1. Typical and Atypical Diffusion-Weighted Imaging Features in Brain Abscesses
John D. Grimme1, J. Keith Smith1,
Majda M. Thurnher2 and Mauricio Castillo1
1University of North Carolina
2University Hospital Vienna

2. OUTLINE OF CONTENTS:
Usual MRI findings for cerebral abscess, including diffusion-weighted imaging (DWI)
Examples of typical DWI appearance of cerebral abscess
Examples of atypical DWI appearances of cerebral abscess
Examples of DWI in abscesses due to atypical organisms
Examples where DWI was used in the management of patients with cerebral abscess

3. USUAL APPEARANCE OF CEREBRAL ABSCESS ON MRI
Abscess center is typically hypointense on T1-weighted images (T1WI) and hyperintense on T2-weighted images (T2WI); surrounding vasogenic edema has similar characteristics.
Abscess wall shows ring enhancement following intravenous gadolinium (Gd) administration.
As the abscess matures, the capsule shows decreased low T2 signal.

4. USUAL APPEARANCE OF CEREBRAL ABSCESS ON MRI
On trace DWI abscesses are typically hyperintense, indicating decreased diffusion of water.
This is secondary to increased viscosity of pus which contains, in addition to cellular debris and bacteria, large molecules such as fibrinogen, which bind water molecules and add to the effect of restricted diffusion.
This can be confirmed with an apparent diffusion coefficient (ADC) map where abscesses are of low signal

5. USUAL APPEARANCE OF CEREBRAL ABSCESS ON MRI
Post-Gd T1WI: Abscess in the right thalamus shows low signal intensity within its cavity and an enhancing rim. Note subtle hypointense outer rim, corresponding to edema.

6. USUAL APPEARANCE OF CEREBRAL ABSCESS ON MRI
T2WI: Same patient with right thalamic abscess. There is high signal in the abscess cavity and in the surrounding edema. Note low signal intensity in the rim surrounding the cavity which is thought to be secondary to susceptibility artifact from presence of local free radicals, and indicates a mature abscess.

7. USUAL APPEARANCE OF CEREBRAL ABSCESS ON MRI
DWI: Same patient in previous two slides. There is marked high signal intensity in the abscess corresponding to restricted diffusion of water molecules in the cavity. Note mild hyperintensity surrounding the cavity due to “T2 shine through” from edema.

8. USUAL APPEARANCE OF CEREBRAL ABSCESS ON MRI
Left and right frontal abscesses: Another example of the expected appearance of abscesses on MRI. The abscess cavities show low and high signal on T1- and T2WI, respectively. There is surrounding vasogenic edema and mature capsules. There is corresponding high signal on trace DWI. Dark signal on ADC map confirms restricted diffusion.
35-year-old male presenting with seizure, left sided weakness, and urinary incontinence. Drainage was performed and cultures grew Streptococcus anginosus.

9. ATYPICAL APPEARANCE OF BACTERIAL ABSCESS ON DWI
Mixed signal: Abscess located in the left temporooccipital region. There is a hypointense cavity with an enhancing rim on the post-Gd T1WI. FLAIR image (middle) shows high signal in the rim, surrounding tissues and in the anterior part of the cavity corresponding to areas of edema and pus. There are also isointense areas in the cavity. (Continued)

10. ATYPICAL APPEARANCE OF BACTERIAL ABSCESS ON DWI
Mixed signal: (Continued) DWI (right) shows high signal in the cavity corresponding to the region of hyperintensity on the FLAIR image and decreased signal corresponding to isointense region which indicates either free diffusion or susceptibility, such as that from focal hemorrhage.

11. ATYPICAL APPEARANCE OF BACTERIAL ABSCESS ON DWI
Postoperative abscess: Images from a surgically drained left frontal abscess. There is heterogeneous signal in the abscess cavity on T1WI, FLAIR, and DWI. ADC map (far right) shows no restricted diffusion. Mixed signal on DWI may be related to surgical irrigation, blood products, or a combination of these. Reaccumulation of pus is less likely as there is no restricted diffusion.

12. TUBERCULOMA
Early lesions are usually isointense on T1- and T2WI, and have variable Gd enhancement.
Mature lesions have ring enhancement on post-Gd T1WI and low signal centrally on T2WI.
Normal DWI signal is common even in mature tuberculomas.

13. TUBERCULOMA
Normal DWI signal: Images show a tuberculoma in the left frontotemporal region. There is bright rim enhancement, characteristic hypointensity in its central portion on T2WI and vasogenic edema. The central area is isointenste to gray matter DWI and there is mild “T2 shine through” from edema. ADC map (far right) shows minimally restricted diffusion.
3-year-old female with miliary TB. At biopsy, fluid could noto be aspirated from the cavity.

14. TUBERCULOMA
Two examples of tuberculomas with low signal on DWI: DWI of right occipital (top) and right cerebellar (bottom) show that neither of these lesions have restricted diffusion.

15. ASPERGILLOMA
DWI and ADC axial images at the same level show multi-focal disease with patchy areas of edema. There are two lesions in the left frontal lobe which are brighter than edema on DWI. Restricted diffusion is confirmed on ADC map. This appearance is similar to that seen with bacterial abscesses.

16. ASPERGILLOMA
T2WI, DWI and ADC map of a patient with aspergillosis. There are bilateral foci of patchy increased T2 signal consistent with edema. Much of the bright signal on DWI is from “T2 shine through,” but some areas show restricted diffusion on ADC map.

17. ASPERGILLOMA
Lesions in left basal ganglia and occipital lobes. High signal in basal ganglia and right occipital lobe on DWI (right) is consistent with restricted diffusion in the cavitary lesions seen on the post-Gd T1WI (left) and T2WI (middle). The high signal in the left occipital region on DWI likely represents “T2 shine through.”

18. TOXOPLASMOSIS
Variable appearance on DWI: Post-Gd T1- and DWI in three patients with toxoplasmosis. Lesions are in the left basal ganglia, occipital lobes, and right basal ganglia, respectively. In the first two patients the centers of the lesions are isointense on DWI. The third has low signal on DWI and ADC map (not shown)confirmed restricted diffusion. DWI with ADC maps are useful in differentiating toxoplasmosis from lymphoma in AIDS patients (Camacho, et al.).

19. NOCARDIA
Renal transplant patient: Post-Gd T1WI (top) shows multiple punctate lesions. Note that individual lesions are not discriminated on the trace DWI and that the abnormalities are seen as confluent areas of high signal due to “T2 shine through.”

20. METASTATIC LUNG CANCER MIMICKING ABSCESS
55-year-old female with metastatic non-small cell lung cancer. In addition to lesions in the periventricular white matter, there is a mass in the right occipital lobe with ring enhancement, surrounding edema, a hypointense rim on T2WI (2nd from left) and restricted diffusion on DWI (3rd from left) and ADC map (far right).

21. MONITORING TREATMENT WITH DWI
The following are three examples of cerebral abscesses which were followed with serial MRI examinations, including DWI after surgical drainage.
The appearance of the abscess cavity on DWI can indicate success or failure of treatment.

22. Example 1: 2-year-old female with seizures and right-sided paraparesis
Example 1: 2-year-old female with seizures and right-sided paraparesis. Initial MRI shows a left posterior frontal abscess. The edema is best seen on T2WI (far left) and the mild peripheral enhancement on post-Gd T1WI (2nd from left). There is restricted diffusion in the cavity (DWI image, 3rd from left) confirmed on ADC map (far right). (Continued)

23. Example 1: (Continued) Images one day after craniotomy and drainage (cultures grew Streptococcus viridans). There is persistent edema and rim enhancement, but the abscess cavity is smaller. DWI and ADC map show no restricted diffusion. The low signal on DWI may represent surgical irrigation fluid or a combination of CSF, blood and serum. (Continued)

24. Example 1: (Continued) Images obtained approximately 2 months after surgery. In the region of the abscess there is a non-enhancing linear area without abnormal signal on DWI, consistent with focal gliosis (no recurrence). Clinically, the patient was cured.

25. Example 2: Serial MRI studies (post-Gd T1WI, DWI, ADC) obtained at one-week intervals in a 40-year-old man presenting with fever, headache and vision disturbance. The left occipital abscess was drained. The lesion has similar characteristics to that shown in the previous case with a notable change on DWI following drainage and resolution of the lesion over time.

26. Example 3: Serial MRI studies (post-Gd T1WI, DWI, ADC) obtained at one-week intervals in a 31-year-old man presenting with fever, left arm weakness, left hand paresthesia and seizures. The right frontal abscess was drained and the first post operative study shows low signal on DWI; however, unlike the previous two examples, instead of continued resolution, there was re-appearance of high DWI signal in the cavity on the second post operative study. This prompted a repeated drainage procedure where presence of pus cavity was confirmed. Note resolution of high DWI signal in the last study.

27. CONCLUSIONS Pyogenic abscesses have a typical appearance on DWI
TB and toxoplasmosis usually little restriction of water motion when compared to pyogenic abscesses
Fungal abscess have a variable DWI appearance
DWI can be use to monitor cerebral abscesses during the course of therapy

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