MRI Physics Principles

• Explains MRI pulse sequences' essentials and discusses their applications.
• Introduces spin-echo, fast spin-echo, and gradient echo pulse sequences.
• Image formation has four steps: tissue differentiation, signal recording, filling K-space, and image reconstruction.
• These steps involve RF excitation pulses (90, 180), waiting periods (TR, TE), and gradients (Gz, Gp, Gf).

Tissue Differentiation

• Achieved by manipulating TR, TE, and FA.
• Creates contrast among tissues.

Signal Recording

• The echo is recorded via the receiver coil.

K-space Filling

• Echoes are allocated on the k-space.
• K-space contains raw data.

Image Reconstruction

• Raw data converts into an image with Fourier transformation.
• Fourier transformation allocates colors based on phase and frequency after K-space is filled.
• A pulse sequence is instructions to the MRI scanner for specific image characteristics, using RF pulses and gradients.
• Two groups of pulse sequences exist: spin echo and gradient echo.

Spin Echo (SE) Pulse Sequence

• SE pulse sequence was the first and most basic pulse sequence in MRI.
• Characterized by a 90° flip angle and a 180° rephasing pulse.
• After the first 90° pulse, protons go to the transverse plane and are in phase but dephase (T2*) when turned off.
• A 180° RF pulse rephases the protons flipping them back into the transverse plane.
• Signal decays again after the 180° RF pulse, but slower (T2 decay), because the 180 reduces dephasing mechanisms.
• After the 180, the main dephasing cause is spin-spin interactions.
• TR and TE are the main timings in spin-echo.
• Repetition Time (TR) is the time between applying flip angles (90).
• Echo Time (TE) is the time between applying 90 pulses (FA) and collecting the echo. Half TE is TAU.
• The echo is collected at the signal's max amplitude.
• SE pulse sequences are used in most MRI exams.
• Manipulating TR and TE yields T1, T2, and PD-weighted images and FA is always 90 in SE

Optimal parameters

• Crucial for producing the desired image contrast in Spin Echo pulse sequences.
• MRI Technologists must be alert during image evaluation because changing TR and TE for image contrast can severely affect the SNR and Scan Time.

Parameter ranges for T1WI, T2WI, and PDWI:

• T1WI: Short TR (300-700 ms), Short TE (10-25 ms).
• T2WI: Long TR (2500 ms or more), Long TE (90-120 ms).
• PDWI: Long TR (2500 ms or more), Short TE (10-25 ms).

Gradient System

• Gradients play an essential role in the pulse sequence.
• MRI unit has three gradient sets (Gz, Gy, Gx), each on one of the three magnet axes.
• In horizontal magnets, if the patient is on the table, the Gz gradient is on the Z-axis (horizontal axis) from head-to-foot.
• The Gy gradient is located along the Y-axis (vertical), anterior to posterior.
• The Gx gradient is positioned along the X-axis of the magnet or in the patient's right-to-left direction.
• Gradients produce linear changes in the magnetic field, creating a secondary field.

G radients effects

• Gz alters the magnetic field strength along the Z-axis (horizontal axis) or the head-to-foot direction.
• Gy alters the magnetic field strength along the Y-axis (vertical) or anterior-to-posterior direction.
• Gx alters magnetic field strength along the X-axis or right-to-left.
• The first gradient is for slice selection by Gz to select an axial slice, RF must only excite anatomy protons.
• In 1T magnets, protons precess at 42.6 MHz/T. RF @ 42.6 MHz makes all protons resonate and flip to the transverse plane.
• Turning on a gradient (Gs) alters the main magnetic field altering the proton's precessional frequency.
• Only protons precessing at the RF pulse will resonate and move to the transverse plane, selecting a slice.
• Slice selection = slice encoding and the slice selection gradient (Gs) is turned on every time a RF pulse is sent.
• After slice selection, the signal allocates to correct place on the image.
• Next, apply the phase encoding gradient (Gp), turned on and off quickly, to produce a phase difference within the slice.
• Gp applied between the 90 and 180 RF pulses selects the k-space line to be filled.
• A typical k-space has 256 lines.
• The 180 rephasing pulse is sent, then the Gz is turned on again.
• At echo collection, the third gradient applies called the frequency/readout gradient (Gf).
• The Gf assigns different frequencies to the echo signal to be placed along the other k-space axis.
• The Gf is applied at the echo collection (TE).
• Repetition Time (TR) is the time for all these events to be completed where one K-space is filled
• The TR is repeated according to the number of lines in the K-space; 256 lines in the K-space require 265 repetitions.
• Long scan time is a disadvantage of SE calculated via: ST = TR x PES x NEX.
• Phase Encoding Steps (PES) are the amount of lines on the K-space. An average K-space consists of 256 PES
• Stored data in the kspace has not yet been processed
• During each TR, one line of the kspace (PES) is filled. 256 PES requires 256 TR's
• The echoes with stronger signal and contrast are stored in the central area of the k-space (1, 2, -1, -2).
• The echoes stored on the outer lines of the k-space (127, 128, -127, -128) have greater resolution.
• The imaging parameter that controls how many times the kspace is filled is called Number of Excitations (NEX)
• Higher NEX directly impacts the signal-to-noise Ratio (SNR) and the overall image quality.
• K-space is filled with linear or conventional k-pace filling where each line is individually filled for SE.

Scan Time

Tr = 500 ms, PES = 256, NEX = 1 example calculation below

• ST = 500 ms x 256 x 1 = 128,000 ms
• 128 sec / 1000 = 128 sec
• 128 sec / 60 = 2.1 min.
• Increasing TR, PES, or NEX increases scan time.

Scan times

• Times are long in Spin Echo, especially on T2 and PD images.
• T1 500ms 256 PES and 1 NEX = 2.1 min scan time
• T2 and PD 3000ms 256 PES and 1 NEX = 12.8 min scan time

Dual Echo Spin Echo (DE-SE)

• DE-SE created to save time and combines two TEs to generate two separate images (PD and T2) under the same TR.
• Knee MRI: sagittal T1, T2, and PD (example). The T2 and PD normally combine for 25.6 minutes (12.8 + 12.8).
• DE-SE can reduce the scan time to 12.8 minutes to acquire the T2 and PD within the same sequence (TR)
• Dual echo obtains two echos, one with a short TE produce a PD and a late echo with a long TE for the T2
• Two separate k-spaces are filled, saving time
• Currently DE-SE is not really used

Fast Spin Echo (FSE):

• Also known as Turbo Spin Echo (TSE) or Rapid Acquisition with Refocused Echoes (RARE) is similar to spin echo
• FSE enables faster image acquisition because more echoes are collected per single TR
• Instead of acquiring one echo per TR, several are acquired, so several k-space lines can be filled per TR.
• Accelerated data acquisition decreases the scan time.
• The number of echoes within a TR is called the Echo Train Length (ETL) / Turbo Factor.
• In FSE, the scan time is reduced by the ETL factor

Fast Spin Echo (FSE) vs Spin Echo (SE)

• SE Scan time = TR x PES x NEX and FSE Scan time = TR x PES x NEX / ETL

SE & FSE example

• Example = 3000 x 256 x 1 =12.8 minutes and FSE example = 3000 x 256 x 1 / 3= 4.2 minutes
• T2 & PD images: reduce scan time more than T1
• Short TR can maintain a short TR to max T1
• T1 is limited to long ETL because of the short TR and TE that it needs.
• T2 and PD have no ETL limit because TRs are long used.
• T1WI T2WI & PDWI can be reduced as the scan times are halved.

FSE and SE Image Weighting Scan Times

• T1WI, ETL of 2, SE scan time: 2-4 min, FSE scan time: 1-2 min
• T2WI, ETL of 8, SE scan time: 7-15min, FSE scan time: 1-2 min
• PDWI, ETL of 8, SE scan time: 7-15 min, FSE scan time 1-2 min
• In FSE, the effective TE involves several echoes and will fills the central K-Space.
• Different echoes produce variable image weighting.
• Early and late echoes fills the outside K-Space with resolution but no influence on image contrast.
• Optimized protocols keep the TR at minimum value to save scan time (Except T2 & PD)
• ETLs, added echos, and scans, need time, and the minimum TR is raised as ETL is.
• Short TR maximize T1 because raising ETLs increases min. TR that minimize T1 on the image.
• ETL raising can decrease mins and raise TEs.
• ETL raise means raising T2 on the image
• Technologists manipulating ETLs on T1-weighted images will make images less T1 weighted

Echo Spacing

• Can reduce motion, aka inter-echos spacing
• Time in millisecond ms
• The velocity at which the echoes are sampled.
• Accomplishing flow compensation.
• Accomplishing blurriness and motion ideals
• The later echos have lower amounts of signal and will cause SNR
• Long ETLs can result in low SNR

FSE Contrast Differences

• Achieved scan time reduction.
• Hyperintense fat on 12 = Hyperintense fat on T2
• Muscles and neural tissues show darker in FSE & Multi 180
• FSE will display less magnetic susceptibility
• Using multiples 180 can cause a protons spin reaction, causing fat to brighen.
• Neural looks more clear/differentiated on MRI (Because if Inc. MTF)

FSE Image Blurriness:

• FSE edges looks different from other T2 values the more full the K-line is the blurier the image results

FSE: Safety

• SAR: RF a patient revive in a unit of time.
• Temperature: increase with 180 RF
• Monitoring the SAR
• Single-Short: means the K-space is filled but has too long ETL
• SAR results high in patients
• Driven EQU: DRIVE, RESTORE, FR- FSE, can collect echoes. the technique with additional pulses does can cause SAR.
• Inc. ETL decreases Scan Time, Increases TR & T2, Decrease for T1 SNR & Magnetic, and creates brighten fats

Suppression Techniques

• Help tissues change Image intensity
• suppress image

Inversion Recover: Suppress that signal

• Sequences start after waiting (inversion time).
• suppress that signal.
• (SAT): fats in T2 -FSE& water T2WI (FLAIR).

Inversion Recovery

• A RF disturbs/aligns magnetization.
• Inversion Recovery (IR)
• experience T1.
• Occurs in Z plane and have time
• TI = 180, helps increase tissue the RF pluses, and scan time. The same w/ SE
• TR starts w/ (180°)until pulse

STIR

• FLAIR; fluid that signals from free tissues in pulses and long times

Tissue Info

• Time changes image strength
• Different applications with both

Suppression

• Short Time Inversion recovery
• Suppresses fat signals
• Tissues brighter
• STIR used in Brain-Imaging. FLAIR primary produces T2WI
• Fat appears dark with FLAIR (all characteristics shows)

Comprehensive review questions covering key concepts in MRI physics, including gradient functions, SNR determinants, k-space dynamics, scan time calculations, gradient echo, and contrast optimization.