H proton MR spectroscopy and diffusion-weighted imaging in discrimination between pyogenic brain abscesses and necrotic brain tumors

Background

Differentiation between cerebral abscesses and necrotic brain tumors showing ring enhancement can be confusing at times by conventional MRI. The introduction of advanced imaging techniques, such as MR spectroscopy and diffusion WI, have contributed to the differentiation.The purpose of this study is to test the hypothesis that MR spectroscopy and diffusion weighted can be used to differentiate between necrotizing or cystic brain tumor and brain abscesses.

Methods

The study was conducted on 45 patients (necrotic or cystic tumor (30 cases); brain abscess (15 cases) showing ring-shaped contrast enhancement on conventional MRI. 1.5-T ¹H-MR Spectroscopy and diffusion WI were performed and the results were ensured by stereotactic biopsy or aspiration procedures in surgically indicated cases and/or follow up.

Results

14 patients (out of 15) with pyogenic abscess had lactate, amino acids, and acetate peaks; Succinate peak is seen as extra peak in three of these patients, and lipid peaks are also seen as extra peaks in 3 patients. One patient with brain stem abscess after 20 days treatment by antibiotics shows only lactate and lipid peaks. 2 of them show mild increase in choline with decrease in NAA (brain tissue contamination).17 out of 30 patients with cystic or necrotic tumor showed only lactate peak in MRS. While 13 patients show lactate and lipid peaks, four of them show additional high choline peak with low NAA and creatine peak (contamination with brain tissue).The results were confirmed by Sterotactic biopsy in 27 cases and aspiration in 13 cases and follow up for all cases.The sensitivity, specificity, PPV, NPV and overall accuracy of diffusion and MRS were 88%, 100%, 100%, 93.3% and 95.5% respectively.

Conclusion

¹H-MRS and diffusion WI are fast, easy to perform, noninvasive, and provide additional information that can accurately differentiate between necrotic/cystic tumors and cerebral abscesses.     

Discrimination of an infected brain tumor from a cerebral abscess by combined MR perfusion and diffusion imaging.

The aim of this study was to investigate the signal characteristics of the abscess wall and tumor wall on diffusion-weighted and perfusion-weighted images and thus to evaluate the feasibility of using combined MR diffusion and perfusion imaging to differentiate pyogenic cerebral abscess from infected brain tumor. The tumor wall of various types of cystic or necrotic brain tumor was significantly hyperintense relative to that of cerebral abscess wall on both diffusion-weighted images and regional cerebral blood volume maps.Sixteen patients who had cerebral masses with large cystic or necrotic cavities were imaged to generate diffusion-weighted images and regional cerebral blood volume maps using single-shot echoplanar imaging (EPI) pulse sequences. Apart from qualitative analysis, apparent diffusion coefficients (ADC) as well as regional cerebral blood volume (rCBV) ratios were calculated from the abscess wall and peripheral tumor wall and comparison was made by using Student's t-test. The tumor wall of various types of cystic or necrotic brain tumor had significantly lower ADCs relative to those of the abscess wall (P<0.005) and thus appeared relatively hyperintense on diffusion-weighted images. The mean rCBV ratio relative to normal white matter (2.90+/-0.62) of the peripheral tumor wall of various types of cystic or necrotic brain tumor were significantly larger than the mean rCBV ratio (0.45+/-0.11) of the pyogenic cerebral abscess wall (P<0.001) by Student's t-test. It is concluded that the combined MR diffusion and perfusion imaging might be capable of differentiating an infected brain tumor from a pyogenic cerebral abscess.     

Diffusion-weighted MRI features of brain abscess and cystic or necrotic brain tumors: comparison with conventional MRI

BACKGROUND AND PURPOSE: The purpose of this study was to determine whether diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) can be used to distinguish brain abscesses from cystic or necrotic brain tumors, which are difficult to distinguish by conventional magnetic resonance imaging (MRI) techniques. METHODS: Eleven consecutive patients with brain abscesses [10 pyogenic and 1 toxoplasmosis (in an AIDS patient)] and 15 with cystic or necrotic brain gliomas or metastases were enrolled in this study. None of these lesions had apparent hemorrhage based on T1-weighted image (T1WI). The DWI was performed using a 1.5-T system, single-shot spin-echo echo-planar pulse sequence with b=1000 s/mm(2). The ADC was calculated using a two-point linear regression method at b=0 and b=1000 s/mm(2). The ratio (ADCR) of the lesion ADC to control region ADC was also measured. RESULTS: Increased signal was seen in all of the pyogenic abscess cavities to variable degrees on DWI. In vivo ADC maps showed restricted diffusion in the abscess cavity in all pyogenic abscesses [0.65+/-0.16 x 10(-3) (mean+/-S.D.) mm(2)/s, mean ADCR=0.63]. The case with multiple toxoplasmosis abscesses showed low signal intensity on DWI and high ADC values (mean 1.9 x 10(-3) mm(2)/s, ADCR=2.24). All cystic or necrotic tumors but one showed low signal intensity on DWI and their cystic or necrotic areas had high ADC values (2.70+/-0.31 x 10(-3) mm(2)/s, mean ADCR=3.42). One fibrillary low-grade astrocytoma had a high DWI signal intensity and a low ADC value in its central cystic area (0.44 x 10(-3) mm(2)/s, ADCR=0.49). Postcontrast T1WIs yielded a sensitivity of 60%, a specificity of 27.27%, a positive predictive value (PPV) of 52.94%, and a negative predictive value (NPV) of 33.33% in the diagnosis of necrotic tumors. DWI yielded a sensitivity of 93.33%, a specificity of 90.91%, a PPV of 93.33%, and a NPV of 90.91%. The area under receiver operating characteristic (ROC) curves for postcontrast T1WI was 0.44 and DWI was 0.92. Analysis of these areas under the ROC curves indicates significant difference between postcontrast T1WI and DWI (P<.001). CONCLUSION: With some exceptions, DWI is useful in providing a greater degree of confidence in distinguishing brain abscesses from cystic or necrotic brain tumors than conventional MRI and seems to be a valuable diagnostic tool.     

Differentiation of tuberculous from pyogenic brain abscesses with in vivo proton MR spectroscopy and magnetization transfer MR imaging

BACKGROUND AND PURPOSE: MR imaging features are nonspecific with respect to the causative organism for patients with brain abscesses. On the basis of the hypothesis that the biochemical environment depends on the infecting organism and might be different in tuberculous compared with pyogenic brain abscesses, this study attempted to determine whether pyogenic brain abscesses can be differentiated from tuberculous brain abscesses by use of magnetization transfer (MT) MR imaging and in vivo proton MR spectroscopy. METHODS: Twenty-seven patients with a total of 33 pyogenic brain abscesses and three patients with a total of 12 tuberculous abscesses were evaluated with in vivo MR spectroscopy and MT MR imaging. The diagnosis in all cases was based on the culture of the causative organisms and histopathology whenever done as a part of clinical management. RESULTS: All 27 patients with pyogenic brain abscesses had lipid and lactate levels of 1.3 ppm and amino acid levels of 0.9 ppm with or without the presence of succinate, acetate, alanine, and glycine, while the three patients with tuberculous abscesses showed only such lipid and lactate levels. The MT ratio from the wall of the pyogenic abscesses was significantly higher (P <.001) than that from the tuberculous abscess wall. CONCLUSION: It might be possible to differentiate tuberculous abscesses from pyogenic abscesses by using MT MR imaging and in vivo MR spectroscopy, which could be of value in influencing the management of such cases.     

Pyogenic brain abscess: findings from in vivo 1.5-T and 11.7-T in vitro proton MR spectroscopy

BACKGROUND AND PURPOSE: Metabolites in pyogenic brain abscesses, as detected with in vivo proton MR spectroscopy, are different from those found in brain and can help differentiate pyogenic brain abscesses from necrotic neoplasms. We compared the findings of in vivo with those of in vitro MR spectroscopy and categorized the MR spectral patterns with respect to the causative organisms and abscess size. METHODS: Fifteen patients with pyogenic brain abscesses underwent in vivo 1.5-T (1)H MR spectroscopy and had findings of ring enhancement. The causative organisms were determined from cultures of aspirated pus. Single-voxel (1)H MR spectroscopy was performed with the point-resolved method (1600/270, 135 TR/TE). In six representative patients, in vitro 11.7-T (1)H MR spectra were obtained from the aspirated pus. RESULTS: Three in vivo MR spectral patterns were noted: A) presence of lactate at 1.3 ppm, cytosolic amino acids (leucine, isoleucine, and valine) at 0.9 ppm, alanine at 1.50 ppm, and acetate at 1.92 ppm, with the presence or absence of succinate at 2.4 ppm and lipids (0.8-1.3 ppm), representing mostly obligate anaerobes or a mixture of obligate and facultative anaerobes; B) presence of lactate at 1.3 ppm and cytosolic amino acids at 0.9 ppm, with the presence or absence of lipids but not acetate or alanine (0.8-1.3 ppm), representing mostly obligate aerobes or facultative anaerobes; and C) presence of lactate at 1.3 ppm alone, showing small abscess. Additional resonance peaks of lysine at 1.73 and 3.0 ppm, glutamate/glutamine at 2.09-2.36 ppm, taurine at 3.24 and 3.42 ppm, glycine at 3.55 ppm, and amino acids at 3.75 ppm could be observed in the in vitro MR spectra. CONCLUSION: Results from the in vivo observations were satisfactorily verified by the in vitro experiments. The in vitro measurements may offer complementary information that cannot be extracted from in vivo MR spectra. Determination of the three (1)H MR spectral patterns may be helpful in devising the best possible treatment plans for patients with pyogenic abscesses.     

Brain abscess and cystic brain tumor: discrimination with dynamic susceptibility contrast perfusion-weighted MRI

Differentiating between brain abscesses and cystic brain tumors such as high-grade gliomas and metastases is often difficult with conventional MRI. The goal of this study was to evaluate the diagnostic utility of perfusion MRI to differentiate between these pathologies. MRI was performed in 19 patients with rim-enhancing brain lesions (4 pyogenic abscesses, 8 high-grade gliomas, 7 metastases). In addition to standard MR sequences, trace diffusion-weighted MRI with apparent diffusion coefficient (ADC) maps and perfusion-weighted MRI by using a first-pass gadopentetate dimeglumine T2*-weighted gradient echo single-shot echo-planar sequence were performed. Relative cerebral blood volume (rCBV) ratios were obtained via the values of the capsular portions of the lesions and the normal white matter. All the abscesses had markedly hyperintense signals in trace diffusion images, whereas they had significant hypointense signals in ADC images. In perfusion-weighted images, the capsular portions of the abscesses demonstrated low colored areas compared with the normal white matter and the rCBV ratio calculated was 0.76 +/- 0.12 (mean +/- SD). All but two of the cystic tumors showed low signal intensity on trace diffusion-weighted images and high signal intensity on ADC maps. Hyperintense signal was found in two brain tumors mimicking brain abscesses on trace diffusion images. The rCBV values in high-grade gliomas and metastases were 5.51 +/- 2.08 and 4.58 +/- 2.19, respectively. The difference between abscesses and cystic tumors was statistically significant (P = 0.003). Perfusion MRI may allow the differentiation of pyogenic brain abscess from cystic brain tumors, making it a strong additional imaging modality in the early diagnosis of these two entities.     

Hemorrhagic brain metastases with high signal intensity on diffusion-weighted MR images. A case report

Diffusion-weighted MR imaging has been applicable to the differential diagnosis of abscesses and necrotic or cystic brain tumors. However, restricted water diffusion is not necessarily specific for brain abscess. We describe ring-enhancing metastases of lung carcinoma characterized by high signal intensity on diffusion-weighted MR images. The signal pattern probably reflected intralesional hemorrhage. The present report adds to the growing literature regarding the differential diagnosis of ring-enhancing brain lesions.     

Role of diffusion-weighted MR imaging in discrimination between the intracranial cystic masses

Objective

Discriminating pyogenic brain abscesses from cystic or necrotic tumors is sometimes difficult with CT or conventional MR imaging. Diffusion MR imaging is a valuable diagnostic test in cases of intracranial cystic masses.

Methods

This work was conducted from July 2008 to June 2013 on 90 patients; 43 males and 47 females. Their ages range from 5 to 70 years. All patients were subjected to routine MRI examination and diffusion weighted imaging using 1.5 T MRI scanner. Gadolinium was given to some cases on routine MRI. Diffusion weighted imaging was performed with a single-shot spin-echo echo-planar pulse sequence (b = 0–1000 s/mm²). The apparent diffusion coefficient values and ratio were measured.

Results and conclusions

Patients in this study were categorized into three main groups; first group is brain abscesses (36 cases), 91.6% of them showed restricted diffusion, second group is malignant cystic or necrotic brain tumors, 28 cases of high grade necrotic glioma, 60.7% of them are free diffusion, and third group is benign cystic masses, arachnoid and epidermiod cysts (11 cases); all arachnoid cysts are free diffusion. From these results diffusion-weighted imaging is playing an important role in discrimination of cystic intracranial masses.     

Role of diffusion weighted imaging and proton magnetic resonance spectroscopy in ring enhancing brain lesions

Aim of the work

To assess and compare the usefulness and efficacy of both diffusion weighted imaging (DWI) and proton magnetic resonance spectroscopy (¹HMRS) in brain lesions with ring enhancement in post contrast T1WI and to determine which method is more effective.

Subjects and methods

Thirty patients with ring-enhanced brain lesions were classified into 2 groups, abscess group (11 patients) and tumor group (19 patients), were examined using diffusion-weighted imaging (DWI) and H-proton magnetic resonance spectroscopy (¹HMRS).

Results

Restricted diffusion and low ADC value were seen in 9 (81%) patients of brain abscesses, however, free diffusion and high ADC value were found in 18 (94%) patients with necrotic brain tumor. The abscess group showed aminoacids, acetate and lactate in 9 patients and extra peak of succinate was found in 1 patient; however in the tumor group lactate alone was found in 12 patients, lactate and choline were seen in 5 patients, none of the patients showed amino acids, succinate or acetate.

Conclusion

Both DWI and ¹HMRS are useful and efficient imaging techniques in ring enhancing brain lesions and differentiate between pyogenic brain abscesses and necrotic tumors, but DWI is accurate, has less imaging time than ¹HMRS, also is available in many imaging centers.     

Comparative evaluation of fungal, tubercular, and pyogenic brain abscesses with conventional and diffusion MR imaging and proton MR spectroscopy

BACKGROUND AND PURPOSE: It is difficult to differentiate the cause of brain abscesses with the use of CT and MR imaging. We did a comparative evaluation of pyogenic, tubercular, and fungal brain abscesses by using conventional, diffusion-weighted imaging (DWI), and proton MR spectroscopy (PMRS) with an aim to define the unique features that may differentiate among the pyogenic, tubercular, and fungal brain abscesses. MATERIALS AND METHODS: We performed a retrospective analysis on 110 patients with surgically proved brain abscesses. Imaging studies included T2, T1, postcontrast T1, DWI, and PMRS. Apparent diffusion coefficient (ADC) of the wall and cavity of the abscesses were quantified. The morphologic, physiologic, and metabolite features of pyogenic (n=91), tubercular (n=11), and fungal (n=8) abscesses were compared. RESULTS: The pyogenic abscesses had smooth (55/91) and lobulated (36/91) walls, whereas the tubercular abscesses had smooth (4/11), lobulated (6/11), or crenated walls (1/11) with no intracavitary projections. The fungal abscesses showed irregular walls (lobulated 4/8, crenated 4/8) with intracavitary projections (8/8). The wall as well as the cavity showed low ADC in the pyogenic and tubercular abscesses. In the fungal abscesses, the wall and projections showed low ADC (8/8); however, the cavity itself showed high ADC (8/8). PMRS showed cytosolic amino acids (89/91), acetate (25/91), and succinate (18/91) in the pyogenic abscesses, whereas lipid/lactate (11/11) was seen in the tubercular abscesses. The fungal abscesses showed lipid (4/8), lactate (7/8), amino acids (4/8), and multiple peaks between 3.6 and 3.8 ppm assigned to trehalose (5/8). CONCLUSION: Based on the morphologic, ADC, and metabolite information, it may be possible to differentiate among the pyogenic, tubercular, and fungal brain abscesses.     

Use of diffusion-weighted MR imaging in differential diagnosis between intracerebral necrotic tumors and cerebral abscesses.

The differential diagnosis between intracerebral necrotic tumors and cerebral abscesses is frequently impossible with conventional MR imaging. We report two cases of cerebral abscesses that showed high signal on diffusion-weighted echo planar imaging and a strongly reduced apparent diffusion coefficient. This appearance was not present in our cases of necrotic/cystic gliomas (eight cases) and necrotic metastases (two cases). We believe that diffusion-weighted MR imaging may be a diagnostic clue in cases of cerebral "ring-enhancing" masses.     

In vivo differentiation of aerobic brain abscesses and necrotic glioblastomas multiforme using proton MR spectroscopic imaging

BACKGROUND AND PURPOSE: Abscesses caused by aerobic bacteria (aerobic abscesses) can simulate intracranial glioblastomas multiforme (GBMs) in MR imaging appearance and single voxel (SV) proton MR spectroscopy of the central cavity. The purpose of our study was to determine whether MR spectroscopic imaging (SI) can be used to differentiate aerobic abscesses from GBMs. Our hypothesis was that metabolite levels of choline (Cho) are decreased in the ring-enhancing portion of abscesses compared with GBMs.MATERIALS AND METHODS: Fifteen patients with aerobic abscesses were studied on a 1.5T MR scanner using an SV method and an SI method. Proton MR spectra of 15 GBMs with similar conventional MR imaging appearances were used for comparison. The resonance peaks in the cavity, including lactate, cytosolic amino acids, acetate, succinate, and lipids, were analyzed by both SV MR spectroscopy and MRSI. In the contrast-enhancing rim of each lesion, peak areas of N-acetylaspartate (NAA), choline (Cho), lipid and lactate (LL), and creatine (Cr) were measured by MRSI. The peak areas of NAA-n, Cho-n, and Cr-n in the corresponding contralateral normal-appearing (-n) brain were also measured. Maximum Cho/Cr, Cho/NAA, LL/Cr-n, and Cho/Cho-n and minimum Cr/Cr-n and NAA/NAA-n ratios in abscesses and GBMs were compared using the Wilcoxon rank sum test. After receiver operating characteristic curve analysis, diagnostic accuracy was compared.RESULTS: Cytosolic amino acid peaks were found in the cavity in 7 of 15 patients with aerobic abscesses. Means and SDs of maximum Cho/Cr, Cho/NAA, LL/Cr-n, and Cho/Cho-n and minimum Cr/Cr-n and NAA/NAA-n ratios were 3.38 ± 1.09, 3.88 ± 2.13, 2.72 ± 1.45, 1.98 ± 0.53, 0.53 ± 0.16, and 0.44 ± 0.09, respectively, in the GBMs, and 1.77 ± 0.49, 1.48 ± 0.51, 2.11 ± 0.67, 0.81 ± 0.21, 0.48 ± 0.2, and 0.5 ± 0.15, respectively, in the abscesses. Significant differences were found in the maximum Cho/Cr (P = .001), Cho/NAA (P = .006), and Cho/Cho-n ratios (P < .001) between abscesses and GBMs. Diagnostic accuracy was higher by Cho/Cho-n ratio than Cho/Cr and Cho/NAA ratios (93.3% versus 86.7% and 76.7%).CONCLUSION: Metabolite ratios and maximum Cho/Cho-n, Cho/Cr, and Cho/NAA ratios of the contrast-enhancing rim were significantly different and useful in differentiating aerobic abscesses from GBMs by MRSI.

Brain abscess can be a lethal condition if appropriate treatment is delayed. Thus, early diagnosis of brain abscess is desired and is a challenge for clinicians and radiologists. Radiologically, a brain abscess in the capsule stage appears in CT and in MR imaging as an expansile, rim-enhancing mass surrounded by edema, which is similar in appearance to necrotic malignant tumors, especially glioblastoma multiforme (GBM).^1,2 Clinically, both brain abscesses and GBMs may cause nonspecific headaches in the absence of fever, focal neurologic deficits, epileptic seizures, and disturbances in higher-level cortical function. In addition, laboratory examination often shows normal white blood cell count.^1,2 Further reduction of mortality from brain abscesses requires more rapid, accurate, and safe diagnostic techniques. Application of diffusion-weighted imaging (DWI) in distinguishing between pyogenic brain abscesses and cystic or necrotic brain tumors has been reported to be useful in many publications.^3–8 The cystic or necrotic portion of tumors almost always has a low signal intensity on DWI and a higher apparent diffusion coefficient (ADC) value; however, some exceptions have been reported for necrotic brain tumors.^7–12 Hyperintensity with restricted diffusion is diagnostic, but not pathognomonic, for pyogenic abscess on DWI. However, exceptions of DWI studies and substantial variability in the ADCs have been reported for pyogenic abscesses.^8,12–16Single-voxel proton MR spectroscopy of the central cystic portion has been reported to allow the broad group of pyogenic abscesses to be distinguished from malignant gliomas.^17–22 Acetate, succinate, and amino acids have been identified in cerebral abscesses in humans and were used as bacterial marker metabolites for noninvasive diagnosis by MR spectroscopy.^17–22 Abscesses caused by anaerobic bacteria have been distinguished from those caused by aerobic bacteria based on metabolites detectable with MR spectroscopy (acetate and succinate).^21–23 However, spectral patterns recorded for the cystic or necrotic components of GBMs and abscesses caused by aerobic bacteria (aerobic abscesses) were similar by single-voxel MR spectroscopy.^18,21,24 Diagnosis based on such subjectively selected metabolites or metabolite ratios was not possible.²⁴At pathologic examination, the enhancing rim of GBMs represents infiltrating tumor cells,^25,26 and an increased Cho/Cr ratio was observed. Pathologically, the enhancing rim of a pyogenic abscess represents an inflammatory infiltrate composed of neutrophils and, later, macrophages and lymphocytes. Granulation tissue surrounds the area of inflammation and eventually develops into a fibrous capsule. This capsule, in turn, is surrounded by gliotic, edematous brain tissue. In the experimental setting, brain abscesses have been shown to have relatively high amounts of mature collagen and decreased neovascularity.^27,28 The Cho/Cr ratio of rim-enhancing lesion of abscesses would be presumed to be less than that of GBMs.Recent developments in MR spectroscopy have made it possible to obtain spectroscopic MR imaging with high spatial resolution and multiple spectra simultaneously from contiguous voxels.^29,30 To date, proton MR spectroscopic imaging has not been used to differentiate aerobic abscesses from GBMs. We aimed to test the feasibility of MR spectroscopic imaging to distinguish between aerobic abscesses and GBMs by the evaluation of the contrast-enhancing rim of the lesions. Our hypothesis was that metabolite levels of Cho are decreased in the rim-enhancing portion of aerobic abscesses relative to the Cho seen in the rim-enhancing portion of necrotic GBMs.     

Role of diffusion-weighted imaging and in vivo proton magnetic resonance spectroscopy in the differential diagnosis of ring-enhancing intracranial cystic mass lesions

OBJECTIVES: Proton magnetic resonance spectroscopy (PMRS) and diffusion-weighted imaging (DWI) were compared to determine which technique is more effective in the differential diagnosis of cystic intraparenchymal ring-enhancing lesions with variable perifocal edema. METHODS: Fifty-two patients (abscesses [n = 29], tumor cysts [n = 20], and benign cysts [n = 3]) formed the basis for comparative evaluation in this study. The criteria for abscess diagnosis were apparent diffusion coefficient (ADC) values less than 0.9 +/- 1.3 x 10 mm/s and presence of lactate cytosolic amino acids (AAs) with/without succinate, acetate, alanine, and glycine on PMRS. Criteria for nonabscess cyst identification were ADC values of 1.7-3.8 x 10 mm/s and presence of lactate and choline on PMRS. On the basis of these criteria, patients were categorized into abscess (n = 29) and nonabscess (n = 23) groups. Sensitivity and specificity of PMRS and DWI with respect to the final diagnosis were calculated based on the efficacy of these techniques. RESULTS:: Apparent diffusion coefficient values in 21 patients with abscesses were observed within the range of defined criteria, whereas in 8 patients, ADC values were beyond the range of defined criteria. Lactate and AAs with or without other metabolites were observed in 25 of 29 cases of abscesses on PMRS. In the nonabscess group, ADC values of cystic lesions in all patients were consistent with respect to the defined criteria. Only lactate was seen in 14 of 23 patients, whereas both lactate and choline were visible in 6 patients. In 3 patients with neurocysticercosis, AAs (n = 2), lactate (n = 3), acetate (n = 1), succinate (n = 3), choline (n = 2), and alanine (n = 3) were seen. The sensitivity of DWI and PMRS for differentiation of brain abscess from nonbrain abscess was 0.72 and 0.96, respectively, whereas the specificity was 1 for both techniques. CONCLUSION: Demonstration of restricted diffusion on DWI with reduced ADC is highly suggestive of brain abscess; however, in the absence of restriction, PMRS is mandatory to distinguish brain abscesses from cystic tumors.     

Magnetization transfer imaging diagnosis of intracranial tuberculomas

Tuberculomas (TB) can mimic brain tumors and abscesses. We performed prospective magnetization transfer ratio (MTR) analysis on 60 patients with 52 intracranial TB, 13 pyogenic abscesses and 65 tumors to determine the efficacy of magnetization transfer (MT) imaging in the differential diagnosis of brain tumors and infection. The nonenhancing cores of TB had significantly higher (P=.026) MTR (0.14+/-0.29) than necrotic components of high-grade gliomas (-0.19+/-0.22). The MTR of cores of TB were also higher than those of the cystic areas of low-grade gliomas (-0.53+/-0.32), benign (-0.09+/-0.21) and malignant (-0.07+/-0.25) tumors, and abscesses (-0.03+/-0.13), but the differences were not significant because of the small number of tumors and abscesses. There was also no significant difference between the MTR of abscesses, malignant and benign tumors. Using the criteria of MTR of necrotic center > 0.14 (mean MTR of TB) for diagnosing TB, MTR <-0.03 (mean MTR of abscesses) for diagnosing tumors and MTR between these values for diagnosing abscesses, MTI had diagnostic sensitivity of 68.42%, specificity of 80.49%, and accuracy of 76.67%. The improved diagnostic accuracy of MRI with the addition of MTR analysis from 86.67% to 91.67% and from 85.71% to 87.50% for both radiologists respectively was not significant. MTR analysis helped us to differentiate solitary TB or abscess from low-grade glioma in five patients and to diagnose multiple TB, abscesses, and metastases in four.     

Diffusion-weighted echo-planar MR imaging in differential diagnosis of brain tumors and tumor-like conditions

We assess diffusion-weighted MR images in the differential diagnosis of intracranial brain tumors and tumor-like conditions. Heavily diffusion-weighted (b = 1100 or 1200 s/mm²) axial images were obtained with single-shot echo-planar technique in 93 patients with pathologically confirmed various intracranial tumors and tumor-like conditions with diffusion gradient perpendicular to the images. We compared signal intensity of the lesions with those of gray and white matter, and cerebrospinal fluid (CSF). In 29 cases (31.1 %) the lesions were isointense to gray and/or white matter. However, 5 cases (5.4 %) showed extremely increased signal intensity: two epidermoid cysts; two chordomas; and one brain abscess. The entire portion of a tumor was markedly hyperintense in 10 cases (10.8 %): four malignant lymphomas; four medulloblastomas; one germinoma; and one pineoblastoma. A CSF-like hypointense signal was seen in many cystic tumors, and cystic or necrotic portions of tumors. A neurosarcoid granulation was the only solid lesion showing characteristically a hypointense signal like CSF. The combination of markedly hyperintense and hypointense signals was seen generally in hemorrhagic tumors. Diffusion-weighted echo-planar MR imaging is useful in the differential diagnosis of brain tumors and tumor-like conditions, and suggests specific histological diagnosis in some cases. Received: 30 July 1999; Revised: 2 November 1999; Accepted: 9 December 1999     

Differentiation of pyogenic and fungal brain abscesses with susceptibility-weighted MR sequences

Introduction

Conventional magnetic resonance imaging (MRI) techniques are insufficient to determine the causative agent of brain abscesses. We investigated: (1) the value of susceptibility-weighted MR sequences (SWMRS) in the differentiation of fungal and pyogenic brain abscesses; and (2) the effect of different SWMRS (susceptibility-weighted imaging (SWI) versus venous blood oxygen level dependent (VenoBOLD)) for the detection of specific imaging characteristics of pyogenic brain abscesses.

Methods

We studied six patients with fungal and ten patients with pyogenic brain abscesses. Imaging characteristics on conventional MRI, diffusion-weighted imaging (DWI) and SWMRS were recorded in all abscesses. All lesions were assessed for the presence of a “dual-rim sign” on SWMRS.

Results

Homogenously hyperintense lesions on DWI were present in 60 % of patients with pyogenic abscesses, whereas none of the patients with fungal abscesses showed such lesions. On SWMRS, 90 % of patients with pyogenic abscesses and 60 % of patients with fungal abscesses had only lesions with a low-signal-intensity rim. On SWI, the dual-rim sign was apparent in all pyogenic abscesses. None of the fungal abscesses on SWI (P?=?0.005) or any of the pyogenic abscesses on VenoBOLD (P?=?0.005) were positive for a dual-rim sign.

Conclusions

In fungal abscesses, the dual-rim sign is not present but a prominent peripheral rim or central susceptibility effects on SWI will be seen. The appearance of pyogenic abscesses on SWMRS depends on the used sequence, with the dual-rim sign a specific feature of pyogenic brain abscesses on SWI.     

Diffusion weighted MR: principles and clinical use in selected brain diseases

PURPOSE: To define the principles and technical bases of diffusion weighted MR imaging of the brain and report our experience in the evaluation of selected brain disorders including age-related ischemic white matter changes (leukoaraiosis), neoplastic and infective cysts and wallerian degeneration. MATERIAL AND METHODS: Between May 1999 and June 2000 we examined seventeen patients: 10 patients with leukoaraiosis and deterioration of cognitive and motor function, 5 patients with focal cystic lesions (one anaplastic astrocytoma, one glioblastoma, one metastasis from squamous cell lung carcinoma, one pyogenic abscess and one case with cerebral tubercolosis) and 2 patients with wallerian degeneration (one with post-hemorrhagic degeneration of right corticospinal tract and one with post-traumatic degeneration of left optic tract). All patients underwent a standard cranial MR examination including SE T1-, proton density, T2-weighted, FLAIR and diffusion weighted images. Post-contrast T1-weighted sequences were also obtained in the patients with cystic lesions. Diffusion weighted images were acquired with double shot echoplanar sequences. Diffusion sensitizing gradient along the x, y and z axes and b values ranging 800 to 1200 s/mm2 were used. For each slice a set of three orthogonal diffusion "anisotropic" images, an "isotropic" image and a standard T2-weighted image were reconstructed. Postprocessing included generation of the apparent diffusion coefficient maps and of the "trace" image that reflects pixel by pixel the diffusional properties of water particles only. Values of mean diffusivity within regions of interest were computed in the "trace" image and compared with those obtained in contralateral brain areas. In patients with leukoaraiosis the diffusivity in posterior periventricular white matter was compared with that measured in 10 age-matched control subjects without leukoaraiosis. RESULTS: In patients with leukoaraiosis the areas of increased periventricular signal intensity on T2-weighted images showed a significantly higher (p < 0.001) diffusivity (mean values 124.7 +/- 21.3 x 10(-5) mm2/s) as compared to control subjects (mean values 85 +/- 7 x 10(-5) mm2/s). Diffusion weighted images in 2 patients revealed the presence of a small focal area of increased signal and reduced diffusivity in "trace" images consistent with recent ischemic lesion. In neoplastic cystic lesions the central necrotic/cystic content was always hypointense on diffusion weighted images and showed increased diffusivity on "trace" images. On the other hand the central necrotic content of the pyogenic brain abscess was hyperintense and showed low diffusivity. In patients with wallerian degeneration diffusion weighted images and "trace" images demonstrated loss of anisotropy and increased diffusivity in the affected white matter tract relative to the contralateral. DISCUSSION: The increased diffusivity observed in areas of leukoaraiosis and the identification of subclinical acute ischemic lesions by diffusion weighted images might be more useful than standard MR sequences for monitoring the disease progression. Diffusion weighted images allow differentiation of the different parts of focal cystic lesions (edema, solid and cystic/necrotic portion) and are useful to differentiate pyogenic brain abscess from necrotic tumors. In patients with wallerian degeneration the loss of anisotropy and the increase of diffusivity values in the affected tract are probably related to myelin breakdown and allow better recognition of the affected tract relative to standard MR images. CONCLUSIONS: Diffusion weighted MR imaging can be performed during a standard cranial MR examination and add useful clinical information in several brain disorders besides acute ischemic stroke.     

Proton MR spectroscopy in patients with pyogenic brain abscess: MR spectroscopic imaging versus single-voxel spectroscopy

Purpose

Single-voxel spectroscopy (SVS) has been the gold standard technique to diagnose the pyogenic abssess. Two-dimensional magnetic resonance spectroscopic imaging (MRSI) is able to provide spatial distribution of metabolic concentration, and is potentially more suitable for differential diagnosis between abscess and necrotic tumors. Therefore, the purpose of this study was to evaluate the equivalence of MRSI and SVS in the detection of the metabolites in pyogenic brain abscesses.

Materials and methods

Forty-two patients with pyogenic abscesses were studied by using both SVS and MRSI methods. Two neuroradiologists reviewed the MRS data independently. A κ value was calculated to express inter-reader agreement of the abscesses metabolites, and a correlation coefficient was calculated to show the similarity of two spectra. After consensus judgment of two readers, the binary value of metabolites of pyogenic abscesses (presence or absence) was compared between SVS and MRSI.

Results

The consistency of spectral interpretation of the two readers was very good (κ ranged from 0.95 to 1), and the similarity of two spectra was also very high (cc = 0.9 ± 0.05). After consensus judgment of two readers, the sensitivities of MRSI ranged from 91% (acetate) to 100% (amino acids, succinate, lactate, lipid), and the specificities of MRSI were 100% for detecting all metabolites with SVS as reference.

Conclusion

SVS and MRSI provide similar metabolites in the cavity of pyogenic brain abscess. With additional metabolic information of cavity wall and contralateral normal-appearing brain tissue, MRSI would be a more suitable technique to differentiate abscesses from necrotic tumors.     

Biological correlates of diffusivity in brain abscess.

Restricted diffusion in brain abscess is assumed to be due to a combination of inflammatory cells, necrotic debris, viscosity, and macromolecules present in the pus. We performed diffusion-weighted imaging (DWI) on 41 patients with proven brain abscesses (36 pyogenic and five tuberculous), and correlated the apparent diffusion coefficient (ADC) from the abscess cavity with viable cell density, viscosity, and extracellular-protein content quantified from the pus. On the basis of the correlation between cell density and ADC in animal tumor models and human tumors in the literature, we assumed that the restricted ADC represents the cellular portion in the abscess cavity. We calculated restricted and unrestricted lesion volumes, and modeled cell density over the restricted area with viable cell density per mm(3) obtained from the pus. The mean restricted ADC in the cavity (0.65 +/- 0.01 x 10(-3) mm(2)/s) correlated inversely with restricted cell density in both the pyogenic (r = -0.90, P = <0.05) and tuberculous (0.60 +/- 0.04 x 10(-3) mm(2)/s, r = -0.94, P = <0.05) abscesses. We conclude that viable cell density is the main biological parameter responsible for restricted diffusion in brain abscess, and it is not influenced by the etiological agents responsible for its causation.     

Role of diffusion-weighted MR in differential diagnosis of intracranial cystic lesions

AIM: The purpose of this study was to evaluate the role of diffusion-weighted imaging (DWI) in characterizing cerebral cystic lesions. The usefulness of the apparent diffusion coefficient (ADC) map in lesion characterization was also evaluated. METHODS: We compared the findings of conventional MR images with those of DWI: 63 cystic masses in 48 patients were examined with routine MR imaging and echo-planar DWI. The routine MR imaging included at least the axial T2- and T1-weighted sequences, and post-contrast T1 axial sequences. The DWI included an echo-planar spin-echo sequence with three values (0, 500 and 1000s/mm(2)) sensitizing gradient in the x, y, z direction, and it obtained an ADC map. RESULTS: The sensitivity of DWI for differentiating abscesses from primary brain tumours was 100%; for differentiating abscesses from metastatic tumours was 73%; for differentiating benign from malignant lesions was 90%. CONCLUSION: Although some metastatic lesions may appear hyperintense on DWI thus imitating an abscess, evaluation of the lesions with both DWI and conventional MRI may have an important contribution to the differentiation of tumours from abscesses.