Chapter 1: Introduction, Chart overview and History
The final major MR RF sequence category we will discuss here is Echo Planar imaging (EPI).
This will be the 3rd and last part of the module about the MRI RF Sequences. Although, in the future, it would be very interesting to put RF sequences for proton spectroscopy together as applications.
And in repetition to the GRE- and SE based RF sequence e-module:
Regarding all the acronyms and abbreviations I like to suggest: use the internet: Glossary of MRTerms-ACR. The MRI Acronym pocket guide from Hitachi Medical covers 5 MRI manufacturers.
In my opinion, they really have a good overview and they are open for error corrections. However, they only cover Siemens, GE and Philips. I also recommend the Radiology Science Dictionary: keywords, names and definitions (by David Dowsett).
Here you can also add terminology. Or you look on your MRI manufacturer’s website. They often have acronym comparisons (like the Hitachi pocket guide comparing GE, Siemens, Philips, Toshiba (CANON Medical now) and Hitachi of course).
For medical abbreviations: https://radiopaedia.org/articles/medical-abbreviations-and-acronyms
I could not find a website, which covers all the RF sequences implemented on the different MRI systems. I absolutely belong to that group but will do my utmost best to stay updated! Please let me know if there are different ones implemented on your MRI system and I am open for error-corrections.
There are many great online resources for lifelong MRI education. Find your way: explore!
I am very well aware I cannot cover all RF sequences and constantly new RF sequences are developed and implemented. I hope I cover the most-important and most-used ones.
And there are always new technologies in development and scientific “energies” underway: try to stay updated!
At the moment, EPI (Echo-planar imaging) is the fastest and most flexible approach to MR imaging, offering considerable freedom in the selection of contrast and resolution parameters. It is, however, a technologically challenging method that requires that the MRI system operates at its performance limits in gradient amplitude and rise times, system stability, and overall noise figure. Furthermore, EPI can suffer from serious artefacts in shape distortion and image ghosts that require extra attention. With the fast developing technology nowadays, I really believe that in the near future EPI RF sequences will be used more and more. It has already proven its worth for MRI applications as all manufacturers have installed advanced EPI RF sequences on their MRI systems.
But there are still major advances to be made, specifically with very advanced k-space trajectories, ultra-high performance local gradient coils, which will improve IQ by reducing distortions and other bandwidth-related artefacts, and hopefully by avoiding sensory stimulations.
So the future of EPI RF sequences is bright!
After establishing a good overview and in-depth knowledge of the RF sequences, which you should work on regularly, you can start to fine-tune several parameters in relation to clinical trial protocols.
Several items related to this module were already discussed in the previous modules. I hope you will now be able to use the specific knowledge of the SE, GRE and IR RF sequences and several k-space trajectories, although a short repetition of the last subject will be included.
Check your own MR system(s) for availability of the RF sequences discussed. The acronyms and abbreviations can also be found on the internet.
1.02 Overview chart
EPI ECHO PLANAR IMAGING
EPI (Echo Planar Imaging) is one of the first NMR techniques (1970s) introduced by Peter Mansfield, an English physicist who was granted the 2003 Nobel Prize in Physiology or Medicine, shared with Paul Lauterbur, for discoveries concerning NMR.
Mansfield was a professor at the University of Nottingham and leaded the SPMIC (Sir Peter Mansfield Imaging Centre), a transnational imaging center named after him. He brought together the scientists developing new medical imaging techniques with the clinicians and scientists who use them, involved in numerous national and international collaborations.
Later the term NMR imaging was changed to MRI, even more abbreviated to MR. The word “nuclear” was removed to avoid confusion with other medical imaging modalities using ionizing radiation.
Sir Peter Mansfield is not only credited for EPI, but also for inventing ‘slice selection’ (the selective excitation method with gradients) for MRI and understanding how the radio signals from MRI can be mathematically analysed, making interpretation of the signals into a useful image.
After a variety of further improvements this RF sequence finally led to Rzedzian’s SE-EPI (1987) and Howseman’s blipped EPI (1988). The EPI RF sequence could fully be used and lost its limitations through improvements in hardware and software.
Mansfield was and is interested in a wide range of problems in MR. He had already made significant progress in solid state spectroscopy by discovering echo’s in the presence of strong dipolar coupling (1965), initiate NMR microscopy (Botwell, 1990) and show keen interest in peripheral nerve stimulation (1993).
EPI allows T2*w images to be collected much faster than previously possible. It also has made fMRI and DWI feasible.
DWI was introduced by Le Bihan (1985) and DTI by Basser (1994) but the first spine diffusion measurements (with SE and time dependent field gradients) were already measured in 1965 by E. Stejskal and J. Tanner. Temperature Imaging was introduced in 1988 by Jolesz and MT (Magnetization Transfer) Imaging by Wolff and Balaban in 1989.
BOLD Imaging a group at Bell Laboratories led by Seiji Ogawa in 1990.
Other prominent pioneers of BOLD fMRI include Kenneth Kwong and colleagues, who first used the technique in human participants in 1992. ASL (Arterial Spin Labelling) was developed in 1992 by Detre.
Chapter 2: Hardware, Software requirements and Safety
2.01 Hardware Requirements for EPI RF Sequences
EPI is difficult and demands a lot of the RF sequence, implemented on the MRI system.
Although hardware improvement has helped al lot already:
- Active shielding of the magnet (van Geuns, 1983)
- Electronic decoupling of surface coils (Boskamp, 1984)
- Increase in Field strength (Field strength debate, Crooks, 1984)
- Quadrature transmission coil (Hayes, 1985)
- Active shielding of gradient coils (Mansfield, 1986)
- Distributed winding of gradient coils (Turner, 1988)
- Digital spectrometer (Mehlkopf, 1988)
- Multi element receive coils (Roemer 1990)
In addition, faster reconstruction and faster data storage is needed. The data acquisition system needs a high performance, because the data are sampled so rapidly. Very fast ADC’s (analogue- to digital converters), up to 2 MHz, are required. Fortunately, the semiconductor technology has improved a lot, making the ADC subsystems more widely available.
Good IQ images require high performance gradients with rapid rise times and high peak amplitudes. So high accuracy of the gradient wave forms adds another complicating factor: any deviations from the ideal waveform could result in phase errors in the images. As a consequence, eddy currents (which should be low), physical instabilities or amplifier distortion are shown. Hopefully the ultra-high performance local gradient coils will bring us solutions and improvements.
The advanced k-space trajctories, mentioned in the previous chapters of the GRE and SE based RF sequences, like Compressed Sensing techniques and MultiBand are of course also essential to shorten the single shot EPI and decrease the distortion in the images.
2.02 K-space trajectories
In conventional imaging, each raw data line is separately acquired after an RF excitation. As a result, the TR goes by between the collection of each data line. In EPI, the lines/profiles are acquired continuously, in a raster-like pattern, with as little as 300 µsec passing from line to line.
The echo planar (EPI) is a unique imaging method, because it can collect a MR image from a single FID signal in about 20 to 100ms and is the fastest acquisition method in MRI (<100ms / slice), but with limited spatial resolution which is based on:
- An excitation RF pulse, with or without a 180° refocusing RF pulse and possibly preceded by magnetization preparation;
- Continuous signal acquisition in the form of a GRE train, to acquire total or partial k-space (single shot or segmented/ multi-shot acquisition);
- Frequency- and phase-encoding gradients which are adapted to spatial image encoding, with several possible trajectories to fill k-space using a constant (non-blipped) or intermittent (blipped) phase encoding gradient, a spiral acquisition etc.).
GRE-EPI: a simple RF pulse sequence diagram and k-space sampling pattern with more profiles/lines per excitation. If all profiles / lines are measured after only 1 excitation, it is called a single shot (like in a T(F)SE single shot RF sequence).
The 3 gradients in EPI are generally considered as the slice select (z), blipped (y) and switched (x), because of their respective waveforms (going out from an axial slice).
DWI with readout segmented EPI is a mulit-shot EPI RF sequence courtesy: Pinterest.com
We can divide the rapid EPI RF sequences, in which k-space is traversed in one or several numbers of excitation’s: single shot EPI and multi-shot (segmented) EPI.
“zig-zag” traversal of k-space: the 1st negative-going frequency-and phase-encoding parts of the gradients initiate negative phase shifts to all spins, moving the signal from the middle of k-space to the far left bottom corner to a point which is earlier callled as (−kxmax,−kymax). Following positive and negative frequency-encoding lobes swap k-space from left-to-right and right-to-left, blipped low-amplitude phase-encoding gradient pulses produced a step-wise increase along the ky-axis. The frequency-encoding is along the horizontal axis and the phase-encoding is along the vertical. (courtesy: Q&A)
So a strong switching frequency-encoding gradient was used at the same time with an intermittently “blipped” low-magnitude phase-encoding gradient. During the alternation of the frequency-encoding gradient, gradient echoes were composed.
K-space trajectories of SE-EPI: T is the sampling time. (k)Gx and (k)Gy are gradients responsible for frequency and phase encoding, respectively. The signal is acquired during the frequency encoding gradient.
So the zig-zag method and the blipped gradients using multi shot or single shot in combination with linear, as shown below, are the conventional methods besides the spiral filling of the k-space.
In the blipped variant a positive Gx pulse is given, resulting in traversing of the k-space from left to right, then the first positive G y blip together with a reversal of the G x gradient results in movement in k-space up and from right to left. This is repeated until the complete k-space is sampled.
Below a useful way to increase resolution along the readout axis. The symmetric property of the k-space implies that it is necessary to acquire only half of the entire MR raw data space (also called Half-Fourier, Half-Scan, Half-NEX or partial-k) to form a complete MR image. A very efficient way to achieve high resolution in a single shot EPI experiment is to use a long readout duration along ky and to acquire only the positive (or negative) values in kx. Prior to image formation, it is a relatively simple matter to calculate the data that makes up the uncollected portion of the image and then to Fourier transform the entire raw data set to form a complete image . With the “Mosaic method” regions of k-space are collected in tiles, using the “MESH” method, the k-space regions are collected in a interleaved fashion, usually with a higher amplitude phase encoding step.
Single-shot SE EPI images of the brain with 1.5 mm in-plane resolution and 3 mm slice thickness, collected using the conjugate synthesis method.
Another GRE-EPI RF diagram: after the slice-selective excitation 90° pulse, the transverse magnetization is phase-encoded with a gradient Gy and an echo is shaped in the presence of a constant frequency encoding gradient Gx. TE effective is the time from the excitation pulse to the center of the frequency encoding period and the TR is repeated depending on the number of profiles measured (resolution). All profiles can be measured in 1 shot, after 1 excitation RF pulse or as a multi shot. So the frequency encoding gradient generates a series of gradient echoes, which dephase due to T2 and T2*. TE refers to the TE effective.
Since the beginning of MRI it is known that the rapidly switched magnetic field gradients in the MRI systems result in current induction in the patients. The gradient switching rate (dB/dt) was not likely to be a cause of concern in the early days. So gradient switching rates above the predicted nerve pulse threshold (nerve stimulation) could not easily be achieved. Once the non-linear amplifier methods became feasible, however, it became possible to routinely exceed the threshold of sensation using the gradients. Most of the today’s MRI systems have gradient performance that is limited to just below the typical threshold of sensory stimulation.
So the switching gradient fields can cause PNS (Peripheral Nerve Stimulation): the rapidly changing magnetic fields induced by the gradient coils can make electric fields in human tissue causing stimulation of peripheral nerves and muscles. At threshold levels, PNS is experienced as a mild sensation such as a light vibration, poking of the skin and a tickling feeling (More in the Safety-Module)
2.04 LEARNING – EVALUATION MOMENT:
After this introduction of “ an EPI chart overview , History,Hardware, Software requirements and Safety”, and at the end of the module you should be able:
. To understand the acronyms and abbreviations in this part of the module and from those basic EPI RF sequences
. The possible ways to go through k-space with EPI RF sequences.
. To understand the indication and techniques of those EPI RF sequences.
. To describe various technical items, needed for the use of those basic EPI RF sequences and methods.
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