chapter one generalapproachtobodymri - home - … · fu l i nsp irat o full expiration ... siemens....

BODY APPLICATIONS OF MRI

Chapter One

1

©2009, International Center for Postgraduate Medical Education. All rights reserved.

single breath hold, eliminating most motionartifacts. As a result, the majority of stream-lined body MR protocols can be completed inless than 30 minutes.

For the purposes of this module, Body MRIwill refer to any MR imaging in the torso notrelated to neuro-anatomical or musculoskeletalapplications.

OVERVIEW

Previously most MR systems were unable toacquire images in the body rapidly enough toeliminate artifacts associated with respirationand other physiologic motion. With state-of-the-art scanners and scanning techniques, theacquisition time of most body sequences aresufficiently short to be performed during a

CHAPT ER ONE

General Approach to Body MRIAfter completing this chapter, the reader will be able to:

� Identify the major challenges of body MRI� Identify methods for reducing or eliminating artifacts seen in body MRI� Demonstrate proper coil usage and positioning

POINTS FOR PRACTICE

1. What are the pros and cons of MRI body applications as compared to CT and US?2. Name several strategies for reducing or eliminating respiratory motion artifact.3. What two strategies can be used to suppress signal from subcutaneous fat?4. What are common causes of magnetic susceptibility? How can these best be managed?5. Why do the vast majority of body MRI applications use a combination of GRE and FSE/TSE

sequences?6. What are methods used to reduce the effects of echo spacing in FSE/TSE and EPI-based pulse

sequences? What are the disadvantages of these methods?7. What is the most important parameter for producing high-quality images?8. Why have phased-array coils become a standard part of the MRI system?

Tom Schrack, BS, ARMRITFairfax Radiological Consultants

Fairfax, VA

Body magnetic resonance imaging (MRI) has enjoyed its greatest relative growth in the numberof procedures performed in recent years compared to neurological and musculoskeletal MRIapplications. This growth is attributed to numerous factors, including significant advances in MRhardware and software.

2

Benefits and Challenges of Using MRI

Although computed tomography (CT) andultrasound (US) have benefited from techno-logical advances over the last several years, inmany spheres of diagnosis, body MRI remainsthe most sensitive and specific diagnosticimaging tool available (Figure 1). Patientsundergoing CT or US exams may require amore definitive study, and MRI is increasinglyused as first-line imaging.

Body MRI generally requires more time thanmost other imaging modalities. Because ofthe nature of collecting MR data, it has beendifficult to acquire a diagnostic image in whatmight be considered a “fast” imaging time.In the early days of MRI, obtaining a singleimage in less than a minute was considereda marvel, despite the severely compromisedimage quality. These long imaging times, forexample, six to eight minutes to acquire axialimages of the entire brain, could be toleratedfor many applications because the patient hadonly to lie still for a painless, albeit long andnoisy, imaging exam. Body applications,however, were altogether different.

For many years, MRI body imaging sufferedfrom challenges typically not problematic forother areas of the body. Because of respira-tory, cardiac, and bowel motion, MR systemslacked the hardware and software to scanrapidly enough to acquire meaningful images.Much of MR body imaging was of poorquality, requiring patients to remain in themagnet bore for excessively long periods,even by MR standards. With the addition ofgadolinium-based contrast sequences, theexam became intolerably long. Respiratoryartifacts, referred to as ghosts, severelyaffected images. Compromises in imagingmatrix, slice thickness, and signal-to-noiseratio (SNR) in favor of speed left images pixi-lated with noise and low in spatial resolution.As a result, MRI, for all its promise in other

Figure 1. (a) Contrast-enhanced liver CTdepicts a subtle abnormality of the medialsegment of the left lobe of the liver(arrows). T1-weighted gradient-echo MRIperformed. (b) Pregadolinium administra-tion. (c) Postgadolinium administrationdepicts a malignant cholangiocarcinoma(arrows).

a

c

b


GENERAL APPROACH TO BODY MRI

Chapter One

3


applications, was not valued as a diagnostictool in body applications until the mid-1990swhen high-speed gradients and advancedpulse sequences were developed.

ADVANCES IN GRADIENT SPEEDAND PULSE SEQUENCING

Gradient Speed

High slew rate gradients allow more slices perrepetition time (TR), resulting in shortenedacquisition times. High slew rate gradients alsoreduce echo spacing in fast spin-echo or turbospin-echo sequences, improving image quality.

Advanced Pulse Sequences

New pulse sequences (pulse sequence data-base or PSDs) collect MR data quickly,reducing acquisition times so that breath-hold 3D imaging is now routine. Other pulsesequences provide effective methods ofreducing or even eliminating breathing arti-facts, and these are discussed later in detail.

Phased-Array Technology

High-channel phased-array coils specificallydesigned for chest, abdominal, and pelvicimaging yield high SNR and coverage thatfree the user from performing in-room coilrepositioning.

Today most MRI body applications are opti-mized so that only two or three pre-contrastpulse sequences are required. These are typi-cally breath-held sequences. Post-contrastimaging can be sufficiently rapid to acquiredynamic contrast uptake images with hightemporal resolution in a 3D acquisition. Whilenot all body applications require post-contrastimaging, its use is no longer an impediment tosuccessful body imaging.

In the sections to follow, typical pulsesequences are presented for their use andspecial characteristics. The newest technolo-gies in body imaging, such as parallel imagingand fast steady-state techniques, ultrafastT2 techniques, and fast 3D T1 methods arediscussed. Individual applications and protocolsalso are included, using image examples andanatomical references. The areas of MR bodyimaging included in this module are:

� Breast� Hepatobiliary System� Adrenal Glands and Kidneys� Female Pelvis� Prostate� Rectal Cancer

ISSUES UNIQUE TO BODY MRI

As previously mentioned, body MRI presentsunique challenges as compared to otherMRI applications. These challenges includerespiratory and cardiac motion, peristalsis,and susceptibility effects of tissue-air inter-faces. Each of these challenges affects theimages differently, but each can be minimizedor resolved using artifact-reduction methods.

RESPIRATORY MOTION ARTIFACTS

It is well recognized that any motion ofimaged anatomy invariably contains acascading artifact known as a ghost acrossthe image. Motion artifacts occur in any MRimage when the position of protons changesrelative to the gradient field from one phase-encoding step to the next. The resultingphase-encoding errors cause misplaced signalin the phase-encoding direction. Respiratorymotion usually presents the most visibleof motion ghosts, since it regularly occursthroughout the imaging process, as comparedto jerk motion, which occurs sporadically.Moreover, respiratory motion artifacts are

4

compounded by changes in breathing depthand rate (Figure 2). While it is optimal to elim-inate respiratory motion artifacts completelyvia suspension of breathing, there are benefi-cial pulses with a duration longer than abreath hold. Fortunately, several strategiesexist to reduce or eliminate respiratory motion.

STRATEGIES TO REDUCE OR ELIMINATERESPIRATORY MOTION

Respiratory Ordered Phase Encoding

With respiratory ordered phase encoding,respiratory bellows are placed around thepatient’s abdomen to record the patient’srespiratory waveform (Figure 3). The wave-form information includes the breathing rateand depth of inspiration/expiration. For eachphase-encoding step, the respiratory positionis recorded. After the scan is completed, thedata are shuffled to correspond as closely aspossible to the position of the abdomen. This“respiratory compensation method” reduces

the number and intensity of respiratoryghosts without increasing scan time. Whilethis method is only somewhat effective, itrepresents one of the earliest advances inbody imaging.

Increasing Signal Averaging

Because they are random, respiratory ghostscan be treated as noise rather than as truesignal. One method for reducing noise is to

Figure 2. 53-year-old female patient with a limited ability to breath hold during liver imaging.(a) Axial T2-weighted fat suppression. (b) Axial T1-weighted spoiled gradient echo. Total breath-hold time was 0:22 and 0:26 seconds, respectively. Arrows indicate excessive respiratory motionartifacts through each image.

Figure 3. Respiratory waveform generatedby the expansion and contraction of respira-tory bellows placed around the patient’sabdomen.

Full Inspiration

Full Expiration


a b


Chapter One

5


significantly increase the SNR by increasingthe number of signal averages, effectivelydecreasing the intensity of the respiratoryghosts into the noise floor of the image. Theobvious downside to this method is greatlyincreased scan times.

Fat Suppression and Fat SaturationTechniques

Respiratory ghosts are highly visible becausethey arise from the bright signal of subcuta-neous fat, primarily located in the anteriorabdominal wall (Figure 2). Ghosts from tissueof very low signal intensities are typically notvisible because their lower signal intensity fallsbelow the noise floor. Therefore, if the high-intensity fat signal is suppressed, the ghostscan be converted to low-signal intensity andfall below the noise floor, becoming lessdisturbing.

One method for suppressing fat signal is toplace pre-saturation pulses into the anteriorabdominal wall, the largest contributor ofrespiratory ghosts. However, the pre-satura-tion bands must be carefully placed as to notsaturate important structures. Moreover, pre-saturation bands cannot be shaped to thecurvature of a patient’s abdomen or be usedto suppress fat signal deeper in the abdomen.Finally, pre-saturation pulses increase SAR,which can reduce the numbers of slices perTR and ultimately increase scan time.

Chemical saturation is the other method offat suppression. Protons of fat are excited justprior to slice excitation, saturating the fatsignal throughout the entire image. While thismethod is effective in reducing the intensity ofrespiratory ghosts, it also reduces the overallSNR on the image. However, high-intensity“non-fat tissues” can still exhibit ghosting.Alternatively, chemical fat saturation alsoincreases SAR and ultimately scan time.

Gradient Reorientation

Like any motion artifact, respiratory ghostsrun in the phase-encoding direction (Figure 2).In an axial body image, the anterior-to-posterior direction is a default direction forphase encoding. Thus, the ghosts appear tocascade through the image in that direction.Re-orienting or “swapping” the phase-encoding and frequency-encoding directions(so that phase runs in the right-to-left direc-tion) forces the ghosts to run in that direction.It should be noted that this method doesnot reduce the number or intensity of therespiratory ghosts, rather only changes theirdirection.

Respiratory Triggering

As in respiratory ordered phase encodingmethod, respiratory triggering also employsbellows, although the data collection schemeis vastly different and far more effective. Inrespiratory triggering, which uses a fast orturbo spin-echo (FSE/TSE) pulse sequence, arespiratory waveform is generated. However,instead of noting the most active part of therespiratory cycle, as in respiratory orderedphase encoding, the system registers themost quiescent part of the respiratory cycle

Figure 4. Respiratory waveform monitoredby the MR system uses the most quiescentperiod of the respiratory cycle to trigger andlaunch data acquisition.

Full Inspiration

Full Expiration

6

(endexpiratory phase). During this periodof the cycle, the system collects numerouslines of k-space (Figure 4). During the rest ofthe cycle, when there is maximum abdominalmotion, no data are collected. In respiratorytriggering, it is essential that the patientbreathes at a consistent rate. The effectiveTR is determined by the patient’s respiratoryrate; therefore, the effective TR is alwaysrelatively high, making T1-weighted imagingimpractical using this method. For non-breath-held methods, respiratory triggeringis nonetheless a highly effective methodfor near-elimination of respiratory ghosts.Recently, navigator-based sequences havebeen developed that track the motion of thediaphragm, allowing data acquisition duringthe endexpiratory phase without the use ofabdominal bellows.

SUSCEPTIBILITY EFFECTS

Magnetic susceptibility refers to the ability ofa material to become magnetized. Interfacesbetween tissue and air and the presence ofmetal can generate magnetic field gradients,significantly altering the local magnetic field.The change in magnetic field alters the reso-nant frequency, leading to dephasing of spins.In regions of altered magnetic susceptibility,the net effect causes dephasing of spins,yielding significant signal loss and dropout.The field gradients that may be set up can alsoresult in geometric distortions in the images.

Primary sources of magnetic susceptibilityeffects in the body include tissue-air interfacesnear the diaphragm and compact bone-tissueinterfaces. Excessive susceptibility in theseareas can lead to image distortion near the

Figure 5. Axial images of the liver of a 43-year-old male. (a) Single-shot echo planar imagingdiffusion-weighted image with a b-value of 20. (b) Same location using FIESTA. Note the amountof image distortion (circles) in the DWI image. This distortion (referred to as geometric distortion)is an example of the sensitivity of EPI to magnetic susceptibility effects, such as by tissue-airinterfaces, compact bone-tissue interfaces, or metal cause susceptibility effects. In EPI andgradient-echo imaging techniques, these areas of large differences in magnetic susceptibility causehigh degrees of dephasing and image “bending” or distortion. Nevertheless, DWI of the liver canbe highly sensitive to liver pathologies and motion-free due to the ultrafast imaging speed of EPI.


a b


Chapter One

7

interface and increased signal dephasing thatresults in very low SNR in that area. Thesedistortions and areas of signal loss are exacer-bated by higher field strengths, lower slewrate gradients (which result in longer echotimes), and the widespread use of gradientecho (GRE) and diffusion-weighted imaging(DWI) pulse sequences (Figure 5).

Methods for reducing magnetic susceptibilityeffects in body MRI are limited. Because thetechnologist has no control over the fieldstrength of the system, one must turn to pulsesequence manipulation to balance the reduc-tion of susceptibility effects with the contrast,resolution, and SNR requirements of the exam.

Echo Spacing

In an echo-train type pulse sequence, forexample, fast spin echo or turbo spin echo,

as well as echo planar imaging (EPI), the timebetween successive echo read-out gradientpulses is termed echo spacing (ESP). Theduration may be milliseconds or microseconds,depending on the sequence. Regardless of thesequence, any increase in ESP increases thevariance of T2 decay within k-space. Thisincreased variance results in several predictableeffects. In FSE/TSE, blurring will be increased.In an EPI sequence, such as DWI, an increasein ESP results in greater geometric distortion(Figure 6). In both sequences where there is atissue-air interface, the area of susceptibilityeffect is greatly increased.

Reducing the ESP can be accomplished byincreasing the receiver bandwidth (RBW),reducing frequency-encoding steps and, tosome extent, increasing field-of-view (FOV).Associated disadvantages are decreased SNRwhen increasing RBW and decreased spatial

Figure 6. Examples of how reducing echo spacing can reduce geometric image distortion. (a) Axialsingle-shot echo planar imaging of the brain. The extreme distortion in the image is due to parame-ters that greatly increased the ESP. These parameters include low slew rate gradients, high spatialmatrix, small field-of-view, and narrow receive bandwidths. (b) Axial SS-EPI image of the brain thatdemonstrates greatly reduced geometric distortion due to reductions in ESP.


a b

8

resolution when decreasing frequencyencoding and increasing FOV. Nonetheless,effective balancing of the advantages anddisadvantages are attainable as demonstratedin the protocol tables for body applications atthe end of Chapter 2.

Gradient Echo vs SE/FSE/TSE Sequences

Spin-echo–based sequences are less affectedby susceptibility artifacts because thedephasing effects of the field changes arerephased by 180° radiofrequency refocusingpulses. By contrast, GRE and echo planarsequences may experience large areas ofsignal loss and dropout from the field changesand dephasing effects because of their useof gradient-echo refocusing pulses. In rareinstances, these sequences may be completelyunusable. Therefore, it might appear to be adisadvantage to use GRE or EPI sequences inbody imaging, but this is not the case.

GRE and EPI sequences have been some ofthe most effective methods for obtaining fast(in a breath-hold time) abdominal imaging.They very quickly provide the requisite SNR,

contrast, and spatial resolution. The vastmajority of body MRI applications employ aneffective combination of GRE and FSE/TSEsequences.

Effective Use of Local GradientShimming

There is no single, more important parameterfor producing high image quality thanmagnetic field homogeneity. Even if all theother components of the MRI system areoptimized, their value is negated if the homo-geneity is poor. Therefore it is essential thatthe user make judicious use of local gradientshimming in any body imaging. Shimmingfurther reduces susceptibility effects andmaximizes the SNR for any given pulsesequence and its parameters.

SURFACE COIL SELECTION AND USE

The surface coil is a well known and common-place accessory in MRI. A surface coil in MR isnothing more than a receiving antenna thatcollects the signal from excited protons and

Figure 7. (a) Shoulder array coil covered. (b) Shoulder array coil uncovered.


a b

carries that signal back into the radiofrequencysubsystem for analysis and conversion intok-space. Ultimately, this signal informationgenerates a matrix of voxels that becomes theanatomical image. Surface coils generally referto the family of coils that act as receive-onlycoils and are designed for a specific anatomicalpart. There are many different types of surfacecoils, including both single-coil and multi-coildesigns. Surface coils are made in all shapesand sizes, depending on the area of interest(Figures 7-10). Surface coil development,

while benefiting all MR applications, had thelargest impact on body imaging. The mostsignificant of these developments was thedevelopment of phased-array coils.

Phased-Array Coils: A TechnologicalRevolution

All other factors remaining the same, thesmaller the coil size, the better the SNR.However, the smaller size also means a smallerdetection area. The earliest coil used in MR forbody imaging was the inserted transmit/receivecoil that makes up the bore of the magnet.While the coil coverage was excellent, thesheer size of the coil resulted in inadequateSNR (its length was as much as six feet). Thedevelopment of localized abdominal surfacecoils greatly improved SNR as compared to thebore insert but still lacked the SNR boost thatother applications were enjoying due to theirmuch smaller size. Phased-array coil tech-nology changed all of that.

In the mid-1990s, phased-array coil tech-nology was a revolution in MR. The benefitwas so evident that today there is no MR


Chapter One

9

Figure 8. (a) 8-channel wrist array coil.(b) 8-channel head array coil. Courtesy ofSiemens.

a b

Figure 9. (a) Example of placement of a head coil. (b) Example of placement of a run-off coil usedfor imaging blood vessels of the legs. Courtesy of Siemens.

a b


10

system manufactured that does not provide aphased-array coil system as standard hardware.

Stated simply, phased-array coil technologyuses the SNR boost of a small surface coil and“strings” together several smaller coils toobtain the coverage of a larger coil. The key isfor each local coil, called a “coil element,” tohave a dedicated receiver channel that analyzesthe signal from that coil only.

The early iterations of phased-array coil tech-nology used four receivers and coils thatcontained six to eight elements or channels.With four receivers in the RF cabinet, thesemulti-channel coils could utilize up to four tosix of the six to eight elements in the coil. Forexample, a phased-array coil designed for thespine might consist of six elements. The usercould select the top two channels for cervicalspine imaging, the middle four channels forthoracic spine imaging, and the bottom threechannels for lumbar spine imaging. Note thatthe total number of coil elements used at onetime is never greater than the number ofreceivers present in the RF cabinet. For body

imaging, the first phased-array design calledfor two coil elements anteriorly and two coilelements posteriorly.

Today, phased-array coil technology hasadvanced to the point where MR systemsroutinely have 16-32 receivers, and phased-array coils can consist of even more elements.Body arrays often are large enough to permitimaging of the abdomen and pelvis withoutthe need to move the coil, resulting in agreater number of smaller elements providinghigh SNR over a large imaging area. However,all of this imaging power does not comewithout some trade-offs.

IMAGE RECONSTRUCTION

The image reconstruction process, referred toas Fast Fourier Transform (FFT), is a highlymathematical computation. Convertingk-space data into image data that are thenprojected as pixels onto a display screenrequires vast processing power. The recon-struction of the image is done by the “arrayprocessor” engine. In phased-array architec-ture, each receiver produces its own image,with the final image a composite of all of thereceivers’ images. If there is only one arrayprocessor for four receivers, the time to recon-struct and display one composite image is fourtimes longer than if only one receiver’s data isused. If there are eight receivers, it will beeight times longer. If the time to reconstruct


Figure 10. (a) 16-channel body arraycoil. (b) 32-channelhead-spine array coil.(c) Neurovasculararray coil. Courtesyof GE Healthcare.a cb

Today, phased-array coil technology hasadvanced to the point where MR systemsroutinely have 16-32 receivers, and phased-array coils can consist of even moreelements.


Chapter One

11


one receiver’s worth of data is one second,then it requires eight seconds if eight receiversare used.

This may seem of little consequence until wereview a real-world example: If the time toreconstruct 60 images is one minute with onereceiver, using eight receivers results in animage construction time of eight minutes.This is a significant increase in reconstructiontime. To address this issue, MR systems todayoften employ parallel-array processors. Likepersonal computers using dual processors,parallel-array processors are two or moreprocessors that operate in tandem, resultingin far faster processing time for image recon-struction and display.

Likewise, continued development of fasterprocessors also aids in keeping reconstructiontimes reasonable.

SUMMARY

Because of the unique challenges of bodyimaging, MR body applications lagged behindother MR applications. These challengesincluded overcoming respiratory motion arti-facts, peristalsis, extremely long scan times,and tissue-air susceptibility artifacts. BodyMR imaging has improved significantly withthe continued advanced development ofkey components of the MR system. Thesedevelopments include faster, more powerfulgradient subsystems; better and more complexsoftware such as parallel imaging; and thedevelopment of phased-array coil technology.MRI body imaging is currently a significantcontributor in the diagnosis and staging ofbody-related pathologies and diseaseprocesses.

12


POINTS FOR PRACTICE

1. What are the pros and cons of MRI body applications as compared to CT and US?

An assessment using magnetic resonance imaging still generally requires more time than CT orUS, but MRI’s specificity and sensitivity make it an exemplary imaging modality. While patientsare subjected to a smaller and somewhat noisier scanning environment during the MR exam,high-speed gradients and advanced pulse sequences have virtually eliminated motion artifact,allowing for more rapidly-acquired, high-quality diagnostic imaging.

2. Name several strategies for reducing or eliminating respiratory motion artifact.� Breath holding� Respiratory triggering� Respiratory ordered phase encoding� Increased signal averaging� Fat suppression� Gradient reorientation

3. What two strategies can be used to suppress signal from subcutaneous fat?

The use of pre-saturation pulses and chemical saturation both help eliminate the bright signalof subcutaneous fat by suppressing high-intensity fat signal and pushing low-signal intensityghosting below the noise floor. However, pre-saturation pulses can increase SAR and reduceslices/TR, ultimately increasing scan time. Chemical fat saturation can reduce the overall SNRand increase SAR, again increasing scan time.

4. What are common causes of magnetic susceptibility? How can these best be managed?

Magnetic susceptibility refers to the ability of a material to become magnetized. Tissue-airinterfaces, compact bone – tissue interface, and the presence of metal are primary sources.While methods for reducing magnetic susceptibility are limited, pulse sequence manipulationhelps balance the reduction of susceptibility effects with the contrast, resolution, and SNRrequirements of the exam. Strategies include:

� Avoiding gradient echo-based pulse sequences� Using FSE/TSE sequences with higher echo train lengths to correct for T2’ (prime) effects� Increasing the receiver bandwidth to decrease the read-out window time� The use of local shimming volumes

5. Why do the vast majority of body MRI applications use a combination of GRE and FSE/TSEsequences?

These types of sequences obtain abdominal imaging in the time of a single breath hold,providing the requisite SNR, contrast, and spatial resolution. GRE sequences can be obtained invery short scan times, eliminating respiratory artifacts and providing in- and out-of-phase imagecontrast. FSE/TSE pulse sequences provide excellent T2 contrast and can be used along withrespiratory triggering to virtually eliminate respiratory artifacts. When used in conjunction withhigh-performance gradients and parallel imaging, FSE/TSE can also be accomplished within afew breath holds.


Chapter One

13


POINTS FOR PRACTICE

6. What are methods used to reduce the effects of echo spacing in FSE/TSE and EPI-basedpulse sequences? What are the disadvantages of these methods?

Increasing receive bandwidth, reducing frequency-encoding steps and, to some extent,increasing field-of-view help reduce echo spacing. Disadvantages of employing these methodsare decreased SNR when increasing the receiver bandwidth and decreased spatial resolutionwhen decreasing frequency encoding and increasing field-of-view. A high-performance gradientsubsystem also effectively reduces echo spacing but may be an expensive hardware purchase.

7. What is the most important parameter for producing high-quality images?

Magnetic field homogeneity plays the most important role in producing high-quality images.The use of local field gradient shimming reduces susceptibility effects and maximizes SNR.

8. Why have phased-array coils become a standard part of the MRI system?

Phased-array coils provide the SNR boost of a small surface coil, linking several smaller coilsto individual receivers to obtain the coverage of a larger coil. This linkage results in a greaternumber of smaller elements providing high SNR over a large imaging area.

All images, tables, and protocols courtesy of Fairfax Radiological Consultants, Fairfax, VA, unless otherwise noted.

14


Notes

chapter one generalapproachtobodymri - home - … · fu l i nsp irat o full expiration ... siemens....

Documents