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Startort: Parkplatz Kasern, Ahrntal m. This results in three different direc- tional DW-MR images for each fo-value used. An area of the body that demonstrates high diffu- sion anisotropy is in the brain, where the organiza- tion of the white matter fibres confers directionality to the water diffusion.

For example, in the body of the corpus callosum, the neural tracts that connect the two cerebral hemispheres will show less restricted diffusion in the transverse orientation, compared to the anterior-posterior or foot-head directions. The principle of diffusion anisotropy is harnessed for MR neurography. DW-MRI performed with the diffusion sensitizing gradient applied in a single direction anterior- posterior perpendicular to the travel of brachial plexus dem- onstrates the nerve roots, cords and trunks of the brachial plexus to advantage.

On the left, there is tumour infil- tration of left brachial plexus arrows arising from breast carcinoma The directionality of water diffusion can be harnessed by using a special diffusion -weighted imaging tech- nique known as diffusion-tensor imaging DTI , where the motion-probing gradient is applied in mul- tiple directions typically 12 or more Bammer et al. DTI has been used in neurological imaging to reveal the complex organization of the white matter tracts in the brain.

The application of DTI is currently being investigated in a number of sites in the body including the kidneys Fukuda et al. In oncology, diffusion anisotropy is usually not encountered because tumours typically grow in a random fashion. For this reason, the orthogonal motion probing gradients are usually applied simul- taneously rather than sequentially to yield only the "trace" or "index" image.

However, certain regions in the body do demonstrate structural organization that can be differentiated by their directional water diffusivity. For example, the prostate gland and uterus show varying degrees of diffusion anisotropy due to the anatomical arrangement of the collecting ducts or muscular fibres. The directional unequal barrier to water motion is also being utilized in MR neurography, where the motion-probing gradient could be applied in a single direction perpendicular to the long axis of the nerves, to maximize neural visualization Fig.

There is growing awareness of the potential of uti- lizing the principles of diffusion anisotropy to improve disease detection and assessment both in oncological and non-oncological practices in the body.

However, much of this work remains develop- mental, although there appears to be substantial promise in the technique Xu et al. It is likely that DW-MRI performed to harness differences in the anisotropic diffusion of tissues is likely to evolve over the next few years.

On most MR systems, a mono-exponential fit is applied to the relationship between the logarithm of the measured signal inten- sity and the fo-value for each voxel on the DW-MR images. The slope of the mono-exponential line fit represents the ADC. The calculated ADC for each voxel is usually displayed as a parametric map, which can be visually appraised.

By drawing regions of interest ROI on the ADC map, the apparent water diffusivity of different tissues can be recorded. The calculated ADC is unique amongst imaging techniques as it provides a quantitative measure of water diffusivity, which indirectly reflects tissue cel- lularity and tissue organization. The calculated ADC is independent of magnetic field strength, and helps to overcome the effects of T2 shine-through.

This is in part due to the propagation of noise from the native DW-MR images and poorly fitted pixels that arise from a variety of artefacts. Areas of true restricted water diffusion will appear high signal intensity on the high fo-value image but 38 D. A year-old man with right renal cell carcinoma. Graph shows the relationship of the logarithm ratio of signal intensities vs. Note the signal attenuation plots for the tumour box vs. The slope of these plot lines represents the ADC for the individual tissues.

Hence, in this example, the renal tumour has a lower ADC compared with normal renal tissue. It is easy to appreciate this by considering a tumour that shows little attenuation of its signal inten- sity at high fo-value compared with normal tissue. The tumour therefore appears higher in signal intensity on the high fo-value DW-MR image. However, because there is little signal attenuation, the slope or gradient of the line that describes the relation between the measured signal intensity and fr-value will be shallow, thus returning a low ADC value Fig.

As discussed in Chap. Where a more sophisticated model is employed, such as by applying the principles of intravoxel incoherent motion I VIM Le Bihan et al. However, for accurate IVIM calculations, multiple fo-values need to be acquired, which is usually not performed in clinical practice. IVIM analysis is also performed off-line, as current vendor softwares do not support this methodology. Thus, although the IVIM methodology may appear appealing, it is currently only used in a research set- ting in the body.

For example, a simple hepatic cyst and a liver tumour will both return high signal intensity on the low fo-value image. When surveying the high fo-value images, the eye is trained to pick out foci of relatively high signal intensity against the low signal intensity background, which should be optimally suppressed. The definition of a "high" fo-value image is rela- tive and may vary to some extent upon the organ or region of study.

However, in pelvic or whole-body imaging Takahara et al. When surveying high fo-value images, it has to be borne in mind that there are a number of normal ana- tomical structures that show impeded water diffusion. The brain, salivary glands, normal lymph nodes, spleen, uterus, ovaries, testes, spinal cord and peripheral nerves all appear relatively bright at DW-MRI on the high fo-value images Takahara et al.

The normal bowel wall also demonstrates varying degrees of high signal intensity, which can be difficult to eliminate. Figure 3. However, not all cellular tissues are malignant, although solid neo- plasm is within the differential diagnoses.

It has to be remembered that abscesses are also cellular in nature, and would show a similar imaging appearance to tumours at DW-MRI. Thus, DW-MRI fr-value images should always be interpreted with the relevant accom- panying clinical details. By contrast, when a lesion returns low signal inten- sity on the DW-MR image and appears bright on the ADC map, this usually signifies cystic or necrotic tis- sues Fig. This combination of appearances is encountered in benign cysts and cystic tumours, and further characterization may be aided by assessment of conventional morphologic imaging for internal lesion characteristics.

When a lesion appears to show high signal inten- sity restricted diffusion on the high fo-value image, but is also bright on the ADC map, then T2 shine-through is the explanation. T2 shine-through occurs not infre- quently in clinical practice, and may be observed in both benign and malignant conditions. Certain condi- tions demonstrate this phenomenon at imaging, including benign cysts and epidermoid tumours in the brain Chen et al. When a lesion appears low signal intensity on the high fo-value image, and appears dark on the ADC map, a few causes could be considered.

Firstly, macro- scopic fatty tissue can show such imaging characteris- tics, particularly when fat- suppressed single-shot echo -planar imaging DW-MRI technique is used for image acquisition Fig.

Secondly, such an appear- ance can result from susceptibility artefacts from a variety of causes e. One good example is when there is haemo- chromatosis or haemosiderosis of the liver or spleen. For these reasons, DW-MRI should be interpreted with care when mac- roscopic fat or haemorrhage is present within a lesion to avoid misinterpretation. One important point to highlight when using DW-MR images and ADC maps for clinical or research applications is that, these images should not be read in isolation.

It would be ideal to combine, where pos- sible, the information from conventional morphologic imaging, DW-MRI fo-value images, as well as the ADC maps to allow the best image interpretation to be made. The advantage of such an approach to image interpretation is illustrated in Fig.

With effective treatment, which results in a decrease in tumour cellularity, a respond- ing tumour would typically show lower signal intensity on the DW-MR images Byun et al. Macroscopic fatty tissue returns low signal intensity on the higher fr-value image and low ADC values. In this example, fat within the right ischiorectal fossa asterisk demonstrates this phenomenon Quantitative ADC Evaluation One of the unique advantages of DW-MRI is that the technique enables the quantitative ADC measurements of tissues to be recorded.

The majority of radiological tools for disease assessment are qualitative, relying on the visual interpretation of imaging features. However, it is likely that quantitative imaging techniques will become increasingly important. In oncol- ogy, each of these techniques can provide quantitative information that reflects a unique aspect of tumour biology, such as tumour vascularity, tumour metabo- lism and tumour cellular ity; and are currently being investigated as potential imaging biomarkers for treat- ment response and prognosis.

As the lesion contour can be more difficult to define on the ADC map, it is customary to draw the ROI on the fo-value image, and then copy this onto the ADC map to record their values. For this purpose, the higher fo-value images are usually used as there is often better suppression of the background signal on these images.

However, it may be possible to define the ROI on the morphologi- cal Tl- or T2-weighted MR image, but this usually requires the matrix size and field-of-view to be equiva- lent across the DW-MR and morphological imaging. However, in the majority of tumours which are heterogeneous, the distribution of voxel values with the ROI is asym- metrical or even bimodal. In these instances, the median ADC value would be more reflective of the central ten- dency and is therefore usually recorded Fig.

The ADC values can also be analysed on a voxel-by-voxel basis. Koh Fig. Quantitative ADC evaluation. This is often done by drawing the out- line on the b-value DW-MR image, which provides bet- ter delineation of the tissue outline, and then copy and paste this onto the ADC map. The distribution of the ADC values could be displayed using frequency or cumulative histograms. Changes in the distribution of the ADC values could be appreciated by comparing the histograms Fig. More sophisticated methods of data analysis are currently being devel- oped to track ADC changes of tumours in response to treatment, such as by the use of the so-called func- tional diffusion maps or threshold diffusion maps.

Further details of these will be described in Chap. It is customary for the ADC to be calculated using all the available fo-values by default on most MR imaging systems.

However, it may be possible to specify which fo-values one would choose to include or exclude from the ADC calculations e. Quantitative ADC assessment of treatment response. A 65 -year- old man with metastatic bone disease from prostate cancer. ADC maps of the pelvis before and at 1 month after treatment using a novel targeted treatment. A region of inter- est ROI has been drawn around site of the disease in the left ilium green outline. Frequency histogram of the ADC voxel values within the ROI confirms an increase in the median value after treatment, with a shift of the histogram towards the right that the ADCs of malignant lesions tend to be signifi- cantly lower compared with benign lesions.

However, a few words of caution should be borne in mind. Firstly, there is frequently substantial overlap in the ADCs between malignant and benign lesions.

Hence, although ADC differences may be observed between cohorts of different types of lesions, it can be difficult to characterize individual lesions based on ADC val- ues alone. Secondly, the mean or median ADC value does vary to some extent depending on the choice of b- values used for image acquisition and for ADC cal- culations Koh et al. ADCs calculated using low fo-values e. Not surprisingly, reported ADC values of diseases derived from using only lower fo-values are usually higher compared with those that are calculated using larger fo-values or over a wide range of fo-values.

Hence, it is important to apply appropriate thresholds derived from similar MR systems and techniques including fo-values when ADC is employed to aid disease evalu- ation. Hence, a tumour and an abscess may both show low ADC values, and cannot be easily distinguished in this context. For this reasoning, it is important to emphasize the need to combine DW-MRI with other imaging techniques and clinical information to allow the best diagnosis to be made.

Koh 3. One study has demonstrated in normal volunteers that the ADC reproducibility is better using the free-breathing technique, compared with either breath-hold or respi- ratory-triggered techniques Kwee et al. A recent two-centre clinical trial study has also shown that good measurement repro- ducibility could be achieved in this setting Koh et al. The readers are encouraged to explore the use of Bland- Altman statistics for the comparison of test-retest ADC results.

An understanding of measurement reproducibility is important because it provides the level of confidence needed for changes in ADC follow- ing treatment or differences in the ADC measurements between two groups to be regarded as significant. Optimal dis- play will help to facilitate image review. A 2 x 2 layout allows the fo-value images, ADC map and mor- phological series e. Tl- or T2-weighted image to be simultaneously displayed for image assessment Fig.

In terms of image display, the b -value images are usually viewed as acquired, with areas of restricted diffusion appearing higher in signal intensity, com- pared with the background which appears black. However, it is sometimes advantageous to display the fo-value images using an inverted grey scale. This results in areas of restricted diffusion appear- ing dark compared to the background that now appears white.

Using the inverted grey scale for display, these images can superficially resemble PET or radionu- clide imaging studies. The ADC map is usually displayed in shades of grey, but on some MR systems a colour scale may be employed. A word of caution needs to be made when a colour scale is used to display ADC values. The eye perceives grey scale as a continuous range, but the perception of colours, depending on the colour scale employed, may render certain colours to be more striking to the eye than others, leading to false impres- sion of the distribution of ADC values on these maps.

For this reason, it can be advanta- geous to perform image fusion to combine the unique functional information of DW-MRI or ADC map with the anatomical details provided by the morphologic Tl- or T2-weighted sequences. To do so, it is custom- ary to ascribe a colour scale to the DW-MRI grey- scale display or ADC data, and then use this to fuse with the morphological image. The relative ease or difficulty in performing such image fusion is dependent to a large extent upon the vendor MR system and workstation available.

Should this prove difficult on existing plat- forms, it should be relatively easy to transfer the data offline to an external computer, and use a third party imaging software e. Fusion of the DW-MR image or ADC map with the morphological image also allows areas of restricted diffusion to be directly compared with morphological changes on conventional imaging and can draw atten- tion to subtle changes which may be unappreciated on the morphological images Fig.

However, it is important to consider that when performing fusion imaging, the same colour scale should be used each time if possible. This is because using a dif- ferent colour scale, window display and percentage fusion each time can result in false perception and erroneous interpretation.

Image Display. Optimal image display can aid disease assessment. Koh has already been shown to be useful for the evaluation of bladder and uterine cancers Lin et al. The images are stacked together and processed using radial maximum intensity projections in the coronal plane. These are then composed together for whole- body display using an inverted grey scale. The technique is currently being widely evaluated for the detection of malignant or metastatic disease in a variety of tumours, both at 1.

The ADC value reflects tissue cellularity, and aids tissue characterization and response assessment. Attention to image display, including the use of fusion imaging, can aid image interpretation and disease assessment. J Magn Reson Imaging Sasaki M, Yamada K, Watanabe Y et al Variability in absolute apparent diffusion coefficient values across differ- ent platforms may be substantial: a multivendor, multi- institutional comparison study.

DW-MR neurogra- phy uses conventional diffusion gradient encoding rather than diffusion-tensor imaging because the former results in superior visualization of the periph- eral nervous system compared to the latter when the same image acquisition time is applied.

DW-MR neurography allows for three-dimensional render- ing of the peripheral nerves, hence has the poten- tial to improve the diagnosis of peripheral nerve disorders, optimize lesion localization, and may also enable a faster and more straightforward eval- uation of the extent of neural dysfunction compared with electrophysiological studies or conventional MR imaging sequences.

Kwee, MD Department of Radiology, University Medical Center Utrecht, Heidelberglaan , CX, Utrecht, The Netherlands Clinical history, neurological examination and elec- trophysiological studies are the mainstays for the evaluation of patients with peripheral neuropathy or plexopathy.

However, in order to improve diagnostic accuracy, lesion localization and treatment planning of conditions that affect the peripheral nervous sys- tem, there is a need to develop non-invasive tech- niques that are able to visualize the peripheral nerves. As MR imaging produces images with excellent soft tissue contrast, it can be considered a primary candi- date for the imaging of peripheral nerves.

Takahara and T. Conven- tional MR neurography utilizes a variety of morpho- logical imaging sequences, including short-tau inversion recovery STIR , fat-suppressed T2-weighted imaging and Tl -weighted imaging, performed in dif- ferent anatomical planes e. By comparison, DW-MR neurography employs a thin-section DW-MR technique to interrogate an entire imaging volume, from which the high fo-value dataset can be post-pro- cessed for three-dimensional neural visualization.

Both conventional and DW-MR neurography can be used to demonstrate the anatomical location of a neurogenic tumour, the relationship between a tumour and its nerve of origin, abnormal nerve thickening due to inflammatory processes and neural discontinu- ity resulting from traumatic injuries.

Compared with DW-MR neurography, conventional MR neurography has lower nerve-to-background contrast, making peripheral nerves more difficult to identify from normal adjacent structures.

As a result, image interpretation of conventional MR neurography can be time- consuming and precise lesion localization difficult. DW-MR neurography overcomes some of the intrinsic limita- tions of conventional MR neurography, in that it is able to more selectively demonstrate peripheral nerves over long trajectories.

In this chapter, we describe and illustrate the background, rationale and practical implementa- tions of DW-MR neurography for the evaluation of the peripheral nervous system. We will discuss the normal anatomy of the main neural plexi revealed by DW-MR neurography and highlight the current and evolving clinical applications of the technique. Concepts of MR Neurography 4. In that article, peripheral nerves were visualized using fat- suppressed T2 -weighted images as high signal intensity struc- tures.

In radiological terms, an imaging technique post-fix by "graphy" usually implies that the imaging test is able to selectively highlight a particular ana- tomical structure or area of interest almost entirely and exclusively, including its three-dimensional ori- entation, with little or no superimposition of sur- rounding structures.

For example, angiography allows for the selective three-dimensional visualization of vascular structures. However, the first MR neurogra- phy images could not fulfil this definition as the imag- ing technique applied could not specifically segmentate out the neural elements, but were in fact a series of anatomically orientated morphological images on which the peripheral nerves could be identified.

In most radiological departments, fat- suppressed T2-weighted or STIR images and three-dimensional or thin-section two-dimensional Tl -weighted images are used for the evaluation of peripheral nerves.

The combined uses of these MR imaging sequences con- stitute conventional MR neurography. Typically, fat- suppressed T2-weighted or STIR images are applied for the detection and delineation of disease, while Tl -weighted images are used to aid diagnosis and anatomical description Maravilla and Bowen , Moore et al.

Figure 4. Three-dimensional gradient echo Tl -weighted sequence has recently been shown to be able to pro- vide clear visualization of the peripheral nerves of the brachial plexus because of the high spatial resolution thin-section partitions that can be achieved using this technique Zhang et al. In patients with tumours of neurogenic and other origin, conventional MR neurography is being used to localize lesions in relation to specific nerve roots Maravilla and Bowen , Moore et al. In patients with traumatic plexopathy, MR neurography can also help to define the level of injury to the periph- eral nerves, to determine whether damage occurs at the level of the nerve roots or more distally Maravilla and Bowen , Moore et al.

In addition, in patients with peripheral neuritis, thickening of the brachial plexus can be seen when it is affected Maravilla and Bowen , Moore et al. Thus, conven- tional MR neurography has enabled the visualiza- tion and anatomical localization of peripheral nerve pathologies, thereby improving diagnostic accuracy and treatment planning. These images were obtained using a channel head and neck receiver coil. All images show the bra- chial plexus arrows with high signal-to-noise ratio.

Images with an even higher resolution can be obtained when using a microscopy coil or by applying a smaller field of view. How- ever, it is difficult to depict the three-dimensional tra- jectory of these nerves, as the surrounding structures, such as cerebrospinal fluid, veins and fat tissue exhibit similar signal characteris- tics to the nerves on these images A major disadvantage of conventional MR neurog- raphy, however, is its inability to render three-dimen- sional or projectional images, such as by maximum intensity projection MIP , to depict the path of peripheral nerves along the lengths of nerve sheaths, due to overlapping of structures from adjacent tissues.

Furthermore, vascular structures, such as veins which frequently accompany peripheral nerves, can be diffi- cult to distinguish from the neural elements e. For these reasons, image inter- pretation of conventional MR neurography images can be time consuming and precise lesion localization may be difficult.

Figures 4. As is well known, the application of motion-probing gradients MPGs using DW-MRI results in signal suppression from structures with relatively unim- peded diffusion, such as blood vessels and cerebro- spinal fluid.

In structures with relatively impeded diffusion, the MR signal will be little suppressed by the MPGs and will appear relatively bright as the diffusion- sensitizing gradient increases. Peripheral nerves consist of numerous neuronal fibres, across which diffusion is relatively impeded ortho- gonal to the long axis of the nerve fibres. The difference in water diffusivity being higher along the long axis of nerves compared to the orthogonal short axes is known as anisotropy. The degree of unequal water diffusion in relation to the axes of peripheral nerves can be quan- tified by calculating the fractional anisotropy FA , and this value ranges from to 1; where represents fully isotropic water diffusion i.

The FA of peripheral nerves has been reported to range between 0. Kwee Fig. Conventional MR neurography of the brachial plexus in a healthy subject performed using coronal a STIR imaging with the corresponding b maximum intensity pro- jection MIP image. The STIR image reveals the nerves of the brachial plexus as moderately high signal intensity linear structures arrows.

However, note that when MIP was per- formed on the image set, other high signal intensity struc- tures, such as cerebrospinal fluid and veins, are highlighted on the MIP image which obscures neural visualization Takahara et al. Typically, MPGs applied in three directions are sufficient, and peripheral nerve visualization may be even better by applying the MPG in only one direction this will be discussed in a later section. Second, DW-MR neurography uses diffuse weighting as a contrast mechanism to visualize neural structures and does not attempt to perform neural "tracking" or "tractography".

In other words, DW-MR neurogra- phy is simpler to perform and analyse than DTI, and does not attempt to study the directionality of water diffusion, but only aims to obtain a high-quality neu- rographic dataset for three-dimensional evaluation and image display.

However, DW-MR imaging sequences that were used for body imaging in the past could not be applied for three-dimensional image displays of peripheral nerves due to suboptimal image quality.

Applying the principles of DWIBS to DW-MRI neurography allows us to acquire thin-section datasets, which make it pos- sible to produce three-dimensional images of periph- eral nerves in virtually any part of the body. However, DTI is usually performed in the central nervous sys- tem and rarely used to image the peripheral nerves. Using this method, DW-MR images are obtained and handled like a three-dimensional dataset by acquiring multiple thin image sections mm rather than a series of two-dimensional thick image sections mm.

Thin image parti- tions are possible using this technique because of the long and efficient scan time afforded by performing the scan in free breathing. Although breath-hold or respiratory triggering were previously thought to be necessary for performing DW-MRI studies in the body, the application of MPGs results in signifi- cant signal attenuation only where there is significant MR Neurography: Imaging of the Peripheral Nerves 55 water diffusion within the image voxel i.

By contrast, little signal attenua- tion is induced by respiratory motion because the phasic motion of breathing can be considered as a type of "coherent motion" during the short diffusion- encoding time of around ms during which the MPGs are applied Muro et al.

For this reason, it is feasible to acquire thin image sections with excellent signal-to-noise during free breathing, which is one of the key features of DWIBS.

It is well known that a STIR pre-pulse has the advantage of robust fat suppression over a larger field of view FOV , even in areas of the body that may experience substantial magnetic field inhomogeneity.

In this way, the images can be post-processed using MIPs with minimal image degradation from overlying imaging artefacts. Hence, applying the concept of DWIBS enables DW-MRI to be effectively performed in the body, including regions such as the neck and shoulder, where both large magnetic field inhomogeneities and respiratory motion can substantially degrade image quality.

Although the concept of DWIBS was primarily intended for cancer screening, the princi- ples of image acquisition can be translated to improve peripheral nerve visualization Takahara et al. The key advantage of applying DWIBS for DW-MR neurography is that thin image sections with multiple signal averages can be obtained, thus providing a high-quality dataset for three-dimen- sional display of nerve trajectories.

The readers may also want to refer to other review articles related to the subject Koh et al. Table 4. Typical scan parameters for performing DW-MR neurography on a 1. Generally, imaging using a 1. However, in the near future, 3. The key points for the parameter When performing fat- suppressed studies in the body, a chemical shift selective technique e.

This technique works well in most stan- dard two-dimensional axial image acquisitions. However, more robust fat suppression is often required in the neck and chest, especially when mul- tiple image sections need to be acquired over a large FOV which would allow for three-dimensional image display. This is because unsuppressed fat signal in the peripheral regions over the large FOV e.

On certain vendor MR platforms, it is possible to make use of more sophisticated soft- ware to enhance the rendering and visualization of the neural elements.

This software tool is implemented using Interactive Data Language release 5. Note that using CHESS fat- suppression technique resulted in poor fat suppression in the subcutaneous areas, leading to the obscuration of the neural elements on the MIP image.

If CHESS fat suppression is em- ployed, post-processing would be necessary to remove the signal emanating from the body surface due to suboptimal fat suppression. There- fore, in this case, there is less difference in the quality of the images obtained using either the STIR or CHESS fat- suppression technique The soap-bubble software tool was originally developed for the visualization and quantitative analysis of three-dimensional coronary MR angio- grams Wrazidlo et al.

Because of the complicated anatomical orientation, it is usually not possible to visualize all the coronary arteries in one plane, even by the use of curve or slab multi-planar reformats MPRs. The soap-bubble soft- ware was designed to solve this problem.

The name soap bubble originates from the idea that a volume to be rendered by MIP can be considered as an ellipsoid or "soap bubble". By flattening the surface of this soap bubble, all the structures of interest within the image volume e. Since the anatomical orientation of nerves does not normally allow visualization of their course within a single plane using slab or curved MPRs, the soap-bubble procedure is thus also a post-processing proce- dure that can help neurological visualization and assessment.

From a practical point of view, the soap-bubble procedure is performed as follows: on the coronally reformatted DW-MR neurographic images, points are initially seeded on all visible nerve roots of the nerve plexus of interest. Sequential points along each nerve root are then automatically "seeded" by applying ves- sel-tracking function, in order to accelerate the rather time-consuming process of manual seed definition Fig.

Unsatisfactory points of image seeding can be deleted and new ones manually inserted as neces- sary. Subsequently, a deformed plane containing all the defined seed points is created. This post-process- ing procedure typically takes up to about lOmin per nerve plexus, depending on the number of visualized nerve roots. Since the soap-bubble MIP can help to eliminate any surrounding or superimposing struc- tures which also have a high signal at DW-MRI such as lymph nodes and bone marrow , the utilization of the soap-bubble MIP function can thus enable bet- ter visualization and display of an entire nervous plexus compared with conventional slab or curved MIP Fig.

MPGs applied in six directions was compared by Tsuchiya et al. They found that in a fair comparison i. Soap-Bubble MIP procedure. The top left window shows the seed- ing points being created along the nerves of the right brachial plexus. The user can apply the neural track- ing function with a free choice for the distance of tracking.

In addition, and more importantly, DW-MR neurogra- phy with a MPG applied in just one direction proved to be even more superior to both DW-MR neurogra- phy using three or six motion probing directions when the same image acquisition time was used [per- sonal communications]. This technique using just one MPG applied orthogonal to the long axis of the nerve was thus named unidirectionally encoded DW- MR neurography or simply unidirectional imaging UDI and this is the technique that we would cur- rently recommend for the visualization of nerve roots and peripheral nerves Takahara et al.

Normal Anatomy 4. However, radiologists may not be well acquainted with the anatomy of the peripheral nervous system, especially those of the important neural plexi. Note that the use of UDI re- sults in the best visualiza- tion of the brachial plexus standard radiological textbooks discussing image- based radiological anatomy. This is perhaps because until recently, these structures are not well demon- strated by conventional MR imaging.

We would like to briefly review the anatomy of the two important neural plexi that can be now visualized using DW-MR neurography: the brachial plexus and the lum- bosacral plexus. Normal brachial plexus at a DW-MR neurography and b schematic drawing of the brachial plexus Reprinted with permission from Gray and Lewis The Brachial plexus consists of anterior rami of C5 to Tl.

C5 and C6 con- joint first arrowhead , followed by a part of C7 arrow , which forms the lateral trunk. Also, fine nerve branches are not visible. The ganglia are well visualized from C3 to Tl in this image, which may aid in localizing the level of a neurogenic lesion. Note that the first large nerve that is well visualized is C5 thoracic nerve Tl.

The plexus gives rise to the motor innervation of all the muscles in the upper limb except the trapezius and levator scapula mus- cles Maravilla et al. At DW-MR neurography, the pre-ganglionic part of the nerve roots is usually not visualized because of its small size and the variable cerebrospinal fluid flow which induces signal loss.

The postganglionic portions of the C5 through C7 nerve roots are usually very well visualized, although visualization of the C8 and Tl nerves can be variable because of their small sizes Takahara et al. The nerve trunks resulting from the combination of nerve roots are usually well seen, but discrimination of the nerve divisions may be difficult.

The LST can be consistently found medial to the psoas muscle and enters the pelvis just anterior to the sacral ala, medial to the sacroiliac joint. As the LST enters the pelvis, it unites with the anterior divi- sion of SI and portions of the anterior divisions of S2 and S3 to form a flattened band.

Some parts of S2 to S4 join to form a small inferior band, but this is not visualized at DW-MR neurography because of its small size. The nerves of the sacral plexus continue as the sciatic nerve, which divides at the back of the lower thigh into the tibial and common peroneal nerves Maravilla et al. Note the details of the lumbosacral plexus as described. Normal lumbosacral plexus at a DW-MR neurog- raphy and b schematic drawing of the lumbosacral plexus.

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