Diffusion weighted magneticresonance imaging in diagnosis andcharacterization of brain tumors in correlation with conventional MRI Essay Submitted by Mahmoud Abdou Mohammed Abdullah M.B.B.Ch For partial fulfillment of master degree in Radio diagnosis
Supervised by PROF. AHMED FARID YOSEF Professor of radio diagnosis Faculty of medicine - Banha UniversityDR. ISLAM MAHMOUD ELSHAZLY Lecturer of radio diagnosis Faculty of medicine – Banha University Faculty of medicine Banha University 2012
IntroductionThe development of techniques capable of accurately depictingtumor grades in vivo is important for determination of the mostappropriate treatment of tumors.An unfortunate choice of biopsy site or insufficiently large samplesmay result in an incorrect histological diagnosis. The diagnosis ofbrain tumors by magnetic resonance imaging (MRI) is usually basedon basic unenhanced T1- and T2-weighted images and post contrastT1-weighted images. Conventional MRI techniques are not sufficientfor the grading and specification of brain tumors.
ع ع ع ع ع عIn diffusion-weighted imaging (DWI), the image contrast is determined by the random translational (Brownian) motion of water molecules. The quantification of diffusion using DWI has been attracting growing interest as an easy method to further characterize the nature of brain tumors. Diffusion weighted imaging may help to distinguish tumoral invasion from normal tissue or edema. This distinction if possible, would be very important for planning surgical resection, biopsies and radiation therapy.
BRAIN ANATOMY The cerebrum ع ع ع ع ع ع ع عThey are large, oval structures that superficially resemble the surface of a shelled walnut. The midline longitudinal cerebral fissure, occupied in life by the falx cerebri, incompletely separates the two cerebral hemispheres from one another. The floor of the cerebral fissure is formed by the corpus callosum, a large myelinated fiber tract that forms an anatomical and functional connection between the right and left hemispheres. Each cerebral hemisphere is subdivided into five lobes: the frontal, parietal, temporal, and occipital lobes, and the insula.
Additionally, the cortical constituents of the limbic system are alsoconsidered to be a region of the cerebral hemisphere and someconsider it to be the sixth lobe, the limbic lobe. The temporal lobe isseparated from the parietal lobe by the lateral fissure (fissure ofSylvius). The central sulcus (central sulcus of Rolando), separates thefrontal lobe from the parietal lobe. The division between the parietaland occipital lobes is defined on the lateral aspect as the imaginaryline between the preoccipital notch and the parieto-occipital notch.On the medial aspect, they are separated by the parieto-occipitalsulcus and its continuation, the calcarine fissure.
The Posterior fossa The posterior fossa is divided into two compartments by the 4thventricle. Anteriorly the brain stem occupies about one third andposteriorly the cerebellum occupies the posterior two thirds of theposterior fossa. The brain stem has three anatomically recognizablecomponents; the midbrain, pons and medulla. The two cerebellarhemispheres are joined by themidline structures of the vermis.
The ventricular system The ventricular system is composed of four fluid-filled cavities(ventricles), which are located deep within the brain. The lateralventricles consist of central portion called the body and threeextensions: the anterior, occipital and temporal horns. The junction ofthe body and occipital and temporal horns form the triangular areatermed the trigone (atria). The lateral ventricles open downward intothe third ventricle through the paired interventricular foramen(foramen of Monro). The third ventricle is located midline just inferiorto the lateral ventricles.
The third ventricle communicates with the fourth ventricle via anarrow passage way termed the cerebral aqueduct (aqueduct ofSylvius). The fourth ventricle is a diamond-shaped cavity locatedanterior to the cerebellum. The lateral angles of the fourthventricle extend to form the lateral apertures (foramina ofLuschka). The inferior angle of the fourth ventricle has an openingcalled the median aperture (foramen of Magendie), which iscontinuous with the central canal of the spinal cord. The aperturesallow passage of CSF between the ventricles and subarachnoidspace.
Axial T2 of the brain at the level of lateral ventricles
Pathology of Brain Tumors Brain tumors may be primary (i.e. originating from brain itself), orsecondary (i.e. metastatic from another primary site of cancer). Bothprimary and secondary brain tumors are capable of producingneurological impairment according to their site. A benign tumor iscomposed of slow-growing cells, but can be life threatening whenlocated in vital areas. Primary malignant tumors are usually invasiveand composed of fast growing cells. Primary tumors, whetherbenign or malignant, rarely spread outside of the central nervoussystem (CNS). Therefore, most symptoms tend to be neurologic inorigin.
WHO classification of tumors of the nervous systemgrouped by their tissue of origin into the following :major categoriesI. Tumors of neuroepithelial tissue include: Glial tumors. Neuronal and mixed neuronal glial tumors. Non-glial tumors.II. Tumors of the sellar region: Pituitary adenoma. Pituitary carcinoma. Craniopharyngioma.III. Hematopoeitic tumors: Primary malignant lymphoma.
IV. Germ cell tumors: Germinoma. Embryonal carcinoma. Yolk sac tumor. Choriocarcinoma. Teratoma. Mixed germ cell tumor.V. Tumors of the meninges: Menengioma. Mesenchymal tumors (chondrosarcoma, Hemangiomapericytoma.). Primary melanocytic lesions.VI. Tumors of uncertain histogenesis Hemangioblastoma
VII. Tumors of the peripheral nerves that affect the CNS: Schwannoma. Neurofibroma. Malignant schwannoma.VIII. Local extensions from regional tumors: Paraganglioma (chemodectoma). Chordoma. Others.IX. Metastatic tumors.X. Cysts and tumor like lesions: Arachnoid cyst. Epidermoid cyst. Dermoid cyst.
:Tumors of neuroepithelial tissue:A- Glial tumorsAstrocytic tumors- 1Oligodendroglial tumors- 2Mixed gliomas- 3Ependymal tumors- 4 :B- Neuronal and mixed neuronal glial tumors.e.g gangilioglioma and central neurocytoma :C- Non-glial tumors (:Choroid plexus tumors (CPT- 1 :Pineal parenchymal tumors- 2.Tumors with neuroblastic elements e.g: meduloblastoma and PNET- 3
Physical Principles of Diffusion–WeightedImagingFICKS LAW: It which states that local differences in solute concentration will give rise to a net flux of solute molecules from high concentration regions to low concentration regions. However, even with no concentration gradients the water molecules are still in random motion. This is because; diffusion motions are caused by the intrinsically possessed kinetic energy of the liquid medium. The phenomenon of diffusion was named "Brownian motion" after the person who first described it, Robert Brown.
:Free and restricted diffusionIn a glass of water, the motion of the water molecules is completelyrandom and is limited only by the boundaries of the container. Inbiologic systems totally free diffusion generally does not occur due tothe presence of restrictions such as cell membranes or molecularboundaries. The extent of translational diffusion of moleculesmeasured in a biologic system is therefore referred to as the apparentdiffusion coefficient (ADC). The intra-cellular diffusion coefficient islower than in the extra-cellular space due to intracellular barriers asorganelles, membranes and macromolecules.
DIFFUSION–WEIGHTED IMAGING USING PULSED GRADIENT In their fundamental studyStejskal and Tanner described an experimental method to sensitively measure diffusionwith MRI. Stejskal and Tannerused a pair of pulsed magnetic field gradients, symmetrically positioned around the 180°refocusing spin echo pulse (as shown in this figure).
The first gradient pulse induces a phase shift for all spins. The secondgradient pulse will invert this phase shift thus cancelling the phase shiftcompletely for static spins. Spins having completed a change oflocation due to Brownian motion will however experience differentphase shifts by the two gradient pulses. Thus they are incompletelyrefocused and consequently lead to a signal loss. According to Fick’slaw, true diffusion is the net movement of molecules due to aconcentration gradient. With MR imaging, molecular motion due toconcentration gradients cannot be differentiated from molecularmotion due to pressure gradients, thermal gradients, or ionicinteractions.
Disadvantages of pulsed gradient diffusion weighted imaging: Measurement of diffusion properties requires an imaging sequence sensitive for the detection of motion. However, such a sequence will also have sensitivity to bulk motion as CSF pulsations, involuntary twitches and cardiac cycling. Attempts to minimize bulk motion include use of a head holder and cardiac gating, but CSF pulsations remain problematic. To avoid these non-diffusional motions, ultra-fast techniques are used.These ultra-fast techniques are:I- ECHOPLANAR IMAGING (EPI).II- HASTE.
(I- ECHOPLANAR IMAGING (EPI With the development of high-performance gradients, DWI can beperformed with an echo-planar spin-echo T2-weighted sequence. Thesubstitution of an echo-planar spin-echo T2-weighted sequencemarkedly decreased imaging time and motion artifacts and increasedsensitivity to signal changes due to molecular motion. As a result, theDW sequence became clinically feasible to perform. In EPI, multiplelines of imaging data are acquired after a single RF excitation. Like aconventional SE sequence, a SE EPI sequence begins with 90° and180° RF pulses.
Conventional SE imaging. Within each TR period, thepulse sequence is executed and one line of imaging data iscollected. The frequency-encoding gradient (Gx), phase-encoding gradient (Gy), and section-selection gradient (Gz) .are shown during one TR period. RF = radio frequency
Echo-planar imaging. Within each TR period, multiple lines of imagingdata are collected. Gx = frequency-encoding gradient, Gy = phase-encoding gradient, Gz = section-selection gradient
Single-shot and multi-shot EPI EPI can be performed by using single or multiple excitationpulses ("shots"). The number of shots represents the number ofTR periods required to complete the image acquisition. In single-shot (snapshot) EPI, all of the k-space data are acquired withonly one shot. Distortions and signal loss occur predominantly atboundaries between tissue and air, due to the local change ofmagnetic field strength. To achieve higher resolution and reducethe image distortion and signal loss, multishot EPI can beperformed.
Comparison between single-shot and multishot echo-planarimaging. Axial images were obtained with 1 shot (a), 8 shots (b),16 shots (c), and 32 shots (d). The geometric distortion of theanterior aspect of the brain (arrow) is reduced as the number of.shots increases
:II- HASTEAnother non-EPI fast technique is diffusion weighted half-Fourier single-shot turbo spin echo, in which only half of the k-space is traversed and the other half constructed by mirroring,with the advantage of reducing susceptibility artifacts. Imagescovering the whole brain can be obtained in one minute and ittakes minutes to acquire data for calculation of the diffusioncoefficient. This technique can be implemented on mostconventional MRI systems
ISOTROPIC AND ANISOTROPIC DIFFUSION In isotropic diffusion, there is no preferred direction of watermotion. However, for white matter, consisting of dense fiberbundles, water moves more easily parallel to the fibers thanacross them. The anisotropic nature of diffusion in the brain canbe appreciated by comparing images obtained with DW gradientsapplied in three orthogonal directions. The signal intensitydecreases when the white matter tracts run in the same directionas the DW gradient because water protons move preferentially inthis direction.
Anisotropic nature of diffusion in the brain. Transverse DWI with the diffusion gradients applied along the x (Gx, left), y (Gy, .middle), and z (Gz, right) axes demonstrate anisotropyNote that the corpus callosum (arrow on left image) is hypointensewhen the gradient is applied in the x (right-to-left) direction, thefrontal and posterior white matter (arrowheads) are hypointensewhen the gradient is applied in the y (anterior-to-posterior)direction, and the corticospinal tracts (arrow on right image) arehypointense when the gradient is applied in the z (superior-to-.inferior) direction
CREATION OF ISOTROPIC DW IMAGE DW gradient pulses are applied in one direction at a time. The resultantimage has information about both the direction and the magnitude ofthe ADC. To create an image that is related only to the magnitude of theADC, at least three of these images must be combined. The simplestmethod is to multiply the three images created with the DW gradientpulses applied in three orthogonal directions. The cube root of thisproduct is the DW image.
b valueThe magnitude of the diffusion weighting is referred to as theb value. The b value increases with the strength of diffusiongradient used, the duration of each gradient lobes, and thetime between the gradient lobes. At small b values, there isminimal sensitivity to diffusional motions and T2 weighteddominates. At high b-values the contrast is largely due todiffusion properties. Unfortunately even with the maximalcurrently applied b values, T2 component is still present in alldiffusion weighted images. As result of this,T2 shine througheffect occur. Increasing b values result in a progressivedecrease in the gray to white matter signal intensity ratio. Isointensity between gray and white matter results at b valuesbetween 1000 and 2000 sec/mm2 (Typical b values in clinical use are300-1000sec/mm2 ). At b values greater than 2000, the gray- whitepattern reverses relative to the usual b value 1000.
CREATION OF AN ADC MAP An ADC map is an image whose signal intensity is equal to themagnitude of the ADC. The ADC is calculated for each pixel of theimage and is displayed as a parametric map. By drawing regions ofinterests on these maps, the ADCs of different tissues can be derived.Areas of restricted diffusion show low ADC values compared withhigher ADC values in areas of free diffusion. Thus areas of restricteddiffusion will appear of high signal DW images, these areas will appearas low-signal intensity areas (opposite to DWimages) on the ADC map.
Importance of ADC mapThe residual T2 component on the DW image makes it important to view the ADC map in conjunction with the DW image. In lesions such as acute stroke, the T2- and diffusion-WI effects both cause increased signal intensity on the DW image.The ADC maps are used to exclude "T2 shine through" as the cause of increased signal intensity on DW images. The ADC maps are useful for detecting areas of increased diffusion that may be masked by T2 effects on the DW image.
Conventional MRI findings of brain tumors Gliomas 1-Astrocytomas : The common signal characteristics of the tumors include low signal in T1 and high signal intensity T2 and appear more homogenous without central necrosisa-Diffuse astrocytoma they appear homogeneously hypointense on T1WI and hyper intense on T2 WI. Contrast enhancement is absent on MRI in diffuse low grade astrocytomas.b. Glioblastoma multiforme(GBM) (WHO grade IV)solitary deep heterogeneous ring enhancing lesion with extensive surrounding vasogenic edema and mass effect. The most common feature of the enhancing ring is irregularity, with a wide ring that varies in thickness and has a shaggy inner margin. The lesion usually extends through the corpus callosum in most cases.
c. Juvenile pilocystic astrocytomas The mural nodules appears homogenously hyperintense to grey matter on T2 WI and hypointense on T1 WI. The associated cyst is even more hyperintense on T2 weighted images and even more hypointense on T1 WI. Edema of the adjacent white matter is usually minimal. Homogenous contrast enhancement of the tumor nodule is characteristic although a calcific focus if present does not demonstrate enhancement.d. Pleomorphic xanthoastrocytoma (PXA) A cystic supratenitorial mass containing an enhancing mural nodule.e. Subependymal gaint cell astrocytoma(SEGA) Occurs almost extensively in patients with tuberous sclerosis in their late teens or 20s. Tumor appears as heterogonous sharply demarcated intraventricular mass that is mildly hyperintense on T2 WI, hypo to iso intense in T1 WI and appears as a markedly enhancing mass
GBM in the left frontal lobe. Axial gadolinium-enhanced T1-weighted image demonstrates a mass withthick, irregular, enhancing walls and areas of central. necrosis
Pleomorphic xanthoastrocytoma (a) Axial T1-WI. Soft-tissue (S) and cystic (C) components are noted. (b) AxialT2-WI shows mild low signal intensity of the soft-tissueportion of the mass, whereas the cystic portions arehyperintense. Small “fingers” of vasogenic edema surroundthe mass. (c) Contrast-enhanced axial T1-WI showsintense enhancement of the soft-tissue portion of the mass. with rim enhancement of the cystic margin
SEGA in a 16-year-old boy with a history of psychomotordevelopmental delay. (a) Axial T1-weighted MR image showsbilateral masses (arrows) near the foramen of Monro. Themasses are slightly hypointense compared with the white matter.(b) On an axial T2-weighted MR image, the masses are slightlyhyperintense compared with the white matter (c) Contrastenhanced axial T1-weighted MR image shows intenseenhancement of both masses
Oligodendrogliomas-2 Oligodendrogliomas appear heterogenous on both T1WI and T2WI; on T1WI, the tumors appear predominantly hypointense to graymatter and on T2WI, they are most often hyperintense, with smallintramural cysts, focal calcification and heterogenicity.3- Ependymal cell tumors:Most ependymomas arise in the floor of the fourth ventricle. Theyhave tendency to extend through the foramina of Luschka andMagendi into the basal cisterns. The tumor is often calcified andmay demonstrate a large cystic component. Inhomogeneousenhancement is usually seen.
:Tumors of Choroid plexus- 4Most choroid plexus tumors occur as the benign, slowly growingchoroid plexus papilloma, (WHO grade I) tumor with a favorableoverall prognosis. The other 20% of cases manifest as a much morebiologically aggressive (WHO grade III) tumor, the choroid plexuscarcinoma, which is far more common in children than adults.Choroid plexus tumors have long been associated withhydrocephalus secondary to an increase in the production of CSF bythe tumor. It shows intense enhancement on contrast enhanced MRIdue to the marked vascularity of these tumors.
Choroid plexus carcinoma. (a) Axial T1-weighted MR imageshows the lobulated mass with heterogeneous signal intensity.(b) On an axial T2-weighted MR image, the mass is slightlyhyperintense compared with the white matter. (c) Contrastenhanced axial T1-weighted MR image shows intense butheterogeneous enhancement within the mass. At surgery, theventricular wall was traversed by the mass, and histologic analysisconfirmed choroid plexus carcinoma.
: Primitive neuro ectdermal tumors PNET of CNS can be divided into infratentorial tumors (medulloblastoma) and supratentorial tumors PNET.Supratentorial PNET : Their most common location is the frontal lobes. They are often large tumors with lesser degrees of surrounding edema, demonstrated heterogeniety of signal intensity on both T1WI and T2WI, the solid portion of the tumor demonstrates strong contrast enhancement.Medulloblastomas: In children, medulloblastomas are usually at the cerebellar vermis, but in adults they tend to be located more laterally in the cerebellar hemispheres. The tumors are mildly hypointense to isointense on T1WI, and isointense to hyperintense on T2WI.
Contrast enhancement of solid portion of the tumor is seen in morethan 90% of patients; it is typically intense and homogenous but may beirregular and patchy.Medulloblastoma (a) Axial T1-weighted MR image shows mild hyperintensity inthe hemorrhagic regions; otherwise the mass is predominantly hypointense. (b)Axial T2-weighted MR image reveals marked hypointensity in the hemorrhagiczones. These features are consistent with intracellular methemoglobin. (c)Contrast-enhanced axial T1-weighted MR image demonstrates heterogeneous.but intense enhancement of the nonhemorrhagic portions
:Dysembryoplastic neuroepithelial tumorsDNET commonly occur above the tentorium, mainly in thetemporal lobe or frontal lobe. They are lesions of long standingduration that most frequently involve the convexity cortex and oftenprotrude beyond the adjacent cortical margin, eroding the overlyinginner table of the calvarium. Demonstrated as a mass centered in theconvexity cortex and bulging externally. It is hypointense to adjacentbrain on T1WI and hyperintense on T2WI with no surroundingedema. The protruding external margin may present as (Soap bubble)appearance, reflecting internal cystic changes. Contrast enhancementis seen in only a minority of these lesions.
DNET: Axial T2-weighted image (a). Contrast-enhanced axialT1-weighted image (b) showing no evidence of enhancementwithin the mass. Note the protruding external margin.
LymphomaMost lymphomas occur in patients who are immunocompromised (suchas patients under chemotherapy and patients of (AIDS). Lymphomastypically appear as homogeneous slightly high signal to isointense massesdeep within the brain on T2 weighted images. They are frequently foundin close proximity to the corpus callosum and have tendency to extendacross the corpus callosum into the opposite hemisphere. Multiplelesions are presented in 50% of cases. They are associated with only amild or moderate amount of peritumoral edema. By time of presentationthey can be quite large and yet produce relatively little mass effect, mostlymphomas show homogeneous contrast enhancement.
Primary central nervous system lymphoma. Axialpostcontrast T1-weighted MR image (a) demonstrates ahomogeneously enhancing mass in the right frontal lobe,which is isointense on the axial fluid-attenuated inversion-recovery MR image (b), with extensive surrounding T2hyperintensity.
Meningioma Most commonly they are seen parasagittally (25%). Other locationsinclude the convexity (20%), sphenoid ridge (15-20%), olfactory groove(5-10%), posterior fossa 10%, intraventricular region 2% andextracranial region 1%. Meningiomas are more common in women thanmen. They have predilection to occur from the third to sixth decades oflife. They are rare in patients younger than 20 years and if presentcommonly are associated with neurofibromatosis type II. The WHOclassified meningiomas into the following three basic groups: benignmeningioma, atypical meningioma and malignant meningioma. Mostmeningiomas are usually isointense with cortex on T1 and T2WI. Onnon enhanced MRI the majority are of homogenous appearance.
The strong, often striking, homogenous contrast enhancement seenin most meningiomas enables their accurate detection and location.A thickened tapered extension of contrast enhancing dura iscommonly identified at the margin of the tumors. Meningioma: Axial post contrast T1 WI showing intense homogenous enhancement with tapered enhancing extension of the related dura (dural tail).
SchwannomaThis tumor arises from the Schwann cells of the nerve sheath of thecranial nerves. Most common site of intracranial involvement is thesuperior vestibular division of the eighth cranial nerve. On axialimages, the tumor often has a comma-like shape with a globularcisternal mass medially and a short tapered fusiform extension laterallyinto the internal auditory canal. Contrast enhancement is seen innearly all Schwannomas; and may be homogenous in two thirds ofcases. On T1WI it appears as homogenous mild hypointense orisointense to adjacent brain; on T2WI, it appears mildly to markedlyhyperintense and may be obscured by the similarity in signal intensityto that of the surrounding CSF.
Schwannoma (acoustic neuroma_ Contrast enhancedaxial T1-weighted MR image shows a homogeneously.enhanced, coma-shaped right cerebello-pontine lesion
Brain metastasisMost metastasis is round well-demarcated lesions located at the junctionof gray and white matter. Leaky tumor vessels result in an extensive zoneof edema surrounding the tumor. Most intra-cerebral metastatic lesionsare hypo intense on T1WI and hyper intense on T2WI. Signal intensitydepends on cellularity of the lesion, the extent of intratumoral necrosis,the presence and age of hemorrhage, the presence and extent ofcalcification. Contrast administration facilitate delineation of the tumormargin. Melanoma has somewhat characteristic appearance if there hasnot been previous hemorrhage, the lesion is hyperintense in T1WI andisointense T2WI most likely because of free radical content of melanin.
Dermoid tumors and Epidermoid tumorsDermoid tumors are thought to arise at the site of neural tube closue atthe midline. This may explain the frequent midline location of dermoidtumors. In contrast, epidermoid tumors are often located lateral to themidline of the cranium. Intracranial dermoid tumors usually present inpatients up to 20 years of age. In contrast, epidermoid tumors are mostoften first diagnosed in patients aged 40-50 years. Most epidermoid cystsshow a distinctive MR imaging appearance consisting of an irregularlyshaped lesion having slightly higher signal intensity than CSF on T1, T2and proton density weighted images. Dermoid without fat or calcificationwithin them may be indistinguishable from epidermoid or arachnoid cysts.
Epidermoid cyst To the left: Axial T1-weighted MRimage shows an epidermoid cyst with characteristic focalmarbling in the left CPA (arrow). To the right: Axial T2-weighted MR image shows the lobulated margins of thecyst impinging on the pons (arrowhead).
Diffusion MRI and Brain TumorsDiffusion of water molecules.a) Restricted diffusion: high cellularity and intact cell membranes. Note water )molecules (black circles with arrows) within extracellular space, intracellular space, andintravascular space, all of which contribute to measured MR signal. In this highlycellular environment, water diffusion is restricted because of reduced extracellularspace and by cell membranes, which act as barrier to water movement. (b) Freediffusion: low cellularity and defective cell membranes. In less cellular environment,relative increase in extracellular space allows free water diffusion than more cellularenvironment would. Defective cell membranes also allow movement of water.molecules between extracellular and intracellular spaces
Role of Diffusion MRI in gliomaExact differentiation and grading of malignant brain tumors are essentialfor proper treatment planning. Although conventional MRI can detect thelocation and extent of the tumor, it is sometimes insufficient fordifferentiation and grading of malignant brain tumors. Also often somelow-grade tumors may demonstrate peritumoral edema, strongenhancement, central necrosis, or mass effect. The enhancing pattern of atumor is not always reliable for distinguishing high-grade and low-gradetumors because tumoral enhancement is mainly due to disruption of theblood brain barrier rather than from tumoral vascular proliferation itselfand these two entities are usually independent of each other.
Furthermore, the peritumoral abnormal high signal intensity on T2-weighted images, is not specific for the tumor because it may reflectvasogenic edema, the tumoral infiltration, or frequently both, and itsexact nature is indistinguishable by conventional MRI .Diffusion criteria of gliomas: The signal intensity of cerebralgliomas on DWI is variable (hyper, iso or hypointense). In high gradecerebral gliomas, areas of tumors that show significant enhancement onT1WI obtained after injection of contrast material has lower ADC valuethan the ADC of non enhancing tumor and peirtumoral edema. Cysticor necrotic portions of tumor show the highest ADC value.
Glioblastoma multiforme. (a) Contrast-enhanced T1-, (b)diffusion-weighted images, and (c) ADC map. The necroticcomponents are hypo intense on DWI, while the non necroticcomponents are slightly hyper intense. The peritumoralvasogenic edema is isointense to the white matter because theeffect of increased diffusion (dark) is compensated for by theincreased T2 value of edema (bright). The peritumoral edema,CSF, and necrotic component of the tumor are hyper intense(high diffusion) on ADC map.
Grading of gliomas Tumor cellularity (and histologic tumor grading) is inversely correlatedwith tumor ADC value in various grades of astrocytomas. Glioblastomamultiforme had the lowest ADC; anaplastic astrocytoma had intermediateADC and low-grade astrocytoma had the highest ADC. Although theADCs of grade II astrocytoma and glioblastoma overlapped somewhat, thecombination of routine image interpretation and ADC had a higherpredictive value. The lower ADC suggesting malignant glioma, whereashigher ADCs suggest low-grade astrocytoma. The ADC of anaplasticastrocytoma (grade III astrocytoma) is intermediate between those ofglioblastoma and grade II astrocytoma.
Delineation of gliomasIn malignant gliomas, peritumoral edema, which can be depicted witheither CT or conventional MR imaging, often has been reported to haveinfiltrating neoplastic cells. Therefore, the tumor border is still inaccuratelydepicted even with imaging techniques. Areas that showed marked signalsuppression with a higher ADC, most likely representing areas ofpredominantly peritumoral edema, and areas that showed a lesser degree ofsignal suppression with similar but slightly lower ADCs than those ofedema, most likely representing areas of predominantly nonenhancingtumor. So DWI is a useful technique to distinguish areas of predominantlynonenhancing tumor from areas of predominantly peritumoral edema.
Role of diffusion MRI in cystic brain tumors Differentiation between brain abscesses and cystic brain tumors such ashigh-grade gliomas and metastases is often difficult with conventionalMRI. Diffusion MRI study provides tremendous contribution todifferential diagnosis of these lesions when conventional approaches fail.The abscess cavity viscosity is highly restricting the microscopic diffusionalmovements of water molecules . High signal intensity on DWI and lowADC value in brain abscesses, in contrast to low signal intensity on DWIand high ADC value in most tumors or high signal intensity on DWI forcystic or necrotic tumors is due to T2 shine-through.
Cerebral abscess. (a) Transverse contrast-enhancedT1WI showing rim enhancement of the abscess wall,(b) DWI showing high signal of the abscess cavity, and(c) ADC map showing low signal of the abscess cavity.matching with restricted diffusion
Glioblastoma multiforme. (a) Contrastenhanced T1WI showing rim enhancement ofthe solid component, (b) DWI showing lowsignal of the necrotic center and (c) ADC mapshowing high signal of the necrotic centermatching with free diffusion.
Role of Diffusion MRI in meningiomaIt is useful to distinguish among benign, malignant and atypicalmeningiomas before resection, because it would aid in the surgical andtreatment planning. Atypical and recurrent meningiomas have moretendency for recurrence. This distinction is neither easily nor reliablyaccomplished with conventional MRI. Using diffusion-weighted MRimaging, atypical and malignant meningiomas tend to be markedlyhyperintense on DWI and exhibit marked decreases in ADC valueswhen compared to normal brain parenchyma on routine MRI.Although benign meningiomas have variable appearances on DWI,. they tend to have higher ADC values compared to the normal brain
Left frontal benign meningioma. Hypointense in T1 WIs (a), isointensein T2 WIs (b), FLAIR (c), uniform contrast enhancement in axial T1WI(d), hypointense in DWI (e), and iso to hyperintense in ADC (f)
Right parietal malignant meningioma. Isointense in T1WI (a),hyperintense in T2 WI (b), hyperintense in FLAIR (c), uniform contrastenhancement in axial T1WI (d), markedly hyperintense in DWI (e), andisointense in ADC (f).
Recurrent malignant meningioma. (a) Axialpost-contrast T1WI shows intense enhancementof meningioma, (b) DWI shows hyper intensesignal, and (c) ADC map shows hypo intensesignal reflecting restricted diffusion due to high.cellularity
Role of Diffusion MRI in lymphomaThe rate of water diffusion in CNS lymphoma, as represented byADC value is significantly lower than that of high gradeastrocytoma. The cellularity of lymphoma, as represented bynuclear to cytoplasmic (N/C) ratio, is significantly higher thanthat of astrocytoma. Lymphomas are generally hyperintense togray mater on DWI and iso to hypointense on ADC maps,findings that are consistent with lower diffusivity. In contrast,high grade astrocytomas are generally hypo- or hyperintense to. the gray matter
Primary CNS lymphoma. (a) Axial contrast-enhancedT1-, (b) Axial FLAIR shows perilesional edema. (c) ADCmap shows low signal intensity within the enhancing tumorand high signal intensity in peritumoral edema. This isdenoting restricted diffusion at the tumor and facilitateddiffusion at the peri-lesional edema du to high N/C ratio.
Role of Diffusion MRI in metastasisThe signal intensity of non-necrotic component of cerebral metastasison DWI is variable (generally iso or hypointense, occasionally hyperintense). The necrotic component of metastasis shows marked signalsuppression on DWI and increased ADC values. The signal intensityof the solid component depend on the tumor cellularity. Metastasisfrom well differentiated adenocarcinomas has significantly higherADC values than in poorly differentiated adenocarcinomas and lesionsother than adenocarcinoma. The signal intensity of the necroticcomponent is related to increased free water.
Multiple metastases. Axial contrast-enhanced T1- (a)diffusion (b) weighted images, and correspondingADC map (c). Free diffusion of the necroticcomponent is noted with low signal in DWI and high.signal in ADC map
Role of Diffusion MRI in Differential diagnosis of cyst like tumor lesionsEpidermoid tumors appear sharply hyperintense on DWI relative to thebrain and CSF; however, have higher signal intensity on ADC maps thanthat of the brain. Apparently, this hyperintensity on DWI should not beattributed to a decrease in ADC, but should be attributed to the T2 shine-through effect, meaning that the T2 properties dominated thecontributions to DW signal intensity and even overwhelmed the effect ofsignal attenuation resulting from the increase in ADC. The differentialdiagnosis of epidermoid and arachnoid cyst is straightforward on DWI.(The epidermoid cyst is bright, while the arachnoid cyst is dark on DWI).
Arachnoid cyst (a) Axial T1 post contrast image, (b) Axial T2WI, (c) DWI showing low signal and (d) ADC map showing highsignal due to free diffusion.
Epidermoid cyst (a) T1 WI , (b) T2 WI , (c) FLAIR, (d) DWI showing high signal due to T2 shine through effect and (e) ADC map showing that epidermoid cyst has diffusion rate relatively more than normal brain parenchyma.
Role of Diffusion MRI in differentiation of Cerebellar Tumors in Children ADC values and ratios are simple and readily available techniques for evaluation of pediatric cerebellar neoplasms that may accurately differentiate the 2 most common tumors, JPA and medulloblastoma. Proposed cutoff values of (>1.4 × 10−3 mm2/s) for JPA and (<0.9 × 10−3 mm2/s) for medulloblastoma seem to reliably provide the diagnosis, which may affect further diagnostic studies, treatment plan, and prognosis. Ependymomas are also significantly different from other tumor types, and in most of cases show ADC values (1.00–1.30 × 10−3 mm2/s).
Scatter diagram of average ADC tumor values for all pilocyticastrocytomas (JPA), ependymomas (Epend) and medulloblastomas(Medullo) (open circles) along with their respective mean (full circles)and standard deviation (bars) values. ADC values are expressed in10−3 mm2/s.
Fifteen-year-old girl with cerebellar JPA. ADC map inaxial plane at level of middle cerebellar peduncles showswell defined, oval mass in right paramedian location withincreased diffusion
Sixteen-year-old boy with ependymomaA, Axial T2-weighted image at level of middle cerebellar pedunclesshows a very heterogeneous abnormality (arrows) within the fourth. ventricleB, Corresponding contrast-enhanced T1-weighted imagedemonstrates enhancement of the solid portion of this mass(. (arrowsC, ADC map at a level similar to that of A and B shows thatdiffusion within the solid portion of the tumor (arrows) is slightly. higher compared with normal cerebellum
22-year-old woman with desmoplastic cerebellarmedulloblastoma. Axial ADC map at level of middlecerebellar peduncles reveals lesion of decreaseddiffusion in left cerebellar hemisphere (arrow). Nosignificant surrounding edema is seen.