a 90 pulse followed by multiple 180 RF pulses: 90 180 echo, 180 echo, 180 echo
• Increasing the ETL cuts scan time in Fast Spin Echo/Turbo Spin Echo, but decreases SNR and can increase SAR

Mechanism

• As ETL increases, the echoes get further away from the 90 and become more T2 weighted
• Fluid gets brighter and SNR decreases

Inversion Recovery (TI)

• Tells which tissue will be suppressed, either fat or CSF
• TI is the time from the center of the 180 Inversion Pulse to the center off the 90 excitation pulse
• Short TI suppresses Short T1 tissue
• Long TI suppresses Long T1 tissue
• If a tissue's vector is at the Null Point at the end of TI, it will be nulled/suppressed
• Null Point = X/Y Plane

Flip Angle

• Also known as the Nutation Angle
• Gradient Echo has a TR and a TE
• TR greatly affects image contrast/weighting in Spin Echo, but TR plays less of a role in GRE
• The flip angle has a big effect on image contrast in GREs
• The flip angle is how far we push (knock down) the Longitudinal NMV into the X/Y plane
• Think of F/A as adding T1 contrast
• F/A in GREs is usually <90 degrees, rarely reaching 90
• A large F/A of 60-70 creates more T1 contrast
• A Small/Shallow F/A of 10-20 creates less T1 contrast, which allows T2 contrast to be seen easier
• GRE is weighted with a combo of F/A and TE to provide T1 and T2* weighting
• TR doesn't do much in GRE, but gives more slices and more saturation
• TR is replaced by F/A for T1 contrast
• Long TEs = Bright Fluid
• Short TEs = Dark Fluid
• F/A in Spin Echo (SE) is 90 degrees
• It means the NMV is fully in the X/Y plane and has more T1 to do to relax back to B₀
• This is why there is a T1 relaxation difference between tissues
• F/A in SE doesn't change much
• F/A in GRE uses many types of F/A to adjust Image Contrast
• For more T2 (brighter fluid) in GRE, use longer TE (more T2) and a shallow F/A (less T1)
• For more T1 (dark fluid) in GRE, use shorter TE (less T2) and a larger F/A (more T1)

Image Weighting

• A way to term if an image has more of one contrast: T1, T2, or PD than the other two
• One of them will outweigh the other two
• Short TR & Short TE = T1 weighted images, with bright white matter
• Gray matter is not as bright, tissues are either bright or dark due to different T1 relaxations
• When TR is short, long T1 tissues don't have time to T1 relax and won't give much signal at TE, so TR filters out long T1 relaxation tissues and fluids are dark
• Long TR & Long TE = T2 weighted images, which are next to bright CSF
• Long TE lets Short T2 tissues relax out of the X/Y plane, allowing Long T2 tissues behind, which gives more signal than the short T2 relaxing tissues
• TE filters out Short T2 relaxation tissues, so Fat/Gad/Proteins are not bright

Field of View (FOV)

• Size of slices in Phase & Frequency directions, NOT in the Slice direction
• FOV contributes to SNR & I.Q. but does not affect Image Contrast
• FOV / Scan Matrix = Pixel Size
• FOV comes from Frequency Encoding Gradient (FEG), and its amplitude controls the size of FOV
• A Lower FEG covers a larger area/larger FOV
• A Steep FEG results in Smaller FOV, and a Shallow FED may cause shorter min. TE
• A larger Pixel = LOW RES, results in a larger SNR
• Larger Pixels/Voxels = more Partial Volume, more anatomical coverage, and less chance of Wrap Artifact
• Partial Volume is a signal intensity of a particular pixel and is the average of all the signal intensities of all the tissues inside
• Ex: If a tissue has bright & dark tissue, the pixel would look gray

Slice Thickness

• Two Methods: SSG (Slice Select Gradient) & (Transmitter B/W)
• SSG is most common and time efficient
• A Steeper SSG = Thinner Slice
• A Shallow SSG = Thicker Slice, and for this, thicker TRANS B/W is needed
• RF pulses are oscillating wave forms (sine wave) with a connection between RF Profile in frequency and the number of times it "crosses zero" for a given sync pulse
• More crossings = more frequencies which increases time to apply said RF pulse
• To save time, the amplitude of SSG is altered to adjust the slice thickness only, and NOT the Trans B/W
• Bandwidth (B/W) is stated as "± MHz"
• Ex: 63± 5 MHz
• Thicker slices increase SNR, while thinner slices increase Resolution and decrease Partial Volume

Pixel vs Voxel

• Pixel is short for Picture Element and has two dimensions (2D): Height & Width, but no depth or slice thickness
• Pixels are 2D units of an image, measured by millimeters by millimeters (mm x mm)
• It's important to have as many pixels in the pathology as possible but still maintain a balance of good scan time and SNR
• Scan Matrix is Phase & Frequency steps, which affects the Pixel Size, and it states how many times you divide your FOV in each direction
• Smaller pixels mean increases SNR and RES
• Phase Encoding Matrix: Number of phase steps impacts scan time, resolution, & SNR
• Phase Encoding increases Scan time
• Formula: TR x Phase x NEX = Scan Time in ms
• Lowering the Phase Matrix to Save Time/increases Scan Time
• Voxels are 3D units of a volume, so thickness is the 3rd dimension
• 2D's also have voxels but have no depth or thickness
• 3D sequences with thin slices and isotropic pixels are ideal for Multiplanar Reconstruction (MPRs)
• Isotropic are ideal for Reformatting
• In a 3D sequence, SNR increases with the Square Root of # of partitions (slices), along with Scan Time

Frequency Matrix

• Tells the receiver how many times to sample the echo
• Higher frequency = more echo samples which means a longer minimum TE

Receiver Bandwidth

• Tells how many frequencies will be mapped across the FOV (Hz/Pixel)
• Rec B/W is a Sampling Rate/Sampling Speed
• Narrow Rec. B/W sampling a higher matrix increases SNR with a longer minimum TE, and more Chem. Shift Artifact, and it Samples Slower
• Wide Rec. B/W is a Fast Sampling Rate, which Samples more noise & more echo
• Wide Rec. B/W decreases SNR and shortens minimum TE (a shorter TE is good)
• Example: 60 MPH is Wide, and 20 MHP is Narrow
• Wide R/W usually seen on T1 & PD sequences because the shorter TE keeps out T2 from echoes
• T1 & PD sequences have higher SNR compared to T2
• Narrow B/W usually seen on T2, Longer ETL sequences, and Fat Suppression (FAT SAT and STIR)

Fat Saturation (SAT)

• Always consider Fat SAT sequences to be signal starved
• Narrow REC B/W increases SNR, and Chem. Shift Artifact, and minimizes TE and Motion
• Wide REC B/W decreases SNR, and Chem. Shift Artifact, and minimizes TE and Motion

NEX/NSA/Acq

• Used interchangeably
• The # of NEX are the many times the scanner would fill each line of K-Space on successive TRs
• Fills each line of k-space once, 2x, or 3x more
• Successive TRs are used for motion control because there would be less motion between successive TRs
• Acq./NEX/NSA affect scan time & SNR
• Doubling the NEX doubles the scan time and increases SNR (by 40%)
• √# of Acq. = SNR
• √1 = 1
• √2 = 1.4
• √3 = 1.7
• However, when you 4x the NEX, SNR doubles
• Example: √4 = 2
• Signal to noise is a constant, while Noise is a variable, so this can go up

Concatenation

• Used to achieve image contrast
• Ex: Needing a large stack of 40 slices to cover the forearm, but TR is too high for T1
• Either split the slices at half of that TR and do the other half of slices at the other half of the TR
• Often used with T1 and works well with FAT SAT

Gauss

• A unit of magnetic field strength
• 1 millisecond = 1/1000 of a second or 0.001 seconds
• Most common conversion is Gauss to mT
• 1.0G = 0.10mT
• To convert Gauss to Tesla: Move the decimal point 4 places left
• 5 Gauss = .5mT
• 1 Tesla = 10,000 Gauss, so 1.5T = 15,000G

Pulse Sequences

• Two kinds: Spin Echo (SE) & Gradient Echo (GRE)
• Others are a variation of the two

Spin Echo

• Conventional Spin Echo (CSE) and Fast Spin Echo/Turbo Spin Echo (FSE/TSE)
• STIR/FLAIR/Fat-Sat
• DWI

Gradient Echo

• Conventional GRE
• TOF-MRA

Identifying Pulse Sequences

• If there's a 180 pulse = SE, if not = GRE
• Frequency encoding is on during readout
• SE has a Higher SNR than GRE because of the 180 pulse, but GRE has a shorter minimum TR & TE because of no 180
• The 180 cleans up the BIG 3: Main Mag. Field Inhomogeneities, Local Mag. Field Inhomo. and Magnetic Susceptibility Differences, and it also boosts the SNR, while GRE is sensitive to this

Magnetic Field Issues

• Main Mag a.k.a. B₀: The patient is the biggest cause of field inhomogeneities
• Local Mag: Small variations in the mag field like within the FOV being scanned, because the patient may have permanent dental work, surgical clips, spinal hardware
• These affect homogeneity in a small (local) area of the main magnet
• Mag. Susceptibilities: Some tissues act like the magnetic field and others repel it slightly, so this can result in a small area of signal loss, and it gets worse in 3T
• GRE displays susceptibility artifacts WORSE because of no 180, especially METAL susceptibility
• Example: Blood has hemoglobin, and hemo has Iron, which is a metal
• Dephasing always = signal loss
• GRE has a 3rd scan parameter: Flip Angle

Flip Angle (F/A)

• How far the Longitudinal NMV will be tipped of flipped (Neutated) into the X/Y (transverse) plane
• Usually less than 90 degrees, ranging from 20 - 70 degrees
• F/A replaces TR to T1 weighting
• The Higher the F/A = the More T1 contrast, and vice versa

Fast Spin Echo (FSE) / Turbo Spin Echo (TSE)

• Also known as RARE (Rapid Acquisition with Relaxation Enhancement)
• It's a faster Spin Echo with a 90 and a 180 pulse, but with additional 180 pulses that cause more echoes per TR, per slice, which shortens scan time
• Echo is singular in SE and Echoes is plural in FSE
• FSE gets more echoes per slice per TR, which makes the echo go further and further away from the 90, which makes the contrasts different and more T2 weighted
• These contrasts are managed by selectively placing them in k-space for the desired image contrast
• The TE in FSE are called Effective TE (ETE), which is the echo of choice for the image contrast
• Having to change the amplitude of the Phase Encoding during the TR for each echo directs each echo to a different line of k-space
• Low Amplitude Gradient places the echo in the center lines

K-Space

• Center lines contribute to Contrast and SNR
• Outer lines contribute to Resolution of the Image (Edge detail)
• Where we place certain echoes is key in image contrast in an FSE/TSE

Inversion Recovery(IR)

• Can be thought of as suppression

Fluid Attenuated Inversion Recovery (FLAIR)

• Uses the 180 RF pulse and T1 relaxation to make fluids DARK on purpose, mostly fat or CSF, and suppresses tissues with similar T1 (fast or slow) without selectivity.

Short Tau Inversion Recovery (STIR)

• Selectively nulls fat
• Short Tau Inversion Recovery or Short Time Inversion Recovery, where TAU means Time
• STIR images have dark fat and bright fluid (CSF or EDEMA)
• FLAIR looks at Fluid and looks at Suppressed FLUIDS
• There's 3 variants for IRs: STIR, FLAIR: T1 FLAIR & T2 FLAIR

T2 FLAIR

• Long TR (9000+ms), Long TE (120-130ms), and Long TI (2000ms)
• TR minimizes T1 contrast
• TE maximizes T2
• TI suppresses CSF
• LONG, LONG, LONG

T1 FLAIR

• Long TR (2000-3000ms), Short TE (10-20ms), and Short TI (800-900ms)
• LONG, SHORT, SHORT
• Long TE = T2 & Short TE = T1
• Short TI suppresses Short T1 tissues, and Long TI suppresses Long T1 tissues
• In T1 FLAIR, Fat is FAST to T1 & T2, and Water is SLOW to T1 & T2
• TI for T1 FLAIR (CSF) = 800-900ms with Short TE
• TI for T2 FLAIR (CSF) = 2000-2500ms with Long TE
• Field Strength does not affect the TRs or TEs used, ONLY the TI
• At 3T = T1 relaxation times get longer compared to 1.5T
• FAT SAT is sometimes called Chem. Shift. Imaging

Fat Saturation (FS)

• Starts with 3 selective RF pulses (short bursts) at the precessional frequency of FAT
• Inversion recovery DOES NOT use selective RF pulses
• The difference in precessional frequency of FAT & WATER is called Chemical Shift
• Water speeds up, and fat as well, but not at the Same rate
• Fat precesses 3.5ppm (part per million) slower (less) than water
• Fat-Sat can be added in almost every sequence
• More Chem shift/p.f differences = the Higher the Field Strength
• Precessional Frequency is Directly Proportional to Mag. Field strength
• Larmor Equation: ω₀ = γΒ₀
• If no field strength noted = 3.5ppm, but if a field strength is noted, use the Larmor equation
• Water spinning at 63.85MHz (Million Hertz) @1.5T would make fat rotating at 223.47MHz ( 63.85MHz x 3.5 )

Time of Flight (TOF)

• Gets signal from Blood because of the Time blood Flies into the slice
• The slice then gets excited, refocused, and gives out signal Before flying out of the slice
• Fast imaging use TRs and TEs that are Shorter than both T1 and T2 tissues
• Stationary background tissue is saturated with RF, and has no time to T1 relax
• Because of Blood flow, fresh unsaturated spins are always entering slices in vessels
• The unsaturated spins are Bright against the Dark background @TE
• With TOF, use GREs with very short TR & TE
• The TR here used is Shorter than T1 and T2 times of stationary tissues, so 15-20ms
• When TR is this short, Steady-State develops in Stationary Tissues
• Protons Flip Down to the F/A with a TR that short, so they won't be able to T1 relax, Nor can they do much T2 relax either, so They Are Stuck!! Ergo: Steady State
• TRs and TEs in GRE are short since there's no 180
• The 180 in SE increases the TR and makes the TE too long for MRAs
• TOF-MRA shows blood that is bi-directional
• Arterial: away from the heart
• Venous: towards the heart

Saturation (SAT) Pulses

• Placed on the side of the blood you Don't want to see
• The location of Sat Pulses change depending on whether you're Above or Below the heart
• The sat-pulse should be Placed Below the slices! when imaging veins below the heart
• Venous Sat shows arteries
• Venous blood crosses an RF zone of the Sat-Pulse and becomes Saturated entering the slices

Diffusion Weighted Imaging (DWI)

• Done in all brain images, used to examine CNS, detect strokes, brain tumors & brain infections
• Contrast in DWI comes from differences in tissues (diffusion)
• Exploits the random motion of water molecules, also known as Brownian Motion/Sodium Pump

Brownian Motion/Sodium Pump

• Random motion of particles in a medium (liquid or gas)
• The pattern of motion is random and also fluctuates and goes where it wants
• When tissues are Not diffusing, they're called Restricted Diffusion
• When gradient is applied to a Stationary tissue, that tissue picks up phase
• Tissues that move in that same gradient pick up More phase, which means Less signal
• Stroking tissue has no blood supply, so Not diffusion and Very Little Brownian motion happening
• The affected stroking tissue picks up little to no phase, so there's no dephasing here, so they won't lose signal, keeping them bright
• Non - healthy tissue does not move and will keep them bright. No Motion Here
• Unaffected (healthy) tissue, with its blood supply, will be diffusing because it moves
• Diffusing dark tissue (dark) next to non-diffusing tissue (bright) = Contrast (Diffusion Contrast)
• Yhis is because you're getting different amounts of signal from different tissues
• Diffuse refers to spread out, move around, not concentrated, not localized

Homeostasis

• Cells prefer maintaining homeostasis, which is a state of equilibrium.
• Cells must diffuse by taking in nutrients, oxygen, move water in/out, and remove waste material
• The moving stuff in/out of cells is called the Sodium Pump, a molecular mechanism in which sodium ions are transferred across a cell membrane by active transport

Trajectory k-space Filling

• Critical for DWI and EPI because speed is a prime factor, but this results in images that don't have High Res
• B-Value measures the degree of DW being applied, due to the: Amplitude (strength of gradient), Length of Time (how long they're applied for), and Duration of Time (time duration between the paired gradients)
• Apply a larger b-value by: Increasing gradient amplitude, applying it longer, and Widening time interval between paired gradient pulses
• 1000 b-value is the most common in the Brain
• A High b-value is More sensitive to a stroke than a Low b-value

Artifacts

• False info that's either not in the FOV or the patient
• Three types of artifact: Physiological (Patient motion), Inherent (Physics of MRI), & Hardware (Coils & other Electronics)

Artifact Direction

• Motion, Radiofrequency, and Wrap/Aliasing occur in the Phase Direction
• Chemical Shift 1st kind, Gradient Warp, Metal, and Annifact occur in the Frequency Direction
• Chemical Shift 2nd kind and Herringbone occur in the Phase + Frequency Direction
• Cross Talk occurs in the Slice Direction
• Cross Excitation and Magnetic Susceptibility occur in All Directions
• Motion is a Data Sampling Artifact

Motion Artifact

• Comes from patient motion, breathing, pulsatile flow, or peristalsis

• To decrease motion:

  • Comfort patient
  • Coach patient
  • Immobilize by using pads or straps to secure coil or body part
  • Image fast by decreasing scan time
  • Use motion suppressing sequences

• Long TRs and Long TEs are prone to motion and more prone to pulsatile

• Gadolinium makes pulsatile motion worse

• To assist in bettering the scan: Turn on flow comp

Flow Compensation (Comp)

• Also known as Gradient Moment Nulling (GMN/GMR)
• Uses paired bi-polar gradients after excitation but before signal readout
• Flow Comp is applied after Slice Selection Gradient (SSG) but before TE, which lengthens minimum TE

Nyquest Theorem

• It means that we need to sample an analog/sine wave twice its frequency so that we could get an accurate picture of its true waveform, and this is how we know exactly where the signal came from
• If a high frequency wave is under-sampled, the wave will be placed in the wrong location in k-space, creating a Wrap
• It would be then that N.T was not satisfied

Slice Wrap

• When Wrap happens in the Slice & Frequency directions, which occurs in 3D sequences
• Wrap usually occurs in the phase direction but it's not limited

Chemical Shift

• Factors:
  • Field Strength: High the frequency = bigger artifact
  • FOV: The smaller the FOV = artifact worsens
  • Receiver Bandwidth (B/W): Narrow B/W makes chem shift worst, and Wide B/W helps lessen the chem shift

Precessional Frequency Spectrum & Chemical Shift

• As field strength goes up, both peaks spread further apart (fat & water)
• Chemical Shift 1st Kind: Happens in the frequency direction, and is most commonly seen in kidneys because they are full of water and surrounded by fat
• Chemical Shift 1st Kind is Only seen in FSE & SE

Chemical Shift 2nd Kind

• Seen in phase & frequency on GRE sequences
• Out of Phase: Echoes in which fat and water vectors are Out of phase and cancelling themselves out & pointing in opposite directions, creating dark lines around all structures

Gradient Warp

• Comes from the Frequency FOV with gradients losing quality
• All gradients are straight for a finite distance, and afterwards they bend or "warp"
• Gradients need 3 qualities: Linear (Straight, no bending/curving), Constant (Maintaining strength), Reproducible (Every time they're on = they're the same)
• Magnetic properties of materials have 3 states: Diamagnetic (weakly repel a mag field), Paramagnetic (weakly attracted to a magnet), and Ferromagnetic (strongly attracted to a mag field)

MARS

• Metal Artifact Reduction Sequences
• To lessen MARS, use Spin Echo sequences: the 180 (refocusing pulse), FSE: help more bc of the multiple 180, WIDEN Rec B/W, Use Shortest TE, Decrease FOV, decrease ETL, and Avoid GREs & Fat Sat sequences
• Corduroy Artifact/ Criss-Cross/ Spike Artifact/Herringbone is made up of Loss of data points or bad" data points”/spikes in the k-space images
• Spikes are external interfering signals that get placed into k-space
• Those bad data points can come from either electromagnetic spikes in gradients or coming from gradients, however it could also be bc the coil wasn't plugged in correctly
• Artifact is not always in just one direction

Annifact Artifact

• Common in Thoracic, Lumbar spine and in Large FOV
• Shows up as a white "ghosting" artifact running up and down anteriorly
• Caused By too many coils turned on
• Fixed By over sampling can minimize this, but Over Sampling increases Scan Time

Heart Orientation & Overview

• Heart is relatively symmetrical, it does not sit symmetrically within the chest but rather is situated just left-of-center in the mediastinum
• The inferior apical portion of the heart is more anterior in the chest, while the superior portions are more posterior
• Proper visualization of the heart requires complex oblique slice orientations
• The heart is not one pump but four, and the four cardiac chambers do not contract and relax in a uniform way
• Instead of “squeezing" and relaxing uniformly, the heart beats in a twisting motion, much like wringing water from a wet towel
• The complicated motion requires precise image acquisition timing
• Flowing blood in the body can cause flow-related artifacts and ghosting
• The heart sits between the lungs, and air-filled lungs increase the risk of susceptibility artifacts
• Size is generally the size of one's fist and weighs approximately 11 ounces (312 grams)
• The normal heart beats approximately 100,000 times a day, moving 2,000 gallons (~9,000 liters) of blood
• The two upper chambers, the left atrium and right atrium, receive blood and then pump it into their corresponding left and right ventricles which then pump blood out of the heart
• The two sides of the heart are separated by the cardiac septum
• The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava
• When the right atrium contracts, blood is forced through the tricuspid valve and into the right ventricle
• Inside each ventricle are webs of cardiac muscle called papillary muscles that aid in "pulling" the ventricles inward during contraction

Cardiac Circulation

• The lining of the heart is made up of four distinct layers:
• Pericardium: A thin protective coating around the heart
• Epicardium: Just beneath the pericardium
• Pericardial Fluid: Lies between the pericardium & epicardium
• This fluid allows the heart to expand and contract without friction
• Myocardium: Powerful heart muscle, and Primary force of the heart
• Right and left coronary arteries wrap themselves around the heart to provide oxygenated blood
• Left coronary artery comes off the aorta
• Right coronary artery follows the right atrium and right ventricle
• Anastomoses are smaller branches of coronary arteries that provide different pathway flow to myocardial cells
• Pulmonary arteries and Aorta are major arteries arising from the heart
• Aorta is the Largest Artery, and Vena Cava is the Largest Vein
• Ascending Aorta: Comes off the left ventricle
• Three Arteries Arise from Aortic Arch: Left Subclavian Artery: Left Common Carotid Artery, and Brachiocephalic Artery/Innominate Artery, which bifurcates into Right Common Carotid Artery & Right Subclavian Artery
• Superior Vena Cava returns Deoxygenated blood from Upper Body to the right atrium
• Inferior Vena Cava returns Deoxygenated blood from Lower Body to the right atrium
• The Pulmonary Veins carry Oxygenated blood from lungs back to the Left Atrium
• The Pulmonary Artery arises directly from the Right Ventricle and caries Deoxygenated blood to Lungs

Cardiac Gating

• Pulse sequence timing used to capture images of the heart free of motion/perfection motion, and it Eliminates motion and Reduces artifacts
• Two types: Electrocardiogram (ECG) Gating and Peripheral Gating
• ECG Gating: Placed cardiac electrodes/ leads, and it uses the peak of R-wave to trigger a cardiac pulse sequence
• The time in between one R-Wave and the next is called, R-to-R interval, which calculates effective TR and time given for data acquisition
• Peripheral Gating: Pulse sensor placed on toe or finger
• S-A (Sinus node/Sinoatrial Node).

Heart Signals

• Initiates electrical pulse in a rhythmic manner, thus causing Right & Left Atria to contract
• A-V (Atrioventricular Node): Electrical pulse that spreads out into the ventricles
• Depolarization: (P-WAVE): Muscle movement creates energy, and that energy changes electrical signals during contraction
• (R-WAVE/ Trigger): Occurs during the Peak of ventricular contraction
• Repolarization: (T-WAVE): Re-energizes atriums and ventricles
• QRS Complex: Ventricular depolarization
• ECG (Electrocardiogram) Records and displays heart's electrical activity
• Systole: Contraction in heart muscle shown in ECG waveforms
• Diastole: Relaxation
• Single-Phase Image: The images look frozen due to the data that was collected in such a way
• Cine: A loop that displays heart In motion as a Multiphase data set
• 16 phases is the minimum number of phases collected in order to accurately view the heart in motion in real time

Heart Scan Delays

• Trigger Delay: Time from detecting the R-wave to the beginning of the data acquisition
• Trigger Window: Time from the last data acquisition to the next R-Wave
• Prospective Gating: Used in single-phased imaging
• Retrospective Gating: Used in multiphase imaging

Cardiac Gating - Patient Safety

• Because both ECG and peripheral gating use conductive cables, both require extra patient care to reduce the risk of burning the patient, by avoiding a cable loop, multiple points of contact between the cable and the patient, and minimizing the length of cable in the bore
• Never use cabling that's cracked or frayed

Basic Cardiac Pulse Sequences & Contrast

• Bright-Blood Imaging: Visualizes blood in the heart and the great vessels using steady-state imaging (FIESTA/FISP/FFE)

  • All pulse sequences are GRE based (short TR = residual transverse magnetization)
  • T2*: Combo of intrinsic T2 of a tissue and T2′ (T2-Prime) = which is dephasing due to susceptibility artifacts
  • Tissues with High T1 and T2 = blood appears Bright (hyperintense)
  • Tissues with Low T1 and T2 = myocardial muscle appears Dark (hypointense)

• Black-Blood Imaging: Visualizes blood darker

• Can assesses surrounding tissues

• Uses FSE/FTS and IR pulses

• Phase Contrast Imaging (PC): Helpful in MRA sequences to eval cardiac function - more specifically valvular, aortic, and pulmonary great vessel function - Relies on gradient manipulation to provide flow data, and quantifies both Flow and Direction

• Angiography looks at the vasculature (usually involving contrast)

• Bipolar Gradient Pulse uses equal yet apposite magnitudes

  • Allows for the selective encoding of motion-related phase shifts in MRI signals is Essential for measuring velocity and diffusion, to allow reduction in motion artifacts
  • Phase shift produced is proportional to the velocity of flowing blood
  • VENC: Like a motion detector in the scanner as scanner need to know highest speed it it need expect

• Gadolinium Contrast Enhanced Imaging (GBCAs): Checks heart blood flow and muscle condition

Cardiac Perfusion

• Perfusion studies examine variations in the distribution of the GBCA (Gadolinium Based Contrast Agents) to identify blood flow patterns and areas of decreased myocardial perfusion
• Heart rests in two different oblique planes
• There are different types of views for the cardiac:

Cardiac Views

• Short Axis View (SA): Slices run perpendicular to the long plane of the LV
• Long-Axis (LA) view of the heart slices run parallel to the long plane of the left ventricle
• High quality long axis views of the heart use the same localizers from the short axis views

Implantable Devices in MRI

• MRI Safe Materials are Cobalt, Copper, Titanium, and Stainless Steel
• Implantable Devices that are MRI Unsafe are Loop Recorders, Cochlear Implants, Brain Aneurysm Clips, Coils/Stents in blood vessels, Neurostimulators, and Cardiac Defibrillators/Pacemakers
• Some ICDs are labeled as MR-Conditional

Superconducting Magnets

• They need extremely low temperatures (almost to absolute zero/ 0 Kelvin) to maintain their superconducting state, thus having high operating costs and complex cooling systems
• Can't shut down quickly

Permanent Magnets

• Their direction of the main magnetic field is parallel to the Long Axis of the body and they use Surface Coils that are Solenoids perpendicular

Pregnant MRI Studies

• High Risk during the first trimester due to developmental effects, but Low Risk during the second and third trimester
• The American College of Radiology states that pregnant patients may safely undergo an MRI at Any Gestation Stage
• Contrast is generally Avoided during pregnancy because it can affect the fetus, since it crosses into the placenta
• Scans should be reduced,