الفهرس 
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Abstract
Intracranial tumors are a significant health problem. The annual incidence of primary and secondary central nervous system neoplasms ranges from 10 to 17 per 100,000 persons. Inflammatory diseases of the central nervous system (CNS) are playing an increasingly important role in the clinical practice of neuroradiology: Infections of the CNS frequently involve immunocompromised patients and are being accompanied increasingly more with the employment of innovative and aggressive immunosuppressive and immunomodulatory therapies. Noninfectious inflammation, such as multiple sclerosis, accounts for about 10% of all neurological diseases Imaging plays an integral role in intracranial tumor management. Magnetic resonance (MR) imaging in particular has emerged as the imaging modality most frequently used to evaluate intracranial tumors, and it continues to have an ever-expanding, multifaceted role.
The diagnosis of brain abscess is usually made based on the clinical presentation and imaging findings. Typically, contrast-enhanced MRI reveals ring enhancement in the capsule stage and solid enhancement in the cerebritis stage of brain abscesses. The abscess may be multiloculated and/or multifocal. Brain abscess and cerebritis may therefore mimic a brain tumour, such as a high-grade glioma or metastasis on conventional imaging
Brain abscesses and brain tumors may have similar clinical presentations. For example, only 50% brain abscess patients have fever, which could be masked by corticosteroid therapy. Also, the differential diagnosis of brain abscesses versus cystic or necrotic tumors may be difficult based on computed tomography (CT) or magnetic resonance (MR) imaging findings. However, the strategies of management for abscess and neoplasm are very different, and it is especially imperative to have a correct diagnosis before any surgical intervention of cystic brain lesions. The MR special techniques, diffusion-weighted imaging (DWI) and proton (1H) MR spectroscopy, are useful as additional diagnostic modalities for differentiating brain abscesses from cystic or necrotic brain tumors. DWI shows high signal intensity in most cases of pyogenic abscesses and low signal intensity in most cases of cystic or necrotic tumors. MR spectroscopy shows characteristic metabolites in pyogenic abscesses, distinct from those in cystic or necrotic tumors.
Hint on Diffusion physics Diffusion occurs as a result of the constant movement of water molecules. Water makes up 60–80% of our body weight.
It can be demonstrated by adding a few drops of ink to a still bucket of water. Initially, the ink will be concentrated in a very small volume, but it will quickly spread out (diffuse) and mix with the rest of the water. In the measurement of diffusion by MR, the “magic ink” is created by the magnetic field gradients.When the patient enters the large tunnel of a static magnetic field, nuclear spins (small magnets inside each proton nucleus) are lined up along the direction of the big magnet.
Hint on MRS physics MRS is a non invasive technique capable of providing metabolic information about different tissues; also it enables tissue characterization on a biochemical level surpassing that of conventional magnetic resonance imaging (cMRI). It detects abnormalities that are invisible to (cMRI) because metabolic abnormalities often precede structural changes,cMRI obtains resonance signals from protons of water and some lipids in the intra-cellular and extra-cellular spaces, whereas MRS obtains resonance signal from protons present in different molecules such as N Acetlye- methyle group of Nacetlye asparate, N Acetlye- methyle group of choline , and N Acetlye- methyle group of creatine MRS spectrum is presented as a series of peaks of differences in Larmor frequencies plotted along a relative scale on the horizontal axis, expressed in terms of units of parts per million (ppm). The ppm scale of proton MR spectroscopy (HMRS) is from zero to four ppm with the zero on the right and the four is on the left because most of the clinically important metabolites have a resonance frequency in that rang like N-acetyle aspartate (NAA) resonates at 2.02ppm, creatine (Cr) at 3.02ppm and choline at 3.22ppm
Hint on The Pathology of Brain Abscess
Brain abscesses occur in all age groups; men are affected approximately twice at much as women. Abscesses are solitary lesions in 70%, and multiple in 30%. Most commonly (30−60%), they are caused per continuation from adjacent inflammatory foci, i.e., as a complication of infections of the paranasal sinuses and/ or mastoid cells, dental infections, osteomyelitis, or purulent meningitis. About 20−30% of pyogenic brain infections are caused hematogenously The development of brain abscesses may be divided into four stages:
•Early cerebritis
•Late cerebritis
•Early capsule formation
•Late capsule formation
Hint on the pathology of brain tumors
Brain tumors are divided into primary (70%) and secondary (30%). About 24,000 primary brain tumors are diagnosed each year in the United States and account for 20% of malignancies before 15 years. The estimated incidence is 8.2 per 100,000 people Gliomas in man and meningiomas in woman are the most common primary brain tumors constituting respectively 60 and 20% of all intracranial tumors in adults In young adults, low-grade gliomas occur more often. In middle age patients, anaplastic astrocytomas are more common In the elderly, glioblastoma multiforme is the most common primary tumor. Increasing age generally correlates with increasing malignancy
Clinical Course Brain tumors produce symptoms and signs due to direct tissue destruction, local brain infiltration, or secondary effects of increased intracranial pressure. The symptoms of patients are dependent on the anatomic location of the tumor.
Hint on Imaging of Brain Abscess
T2-weighted spin-echo MR images demonstrate early cerebritis as an area of increased signal intensity indistinguishable from or slightly hypointense to the surrounding edema
OnT1-weighted images, cerebritis appears isointense to slightly hypointense to adjacent normal brain parenchyma, with associated mass effect manifested by sulcal effacement or ventricular compression Pyogenic abscess has characteristic MR features , the center of the cavity is slightly hyperintense to CSF whereas the surrounding edematous brain is slightly hypointense to normal brain parenchyma on T1- weighted images. On T2-weighted images, the signal intensities are quite variable depending on the TE chosen and the protein composition and fluidity of the material in the central cavity In an untreated abscess, the usual metabolites observed are succinate (2.4 ppm), acetate (1.92 ppm), Iactate/lipids (1.3 ppm), leucine/isoleucine/valine (0.85 ppm), glycine (3.5 ppm), alanine (1.5 ppm), and other amino acids
The response to treatment consisting of medical therapy alone has also been studied by proton MR spectroscopy Dev et al. showed a decrease in the lactate (1.33 ppm)/amino acid ratio (0.85 ppm) after 1 week in two patients studied on serial medical therapy alone Diffusion has been used in differentiation of cerebral abscess from intracerebral necrotic tumors ( Desprechins etal 1999); The abscesses appear bright on diffusion-weighted images and show a very low apparent diffusion coefficient (ADC) while the necrotic tumors appear hypointense on diffusion-weighted images and have a very high ADC.
Hint on MRI Imaging of the Brain Tumors
Magnetic resonance imaging (MRI) is the modality of choice for evaluating patients who have symptoms and signs suggesting a brain tumor,
The standard protocol most commonly used between institutions includes:
•spin-echo T1-weighted image (T1WI),
•proton density-weighted image (PDWI),
•T2-weighted image (T2WI),
•T1WI after the administration of paramagnetic agent.
Most brain tumors appear hypointense relative to normal brain on T1WI and hyper intense on T2WI. On PDWI the tumors show intermediate hyper intensity. However, the presence of fat, hemorrhage, necrosis, and calcification are responsible for the heterogeneous appearance of some tumors.
High grade glioma
Anaplastic astrocytoma (AA) (WHO grade III) and glioblastoma multiforme (GBM) (WHO grade IV), the most common primary malignant brain tumors, are classified as high-grade tumors. Gliosarcoma, a rare (WHO grade IV) tumor composed of neoplastic glial cells and sarcomatous component is also reviewed withthe high-grade tumors. Finally, the gliomatosis cerebri (GC) is considered as high-grade (WHO grade III) astrocytoma because of the poor prognosis of patients with this tumor, while histologically, the tumor consists of low-grade astrocytoma cells
Anaplastic astrocytoma
On MRI, AAs are heterogeneous on both T1- and T2-weighted images. However, the heterogeneous composition of the tumor is better reflected on T2-weighted images compared to T1-weighted images. On T2-weighted images, they often present with a hyperintense central area surrounded by an isointense rim with peripheral high signal intensity reflecting to the peritumoral edema. Following contrast administration they show heterogeneous or patchy enhancement.
Glioblastoma Multiforme
GBM is the most common primary intracranial CNS tumor accounting for more than half of all glial tumors and 15–20% of all intracranial tumors.
On T1-weighted images, they appear with low- or mixed-signal intensity, while on T2-weighted images, high signal is indicative of surrounding vasogenic edema or necrosis
Prominent inhomogeneous enhancement is seen is typical feature of GBM. The vasogenic edema is produced by abnormal neoplastic vessel
In a small percentage of GBM, not more than 2%, a sarcomatous component is apparent, making, macroscopically, a relatively discreet appearance. These tumors arecharacterized as gliosarcomas.
Gliomatosis Cerebri
GC appears as a poorly defined hypo- or isointense lesion Proton-density and T2-weighted images show mild to moderate high signal intensity.
Metastatic deposits
Metastatic CNS tumors primarily affect the brain parenchyma, and to less extent the meninges or in the epidural intracranial
On precontrast T1-weighted MR studies, metastatic brain tumors are usually isointense with respect to normal gray matter and are not clearly visible. Tumor with areas of necrosis can be seen on precontrast studies showing decreased signal intensity in the necrotic region Low signal intensity is also present in regions of peritumoral edema
On T2-weighted MR studies, brain tumors are hyperintense with respect to normal brain due to increase in the T2 value of the overhydrated tumor cells
lymphoma
On T1-weighted MR images, lymphoma is homogeneous and iso- or slightly hypointense relative to the cortex. On T2-weighted images, PCNSL is usually hypo- or isointense to the cortex, and surrounded by mild hyperintense edema
On postcontrast T1WI, lymphoma shows marked homogeneous enhancement
Diffusion weighted imaging of brain tumors
Gliomas
The signal intensity of gliomas on DW images is variable and depends mainly on their T2 and apparent diffusion coefficient (ADC) values.
Thus, some gliomas are hyperintense on DW images with decreased ADC, which reflects a reduced volume of the extracellular space. Other gliomas have a normal or increased ADC, that is the DW signal is a T2 shinethrough effect
A majority of lymphomas are iso- or hypointense on T2-weighted images. Most lymphomas show homogeneous enhancement in immunocompetent patients,but in immunosuppressed patients a rim enhancement is a more common finding
The enhancing components of lymphomas are generally hyperintense on DW images . The ADC of lymphomas is often lower than that in high-grade gliomas
Metastases
The signal intensity of non-necrotic components of metastases on DW images is variable and depends on their T2 and ADC
The common signal intensity of necrotic/cystic components of cerebral metastases may relate to an increase in free water, showing hypointensity on DW images and increased ADC.However, in the presence of extracellular met-hemoglobin and/or increased viscosity, DW images can show hyperintensity with decreased ADC
MR Spectroscopy of Brain Tumors
MRS provides important supplementary information to that of conventional MR imaging. In normal brain, the principal metabolite signals that can be measured by MRS are choline (Cho), creatine (Cr), N-acetyl aspartate (NAA), and lactate (Lac)
The basic metabolite changes common to brain tumors include elevation in choline (Cho), lactate (Lac), lipids (L), decrease in N-acetyl aspartate (NAA), and decrease in creatine (Cr)
As a general rule, as malignancy increases, NAA and creatine decrease, and choline, lactate, and lipids increase. Myo-inositol can also be used to differentiate LGGs and HGGs. LGGs express higher levels of myo-inositol compared with HGGs. This may be due to the lack of activation of phosphatidylinositol metabolism resulting in accumulation of myo-inositol in LGGs
Hint on Differentiating high grade malignant from inflammatory brain lesions
With conventional magnetic resonance imaging (MRI), typical brain abscess has low signal intensity on T1-weighted images, high signal intensity on T2-weighted images, and ring enhancement after contrast medium administration. However, cystic or necrotic brain tumors may have very similar pictures, making the differential diagnosis difficult. Diffusion-weighted imaging (DWI) and proton (1H) MR spectroscopy (MRS) are results of recent progress in MR technique, and could serve as useful diagnostic modalities for between brain abscesses and cystic or necrotic brain tumors.
We will divide the following discussion into three parts:
¾DWI of Pyogenic Brain Abscesses and Necrotic Brain Tumors
¾1H MRS of Pyogenic Brain Abscesses and Necrotic Brain Tumors;
¾DWI vs. 1H MRS of Pyogenic Brain Abscesses and Necrotic Brain Tumor.
DWI of Pyogenic BrainAbscesses and Necrotic Brain Tumor
In 1996, Ebisu et al. first reported case of brain abscess with high signal intensity on DWI and a low ADC value. The pus itself (verified by aspiration) could account for the restricted diffusion and therefore high DWI signal intensity. The other studies showed very similar or the same results.
All tumors show low signal intensities on DWI in the cystic or necrotic area, and their cystic or necrotic areas had high ADC values
In summary, DWI may be a much more valuable diagnostic tool in the differential diagnosis between brain abscesses and cystic or necrotic brain tumors than conventional MRI. However, there are some conflicting DWI reports in the literature, and we still need to be cautious about making the diagnosis with DWI findings.
1H MRS of pyogenic brain abscess and necrotic brain tumors
Findings from several studies have suggested that in vivo proton magnetic resonance spectroscopy (1H MRS), a non-invasive examination, might contribute to the establishment of the differential diagnosis between brain tumors and abscesses
The predominant resonance lines (N-acetyl-aspartate, choline, and reatine/phosphocreatine) that are usually observed in the parenchyma of the normal brain were hardly detectable in either tumor or abscess necrosis. The main characteristic features of pyogenic abscesses are the resonances of amino acids (valine, leucine, and isoleucine) (0.9 ppm), acetate (1.9 ppm), alanine (1.5 ppm), lactate (1.3 ppm), and succinate (2.4 ppm) .
In contrast, Spectra in patients with cystic or necrotic tumor showed only the peak attributed to lactate (1.3 ppm) . Lipid (0.8-1.3 ppm) may also be found in patients with a tumor. Lactate and lipids are nonspecific metabolites produced by anaerobic glycolysis and necrotic brain tissue in brain abscesses.
Both lactate and lipids peaks can also be observed in necrotic tumors. Increases in acetate and succinate presumably originate from the enhanced glycolysis and fermentation by the infecting microorganisms
Discrimination between amino acids (i.e., valine, leucine, and isoleucine at 0.9 ppm) and lipid (at 0.8 to 1.2 ppm) is important. Lipid signals may exist in both brain tumors and abscesses, whereas amino acids are not seen in proton MR spectra of brain tumors in vivo and are only detectable in vitro
It is known that with an echo time of 135, phase inversion occurs as a result of coupling between lactate and amino acids, but not in lipid, a feature which may be helpful in differential diagnosis between brain abscess and tumor
DWI vs. 1H MRS of pyogenic brain abscess and necrotic brain tumor
DWI is a more practical and accurate method compared with 1H MRS
There are several reasons for this conclusion. First, DWI scan time is very short (about 40 sec), whereas MR spectroscopy takes a much longer scan time..
Second, single-voxel 1H MRS is more limited in voxel size (roughly 1.5x1.5 x 01.5-cm to 2x2x 2 -cm) than DWI, and the smaller lesion in MRS will be more affected by the partial volume effect.. Third, the treated pyogenic abscess may have high signal on DWI, whereas MR spectroscopic examination shows only lactate/lipid peaks (no amino acid peak [end-products of bacterial breakdown, a spectrum the same as what is found in necrotic tumor. Nevertheless, there are some advantages of 1H MRS.
First, 1H MRS may provide valuable information on the changes of metabolites after nonsurgical treatment of brain abscesses, and may thus helpful for the evaluation of the efficacy of antimicrobial therapy
Second, 1H MRS may differentiate anaerobic from aerobic brain abscesses on the basis of metabolite patterns. This would be an important information for prompt and appropriate treatment of patients with brain abscesses
. In summary, DWI and 1H MRS are useful tools for the differential diagnosis between brain abscesses and cystic or necrotic brain tumors. The combination of DWI and 1H MRS modalities may provide a more accurate diagnosis of brain abscesses and cystic or necrotic brain tumors than the use of only a single modality.