Книги по МРТ КТ на английском языке / Neurosurgery Fundamentals Agarval 1 ed 2019

virtually all invasive monitoring devices for acutely ill patients are essentially CT compatible.

The spatial resolution of CT is superior to MRI, however, even 1–2 mm aneurysms are readily identified on with modern MR angiography (MRA) techniques.

MRI is advantageous in younger patients undergoing surveillance for small unruptured/untreated aneurysms as no radiation is involved and contrast administration is not required. On both imaging modalities, aneurysms appear as focal dilatations­ or outpouchings in the vessel wall, usually at branching points or areas of stress. While all the vessels should be inspected, common sites of occurrence that should undergo additional scrutiny

include the anterior communicating (AComm) complex, MCA bifurcation, basilar tip, and origins­ of the posterior communicating (PComm) arteries. In general, angiography is better for surveillance of surgically clipped aneurysms as the artifacts produced by the metallic clips prohibit adequate­ assessment of the adjacent vasculature,­ while MRA is superior for follow­ -up of coiled aneurysms.12,​13

5.4.8  Arterial Dissection

Arterial dissections occur as a result of a tear in the intima of the vessel wall, with dissection of blood into the media of the wall creating a false lumen adjacent to the true lumen. On cross-sectional imaging, there is usually irregularity and narrowing of the true lumen, with eccentric thickening or slight dilatation of the outer circumference of the vessel wall by thrombus/subacute

blood in the media of the vessel wall. Conventional MR images may show loss of the expected flow void in the compromised vessel, or narrowing of the flow void with abnormal hyperintense signal in the vessel wall on T2WI or fat-suppressed T1WI.14 Arterial dissections may be complicated by formation of a pseudo-­aneurysm if the dissecting blood weakens the adventitial lining of the vessel wall. Thromboembolic complications can also occur if the intramural hematoma re-enters the true lumen, with

resultant local or distal vascular occlusion and infarction.

5.4.9  Intracranial Infection

Imaging for meningitis is usually performed to exclude any related process such as abscess. The main role of MRI is to exclude complications of meningitis, and contrast-enhanced MRI is the modality of choice. MR findings in meningitis include abnormal hyperintensity of the CSF spaces

from exudate on FLAIR images, with abnormal leptomeningeal enhancement and/or enhancement of the basal cisterns.15 However, the MRI may also be normal in many cases of viral meningitis, and all clinically suspected cases of meningitis should undergo lumbar puncture and CSF analysis to exclude infection. Complications of meningitis include hydrocephalus, empyema formation, and vasculopathy with cerebral infarction.

Requests to exclude cerebral abscesses are a common reason for imaging patients with suspected CNS infections. Mature cerebral abscesses are focal parenchymal collections of purulent material, surrounded by a well-vascularized wall.15 These histologic characteristics are reflected on imaging,

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Fig. 5.12  Acute right pontine infarct. A 53-year-old female with a history of basilar thrombosis who presents with new right hemianopia and change in mental status. (a) CT scan shows an area of low attenuation involving right pons. (b) Fluid-attenuated inversion recovery image shows corresponding signal hyperintensity in the region in question.

(c)Diffusion weightedimaging and (d)apparentdiffusion coefficient mapsshowcorresponding areas of high signal and low signal respectively consistent with restriction of diffusion. Findingsarecompatible with an acute right pontine infarction. Also note additional punctate areas of restricted diffusion intheleftcerebellar hemisphere. (Images are provided courtesy of Thomas

Jefferson UniversityHospital.)

with CT/MR showing a ring-enhancing collection in the parenchyma, with surrounding vasogenic edema. Restricted diffusion on DWI sequences may be seen within the central portion of the abscess reflecting purulent material, a finding that is helpful in distinguishing abscesses from tumors on MRI.16

5.4.10  Brain Tumor

The initial analysis of any brain tumor on imaging begins with an assessment of whether the mass is intra-axial (within the substance of the brain) or extra-axial (intracranial, but external to the substance of the brain). Intra-axial masses include primary brain tumors (of which glioblastoma

multiforme [GBM] is the most common) and metastases. Extra-­axial masses include tumors arising from the meninges (classically the meningioma), calvarium, synchondroses, and metastases. The distinction between intra-axial and extra-axial masses is not always clear-cut, but in general, extra-­axial masses have a well-demarcated interface between the cortex/brain parenchyma and the mass, while intra-­zaxial

masses tend to arise from and expend the brain parenchyma. There may also be a rim or “claw” of brain tissue reaching around the mass, which is another clue that the lesion is intra-axial in origin. In cases where an extra-axial mass subsequently invades the adjacent brain parenchyma, or an intra-axial mass secondarily involves the meninges, it can be difficult to determine the origin of the mass.

Extra-axial Masses

Meningiomas are the classic extra-axial mass, and probably the most common extra-axial masses encountered on imaging in any neurosurgical practice. Meningiomas are usually dural based, with a broad contact surface with the dura. There may be an enhancing dural tail at the edge of the mass, which is commonly associated with (but not specific for) meningiomas. On non-contrast CT, meningiomas are hyperdense relative to the brain parenchyma and may also have calcification. There is usually hyperostosis (sclerosis and thickening) of the bone adjacent to the meningioma. On post-contrast CT and MR, meningiomas usually enhance homogeneously. Dural-based metastases can look similar, so it is always important to ask about a history of cancer when one encounters an enhancing extra-axial mass.

Intra-axial Masses

GBM is the most common primarily malignant brain tumor in adults, and is the quintessential primary intra-axial mass. GBMs are usually centered in the supratentorial cerebral hemispheres and preferentially infiltrate the brain widely through white matter tracts, sometimes involving the contralateral hemisphere via the corpus callosum and anterior commissure. The enhancing component of a GBM is usually just the most obvious (usually necrotic) component of the tumor. Viable nonenhancing tumor is usually present in the surrounding T2/FLAIR signal abnormality in the region of the tumor. Other disease processes that tend to favor involvement of or spread along white matter tracts include lymphoma and demyelinating disease.

The other major consideration for an intra-axial tumor is metastasis, which is

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the most common CNS malignancy in adults. Metastases are frequently multiple, bilateral, of varying sizes, and centered in the gray-white matter junction. However, solitary brain metastases are also common, can occasionally be difficult to distinguish from primary brain malignancy.17

One helpful finding is that metastases tend to incite extensive vasogenic edema in the surrounding brain parenchyma, best visualized on the FLAIR images.

Small cortical metastases may have little, if any, vasogenic edema, and may only be conspicuous as enhancing masses on the post-contrast images. In unclear cases, imaging the chest, abdomen, and pelvis to look for an occult primary can provide some guidance regarding the origin of a solitary brain tumor.

Pearls

•Develop a systematic method evaluating each imaging modality to ensure no pathology is inadvertently missed.

•Evaluate all imaging personally and do not rely solely on radiologic interpretation of imaging.

•Utilize 3D reconstructions to better delineate fractures, extra-axial hematomas, and the location of intracranial pathology.

•Review all sequences when reviewing an MRI as each can distinctly highlight a variety of pathologies.

•Compare changes over time utilizing the same imaging modality. However, be aware that the gantry angle of a CT can change the appearance of a lesion.

5.5  Top Hits

5.5.1  Questions

1.What imaging modality is first required for the evaluation of neurological injury in the acute trauma setting?

a) MRI

b) CT with contrast c) CT without contrast

2.Which is the most sensitive MR sequence for evaluation of acute cerebral infarction?

a) T1 sequence b) DWI sequence c) GRE sequence

3.Why is MRI performed in the setting of meningitis?

a) To rule out meningitis

b) To evaluate for complications of meningitis

c) Because it is cheaper than CT scan

4.Does a negative MRI rule out menin­ gitis in a patient with high clinical suspicion.

a) Yes b) No c) Both

5.5.2  Answers

1.c. This can expediently evaluate for intracranial hemorrhage, contusions and skull fractures in the acute setting. The presence of contrast can confuse evaluation for hemorrhage as contrast is also hyperdense.

2.b. Acute infarctions show restricted diffusion on diffusion maps.

3.b. To evaluate for complications of meningitis such as abscess, empyema, thrombosis and infarction.

4.b. A negative MRI does not rule out meningitis. Lumbar puncture and CSF analysis should be performed.

References

[1]Bonneville F, Cattin F, Marsot-Dupuch K, Dormont D, Bonneville JF, Chiras J. T1 signal hyperintensity in the sellar region: spectrum of findings Radiograph- ics. 2006; 26(1):93–113

[2]Basaran C, Karcaaltincaba M, Akata D, et al. Fat-­ containing lesions of the liver: cross-sectional imaging findings with emphasis on MRI. AJR Am J

Roentgenol. 2005; 184(4):1103–1110

[3]Romero JM, Schaefer PW, Grant PE, Becerra L,

González RG. Diffusion MR imaging of acute is- chemic stroke. Neuroimaging Clin N Am. 2002; 12(1):35–53

[4]Zimmerman RA, Bilaniuk LT. Computed tomographic staging of traumatic epidural bleeding. Radiology. 1982; 144(4):809–812

[5]Provenzale JM, Hacein-Bey L. CT evaluation of subarachnoid hemorrhage: a practical review for the radiologist interpreting emergency room studies. Emerg Radiol. 2009; 16(6):441–451

[6]Heinz ERWA, Ward A, Drayer BP, Dubois PJ. Distinction between obstructive and atrophic dilatation of ventricles in children. J Comput Assist Tomogr. 1980; 4(3):320–325

[7]Tamrazi B, Almast J. Your brain on drugs: imaging of drug-related changes in the central nervous system. Radiographics. 2012; 32(3):701–719

[8]Hemphill JC, III, Bonovich DC, Besmertis L, Manley GT, Johnston SC. The ICH score: a simple, reliable grading scale for intracerebral hemorrhage. Stroke. 2001; 32(4):891–897

[9]Given CA, 2nd, Burdette JH, Elster AD, Williams DW, 3rd. Pseudo-subarachnoid hemorrhage: a potential imaging pitfall associated with diffuse cerebral ede- ma. AJNR Am J Neuroradiol. 2003; 24(2):254–256

[10]Han BK, Towbin RB, De Courten-Myers G, McLau- rin RL, Ball WS, Jr. Reversal sign on CT: effect of anoxic/ischemic­ cerebral injury in children. AJR Am J Roentgenol. 1990; 154(2):361–368

[11]von Kummer R, Meyding-Lamadé U, Forsting M, et al. Sensitivity and prognostic value of early CT in occlusion of the middle cerebral artery trunk. AJNR Am J Neuroradiol. 1994; 15(1):9–15, discussion 16–18

[12]Wallace RC, Karis JP, Partovi S, Fiorella D. Noninvasive imaging of treated cerebral aneurysms, part I: MR angiographic follow-up of coiled aneurysms. AJNR Am J Neuroradiol. 2007; 28(6):1001–1008

[13]Wallace RC, Karis JP, Partovi S, Fiorella D. Noninvasive imaging of treated cerebral aneurysms, Part II: CT angiographic follow-up of surgically clipped aneurysms. AJNR Am J Neuroradiol. 2007; 28(7):1207–1212

[14]Rodallec MH, Marteau V, Gerber S, Desmottes L, Zins M. Craniocervical arterial dissection: spectrum

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of imaging findings and differential diagnosis. Radi- ographics. 2008; 28(6):1711–1728

[15]Foerster BR, Thurnher MM, Malani PN, Petrou M, Carets-Zumelzu F, Sundgren PC. Intracranial infections: clinical and imaging characteristics. Acta Radiol. 2007; 48(8):875–893

[16]Chang SC, Lai PH, Chen WL, et al. Diffusion-­ weighted MRI features of brain abscess and cystic or necrotic brain tumors: comparison with conventional MRI. Clin Imaging. 2002; 26(4):227–236

[17]DeAngelis LM. Brain tumors. N Engl J Med. 2001; 344(2):114–123

6.1  Introduction

The operating room may be a confusing landscape for the trainee to navigate, especially early in one’s training. While much focus is rightfully placed on the techniques and steps to a given operation there are a multitude of additional workings, preparations, and members which are key to any successful procedure. Familiarizing oneself with ancillary staff and appropriate positioning, in addition to technical maneuvers, will both streamline and improve the likelihood of a positive outcome for the operation.

6.2  Operating Room

6.2.1  Etiquette

Understanding the roles of operating room staff members and the expectations of the trainee are essential to a promising operative learning experience. Ultimately, the role of the student is at the discretion of the surgical staff. Politely asking the attending or resident surgeon to “scrub in” is a start. Regardless of being able to assist with the procedure or not, students are expected to assist with preoperative and postoperative care. Assisting with insertion of Foley’s catheters and lines, patient positioning, Mayfield placement, and additional adjunctive measures are helpful. Transferring the patient to a bed and assisting with transport postoperatively are also helpful. Asking thoughtful questions is invited but should not be done during critical portions of the case. If permitted to assist with the procedure, the trainee’s role is highly dependent on the wishes of the lead surgeon and residents.

Being prepared as a trainee is essential, including reading regarding the case at hand, knowing the basic anatomy, instruments, and basic suturing and knot tying.

6.3  Cranial Positioning

6.3.1  Pterional

The pterion is the region where the frontal, parietal, temporal, and sphenoid bones meet. Patients are placed supine in Mayfield three-point fixation. If the head is rotated more than 30°, an ipsilateral shoulder roll is placed to reduce muscular tension and venous outflow obstruction. The thorax is elevated 10–15° to reduce venous distension and the neck is extended 10–15° to aid in retracting frontal lobe so skull base is more accessible; a good landmark is the maxillary eminence which will be the highest point of the field. The pterional craniotomy is tremendously versatile and the degree of head rotation is useful in projecting the approach towards different segments of the anterior and middle fossae ( Table 6.1).1,​2

6.3.2  Frontal

Patients are placed supine in Mayfield three-point fixation with the head rotated 20–30° towards the contralateral shoulder depending on the side of the operation. A shoulder roll may be placed beneath the ipsilateral shoulder. Similarly to the pterional craniotomy, the thorax may be elevated.3

6.3.3  Temporal

Patients are placed supine in Mayfield three-point fixation with the thorax elevated 10–15°. Importantly, the head is rotated nearly a full 90° towards the contralateral shoulder such that it is horizontal; an ipsilateral shoulder roll is essential to achieve this degree of rotation without injury, postoperative muscular rigidity, and blockage of venous outflow through the jugular system whiczh prevents brain relaxation ( Fig. 6.1).1,​4 Due to the significant degree of head rotation required, many ­surgeons prefer to actually position these patients in the lateral decubitus position.

6.3.4  Occipital

Multiple positions are used for the occipital craniotomy, all in Mayfield three-point fixation. Positions include three-quarter prone, semi-sitting position, or lateral oblique.1,​3 Variants will allow positioning for retromastoid, suboccipital, far lateral, and infratentorial supracerebellar approaches.

6.3.5  Transsphenoidal

Patients are placed supine in either Mayfield three-point fixation or with their head placed on a gel donut or horseshoe. As in most craniotomies the head is extended approximately 15°. The head may be kept midline or angled 15–20° towards the ­neurosurgeon. Especially in endoscopic procedures where ENT surgeons are often present, proper positioning of surgeons around the patient is vital for ease of instrumentation and visualization on adjacent screens.5 The surgeon may opt to

prep a portion of the abdomen for harvesting of a fat graft used in closure of the defect. Adjunctive measures utilized include an orogastric tube and packing the nose with pledgets soaked in oxymetazoline prior to surgery to prevent drainage of blood into the esophagus and bleeding, respectively.6

6.4  Spinal Positioning

6.4.1  Anterior Cervical

Anterior cervical approachs are commonly used for anterior cervical discectomy and fusion (ACDF) and odontoid fractures but also in cases of carotid endarterectomy. Patients are positioned supine, in slight extension, with the head supported on a soft gel pad or horseshoe. Interscapular rolls to maximize extension are used based on ­surgeon’s preference. Depending on the pathology, head rotation may be helpful in reaching high cervical lesions. The approach is usually off of midline and many prefer approaching from the left due to relatively increased protection of the left recurrent laryngeal nerve by the espohagus and trachea compared to the right nerve.7 Incision is often made through a pre-existing skin crease to enhance postoperative cosmesis.

Typically the hyoid bone approximates the C3–C4 vertebrae, thyroid cartilage approximates C4–C5, cricothyroid membrane approximates C5–C6, and C6–C7 is often two fingerbreadths above the clavicle.3