MR SIGNAL: SPATIAL RESOLUTION

F.T. MAGNETIC GRADIENTS SLICE-, PHASE-, f- ENCODING GRADIENTS SNR MULTISLICE

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MR SIGNAL: SPATIAL RESOLUTION by Mind Map: MR SIGNAL: SPATIAL RESOLUTION

1. SIGNAL ENCODING

1.1. SIGNAL ENCODING IS ACCOMPLISHED BY MAGNETIC FIELD GRADIENTS

1.2. THE MAGNETIC GRADIENT

1.2.1. GRADIENT IS PRODUCED BY GRADIENT COILS ARE INSIDE OF MAIN MAGNET

1.2.2. SUPERIMPOSE SMALLER MAGNETIC (B1) FIELD UPON MAIN MAGNET FIELD (Bo)

1.2.3. 3 MAIN FUNCTIONS

1.2.3.1. SLICE SELECTION (AND PIXEL SELECTION)

1.2.3.2. PHASE ENCODING (SHORT-AXIS= ANTERIOR TO POSTERIOR)

1.2.3.2.1. Y- AXIS

1.2.3.3. FREQUENCY ENCODING (LONG-AXIS = HEAD TO TOES)

1.2.3.3.1. X-AXIS

1.2.4. APPLYING GRADIENT ALONG AXIS CAUSES SPINS IN EACH (SLICE, PHASE OR FREQUENCY) TO PRECESS AT DIFFERENT f

1.3. SLICE SELECTION GRADIENT

1.3.1. SLICE THICKNESS

1.3.1.1. STEEPNESS OF GRADIENT (SLOPE)

1.3.1.1.1. STEEPER SLOPE = THINNER SLICES SMALL GRADIENT = THICKER SLICES

1.3.1.2. BANDWIDTH OF RF PULSE

1.3.1.2.1. IS RANGE OF FREQUENCIES IN RF PULSE

1.3.1.2.2. Fo = CENTER FREQUENCY

1.3.1.2.3. DELTA f = BANDWIDTH

1.3.1.2.4. CANNOT BE > 0.2

1.3.1.2.5. NARROW BANDWIDTH

1.3.1.2.6. BROAD BANDWIDTH

1.3.1.2.7. Q-VALUE

1.3.2. Bss = Z GRADIENT MAGNETIC FIELD(Z- AXIS EXCITATION) PREFORMS SLICE SELECTION Gss

1.3.3. Z GRADIENT IS APPLIED WITH RF PULSE, RFt

1.3.4. GRADIENT RUNS FROM ONE END OF SCANNER TO THE OTHER (HEAD TO TOE)

1.3.4.1. MAX Bo @ HEAD

1.3.4.2. MIN BO @ FOOT

1.3.4.3. DIFFERENCES OF 1 G PER SLICE

1.3.4.3.1. CAUSES DIFFERENT RATES OF PRECESSION AT EACH SLICE

1.4. PHASE ENCODING GRADIENT

1.4.1. CORONAL = EXCITATION ALONG Y AXIS

1.4.2. APPLIED JUST BEFORE ECHO

1.4.3. B(phi) = PHASE ENCODING MAGNETIC FIELD

1.4.4. G(phi) = PHASE ENCODING GRADIENT

1.4.5. AMPLITUDE OF B(phi) IS VARIED AND DURATION CONSTANT ACROSS SLICE TO CREATE DIFFERENT PHASES WITHIN THE SLICE

1.4.6. ENERGIZED PULSE SENT BEFORE SIGNAL RECEPTION

1.4.7. CHANGES SPATIAL f'S BECAUSE M VECTORS WILL UNTWIST WITH INCREASING B(PHI)

1.4.8. SELECTS VERTICAL SPATIAL f

1.5. FREQUENCY ENCODING GRADIENT

1.5.1. SAG = EXCITATION ALONG X-AXIS

1.5.2. Br = FREQUENCY ENCODING, aka READ OUT MAGNETIC FIELD

1.5.3. Gr =READOUT GRADIENT

1.5.4. APPLIED DURING RECEPTION OF RF PULSE, RFs

1.5.5. 8 ms APPLICATION

1.5.6. CHANGES SPATIAL f BY INCREASING M VECTOR TWISTING WITH INCREASING Br

1.5.7. SELECTS HORIZONTAL SPATIAL f

1.6. DETERMINES SPATIAL RESOLUTION

2. FREQUENCY CONTENT OF SIGNAL

2.1. SPATIAL f = # CYLCES/ DISTANCE

2.1.1. MR = 2 lp/mm

2.1.2. lp/mm or lp/cm

2.2. EACH f IS DISPLAYED WITH IT'S STRENGTH (CONCENTRATION) IN SIGNAL

2.3. VERY NARROW DETECTED PULSE = HIGH # f'S

2.3.1. MORE f'S SAMPLED THE BETTER THE SPATIAL RESOLUTION

2.4. ROUNDED EDGE = BLURRED, SMALL OBJECTS

2.4.1. LACK OF SPATIAL RESOLUTION

2.5. IMPACTS SPATIAL RESOLUTION

2.5.1. HIGHER SPATIAL f CREATES HIGHER SPATIAL RESOLUTION

2.5.2. SAMPLING MORE f'S= BETTER SPATIAL RESOLUTION

2.5.3. TIME RESTRAINTS

2.5.4. SPATIAL RESOLUTION IS THE ABILITY TO IDENTIFY THE DISTANCE BETWEEN TWO OBJECTS

2.5.4.1. MR SPATIAL RESOLUTION IS 2 lp/mm

2.5.4.2. MR CAN DISTINGUISH OBJECTS 0.25 mm APART

3. SIGNAL-TO-NOISE RATIO

3.1. MR SIGNAL VS BACKGROUND NOISE

3.2. SNR = SQUARE ROOT OF = Nex ( # OF EXCITATIONS)

3.3. IMPROVING SNR BY 2 TAKES 4 TIM ES AS LONG

4. MATRIX SIZE

4.1. EACH PULSE SELECTS A PIXEL

4.1.1. SPATIAL RESOLUTION IS LIMITED BY PIXEL SIZE

4.1.2. PIXEL = FIELD OF VIEW / MATRIX SIZE

4.1.2.1. MORE PIXELS = BETTER RESOLUTION

4.1.2.2. LARGER MATRIX = PIXELS ARE SMALLER OR MORE PIXELS IN VIEWING AREA

4.2. MATRIX = (# ACQUISITION) BY (# ACQUISITION)

4.2.1. SEND GRADIENT PULSE TWICE IS MATRIX 2X2

4.2.2. SEND EACH GRADIENT PULSE TWICE IS MATRIX 6X6

4.2.3. LARGER THE MATRIX, BETTER THE SPATIAL RESOLUTION

4.3. MR MATRIX IS USUALLY 256X256

4.3.1. EACH GRADIENT PULSE IS SENT SEPARATELY, SO THATS IS THREE SETS OF 256 AQUISITIONS (768 PULSES FOR THE VOXEL)

4.3.2. 512 X 512 HAS BETTER DEFINITION (SPATIAL RESOLUTION), BUT IS COSTLY FOR TIME

4.3.3. 128 X 128 IS GRAINY= LESS DETAIL, LESS SPATIAL RESOLUTION

5. SAMPLING

5.1. RATE AT WHICH A CONTINUOUS SIGNAL IS DIGITIZED

5.2. OVERSAMPLING

5.2.1. SAMPLING TOO OFTEN (W VERY HIGH f)

5.2.2. TOO COSTLY IN EQUIPMENT, ROM, AND TIME

5.3. UNDERSAMPLING

5.3.1. TOO FEW SAMPLES TO FAITHFULLY RECONSTRUCT THE SIGNAL

5.3.2. ALIASING

5.3.2.1. MR WRAPAROUND EFFECT

5.4. OPTIMUM SAMPLING

5.4.1. SAMPLING RATES > 2 (MAX f) IN SAMPLE

5.4.1.1. A 100 MHz SIGNAL NEEDS > 200 MHz (250-300) SAMPLING RATE

5.4.2. ABOVE NYQUIST LIMIT

6. FOURIER TRANSFORM

6.1. MATHEMATICAL ALGORITHM CONVERTING DATA FROM ANALOG (VOLTAGE/TIME)-- INTO -- (BINARY) DIGITAL

6.2. FOR IMAGE PROCESSING ( AND SPECTRAL)

6.3. USED TO SIMPLIFY SIGNALS BECAUSE EASIER TO PROCESS IN F-DOMAIN

6.3.1. FREQUENCY DOMAIN

6.3.1.1. AMPLITUDE / FREQUENCY

6.3.2. FREQUENCY SPACE

6.3.3. FOURIER SPACE

6.3.4. K- SPACE

6.4. DETERMINES f CONTENT OF SIGNAL

6.5. TYPES

6.5.1. DFT = DISCRETE F.T.

6.5.2. FFT = FAST F.T. (MODERN, FAST)

6.5.3. INVERSE F.T. CONVERTS DIGITAL TO ANALOG

7. IMAGING TIME

7.1. Tr X No OF PHASE ENCODING X Nex = TIME (PER SLICE)

7.1.1. Tr X B(phi) x ACQ

7.1.2. Tr

7.1.2.1. TIME BETWEEN PULSE SEQUENCES; REPETITION TIME

7.1.2.2. TYPICALLY 2500 ms

7.1.2.3. CONTROLS THE CONTRAST (WEIGHTING) OF IMAGE

7.1.2.3.1. SHORT Tr + SHORT Te = T1W CONTRAST

7.1.2.3.2. LONG Tr + LONG Tr = T2W CONTRAST

7.1.2.3.3. = SDW CONTRAST

7.1.2.3.4. < SEE MR: CONTRAST RESOLUTION>

7.1.3. No OR B(phi)

7.1.3.1. NUMBER PHASE ENCODING STEPS (Y-AXIS PARTITIONS)

7.1.3.2. TYPICALLY 256

7.1.3.3. CONTROLS SPATIAL RESOLUTION (MATRIX SIZE)

7.1.4. Nex (ACQ, NSA)

7.1.4.1. NUMBER OF EXCITATIONS (SIGNALS AQUIRED PER B(phi)

7.1.4.2. TYPICALLY 1

7.1.4.2.1. 2 = Nex (ACQ, NSA) TAKES FOUR TIMES AS LONG

7.1.4.3. CONTROLS SIGNAL-TO-NOISE RATIO (GRAININESS)

7.1.5. 2500 ms X 256 x 1 = 670 SECONDS = 11 MINUTES/ SLICE

7.1.5.1. HEAD SCAN IS 20 SLICES

7.1.5.2. 20 X 11 MIN = NEARLY 4 HOURS!!!

7.1.5.3. ONLY WAY TO SHORTEN TIME WITHOUT AFFECTING IMAGE QUALITY IS TO USE MULTISLICE

7.2. MULTISLICE

7.2.1. IMAGE SET TIME = SINGLE SLICE TIME

7.2.2. SENDING IN CONSECUTIVE PULSE SEQUENCES DURING "DEAD TIME" (Te-Tr)

7.2.3. SPIN ECHO MULTISLICE

7.2.3.1. POSSIBLE FOR 30+ SLICES IMAGED AT SAME TIME

7.2.3.2. # SLICES POSSIBLE = Te/Tr

7.2.3.3. MOST COMMON USED IN CLINICAL SETTING

7.2.3.4. SOME TISSUES WILL ALREADY HAVE BEEN EXCITED AND BE IN RELAXATION WHEN SLICE IS SELECTED

7.2.3.5. OVERLAP OF RF PULSE CAN OCCUR AT EDGES OF SLICE

7.2.3.5.1. SOLUTIONS

7.2.3.5.2. DUE TO RF PULSE BANDWIDTH

7.2.4. INVERSION RECOVERY MULTISLICE

7.2.4.1. LIMITED BY Ti TO ABOUT 2 SLICES IMAGED CONCURRENTLY

7.2.5. SPIN ECHO MULTI- ECHO (SE ME)/ MULTISLICE MULTIECHO

7.2.5.1. MULTI- ECHO SENDS IN A SECOND RF PULSE AT LONGER Te

7.2.5.1.1. CONTRAST 1 Tr = 1000ms, Te = 30ms

7.2.5.1.2. CONTRAST 2 Tr = 1000ms, Te = 60 ms

7.2.5.1.3. 15- 16 SLICES FOR 1Tr

7.2.5.2. ADVANTAGE IS GETTING THE SECOND CONTRAST

7.2.5.3. DISADVANTAGE IS IT TAKES LONGER BECAUSE IMAGE ONLY HALF THE SLICES IN SAME TIME