Traditional X-ray pictures of the head and spine have lost their importance these days. They are mainly used now following accidents, in chronic diseases of the spine, tumours arising in the bones or following surgery. They have largely been replaced by computer tomography and magnetic resonance tomography.
Computed tomography (CT) is an X-ray procedure: during the examination, an X-ray tube circles the supine patient while at the same time detectors opposite the tube and rotating with it measure the radiation. These radiation values are converted in computer calculations to layered images. The patient lies on a special table, which is placed in the CT unit to the level of the body part to be examined. Numerous cross-sections only a few millimeters thick of the area to be examined are imaged. Unlike traditional X-ray examinations, the organs can be imaged without the overlying structures and are thus better visible. Even slight differences in tissues become visible and can be made additionally clearer by injection of contrast agent. The "fathers" of computer tomography, G.N. Hounsfield and A.M. Cormack were awarded the Nobel Prize in 1979 for this method, which was first used in humans in 1973.
The CT is a quick and reliable examination method which is eminently suitable for both emergency diagnostics and the diagnostics of several diseases of the nervous system (head, spine, spinal cord or nerves), for example in stroke, cerebral bleeding, accidents, malformations, tumours, vertebral disc prolapse. The use of multislice-CT sets new improved standards in the imaging of vessels, brain perfusion and 3-D-Reconstruction.
Additional application of contrast agent enables direct imaging of blood vessels and of blood flow in the brain: thus stroke can be accurately diagnosed, when brain tissue can be saved by prompt initiation of therapy. Narrowing of blood vessels ("stenoses") or vessel occlusion, which may be causes of stroke, can be detected. After cerebral bleeding, the pathological changes in vessels (vascular bulging = aneurysms) can also be detected quickly and with high spatial resolution.
Moreover, the CT is used to guide numerous procedures: for example, local anaesthetic is injected under CT guidance in nerve root blockade in the treatment of pain syndromes. Tissue samples are taken under CT guidance in the case of tumours or inflammations, which must be clarified by tissue examination.
The advantages of the CT are the widespread use of the equipment, making the examination readily available, the short examination time, the reliability of results in emergency examinations; moreover, the units are generously constructed so that even people who are fearful in closed spaces can be examined, and unlike the magnetic resonance tomography (MRT), patients with pacemakers and other metal foreign bodies can be examined.
The disadvantages of the CT are based on the exposure to radiation (although this is slight and further minimized by current technical developments), and the lower tissue contrast compared to MRT. In applying the contrast agent, allergic reactions or critical overfunction of the thyroid ("thyreotoxic crisis") may occur in very rare cases.
In summary, computed tomography is the first choice in a number of emergency and routine examinations, making MRT unnecessary in these patients.
Here are some examples of the use of computed tomography in Neuroradiology:
CT Fig. 1: CT following a stroke during contrast agent administration to measure blood flow in the brain ("perfusion CT").
Left the grey-scale picture, the stroke area is dark grey compared to healthy brain tissue (arrows). Right the colour-coded image of blood flow in the brain, areas of reduced flow are red.
CT vessel image ("Angio-CT") during contrast agent administration. Three-dimensional reconstruction of the base of the skull with the large brain arteries. The left middle cerebral artery is occluded (arrow).
Left: normal finding of a cranial CT, right: extensive subarachnoid bleeding (light grey edge, see arrows) after a burst aneurysm).
CT of the cervical spine after injection of contrast agent into the spinal canal (CT-"myelography"). Tearing of a nerve root after a traffic accident with leaking of agent into the torn root pocket (arrow). Other side normal.
CT of the transition between the lumbar spine and the sacrum. CT guided so-called root blockage in pain syndrome. The patient is lying on his stomach. A fine puncture needle (long arrow) is inserted through the back muscles to the nerve root. Anaesthesia mixed with contrast agent is injected (short arrow) and the proper position controlled by CT.
Magnetic Resonace Tomography
What is magnetic resonance tomography? Magnetic resonance tomography (MRT or MRI) is a medical imaging procedure to depict organs and tissues using magnetic fields and radio waves. Unlike computer tomography, in which slice images are also produced, the MRT can be used to produce not only horizontal views, but also other levels without changing the position of the patient.
Magnetic resonance tomography has a number of advantages over other imaging procedures, such as computed tomography:
No exposure to radiation.
Very high soft-tissue contrast, even in the vicinity of bones. This makes it possible, for example, to image small pathological processes in the spinal cord, which cannot be recorded in computer tomography due to the adjacent spinal column. Images of any desired level and view without changing the patient's position.
In addition to images of tissues, MRT special procedures also enable visualization of brain functions (see below).
However, the MRT is not generally "better" than CT. There are questions which can be better answered by CT, such as bone lesions, for example at the skull base, bone fractures (breaks), fresh blood. Moreover, monitoring and examining is simpler in unconscious patients. Also, patients with pacemakers can usually not be examined because of possible disruption caused by the magnetic field in the magnetic resonance tomography. Finally, an MRT examination is considerably more expensive than a CT examination.
Fig. 1: View of an MR tomograph. The patient is lying on his back and is wearing a sort of motorcycle helmet (so-called "Coil") for examination of the head (as in this case). Magnetic resonance tomography can be used for various special examinations using special techniques. These are described briefly and examples of images are given below.
Functional MRT (fMRT)
The basis of the fMRT is the fact that the activity of nerve cells results in local changes in the blood flow in the brain. These can be made visible using special examination sequences, since blood containing oxygen has other magnetic properties than blood from which brain activity has depleted the oxygen (the so-called BOLD effect). This makes it possible to visualize the activation area in the brain in a wide variety of motor, sensory and cognitive processes without using contrast agent. This procedure is performed, for example, prior to neurosurgical operations in which tumours are located near functionally-important areas of the brain (for example for movement or speech) which must be spared during the operation.
Diffusion and perfusion imaging:
In so-called diffusion-weighted examinations, the diffusion (molecular movement) of water in the tissues can be made visible. In stroke, there is an early disruption in this diffusion in brain tissue without blood flow. Thus, diffusion-weighted MRT is a very sensitive procedure to diagnose stroke. When the diagnosis is made early enough, destruction of brain tissue can be prevented or limited by means of appropriate therapy. The flow of blood in the brain can be imaged with perfusion imaging, which also provides additional information, for example in stroke.
Here, special examination sequences are used which are particularly sensitive to movements of small particles, such as flowing blood. This enables direct visualization of vessels through which blood is flowing, without the use of contrast agent. Correspondingly, narrowing or blockage of arteries and veins is also visible. Some of the examinations which used to be performed only invasively, using catheter angiography, can be replaced with this less-stressful MR angiography
Brain metabolic products in the living body can be determined with this procedure. The procedure is useful for example for preoperative characterization of brain tumours, and also in the diagnostics of metabolic diseases of the brain.
Where is magnetic resonance tomography uses in neuroradiology?
MRT is especially well-suited for examination of the brain and spinal cord. Flowing blood in the arteries and veins can be imaged, as can e.g. diseases of the spinal column and vertebral discs.
Thus, for example, in stroke, the brain tissue in which blood flow in interrupted, as well as the vessels leading to the brain and possible occlusions in these vessels can be imaged by MR angiography .
Acute stroke with occlusion of the right middle cerebral artery (arrow in the right-hand picture) and image of the infarction area (light in the left-hand picture).
Inflammation foci in multiple sclerosis can also be almost exclusively imaged using MRT.
Fresh episode in multiple sclerosis, in which newly-formed inflammatory foci appear bright (see arrows).
In brain tumours, images of the tumour at various levels and imaging of the displacement of arteries and veins are important to be known for surgery.
Benign tumour (meningeoma) in the tentorium cleft. A vein, the so-called straight sinus, is displaced by the tumour (double arrow).
Diseases of the spinal column or vertebral discs are imaged well in the MRT by imaging at several levels and soft-tissue contrast.
Prolapsec disc in the lowest segment of the lumbar spine (arrow). Image in sagittal direction (spinal axis, left) and horizontal direction (cross-section, right).
However, magnet resonance tomography can be used not only for anatomical-pathological imaging, but also to image brain function (so-called functional imaging) and metabolic processes (MR spectroscopy).
Brain tumour (arrow) in the left temporal lobe near the speech centre of a right-handed patient. The speech regions (dotted arrows) are imaged in colour by means of functional imaging. Their position near the tumour is taken into account in planning surgery.
Especially in hard-to-treat epilepsy, MR spectroscopy can often provide additional information about metabolism in the diseased tissue.
Epileptic focus in the right temporal lobe (so-called hippocampal sclerosis, white arrow) and spectroscopy of regions defined with the boxes in the left image. Reduction of the so-called N-acetyl-aspartate (NAA, black arrow), a marker molecule for neurons, can be seen.
How is magnetic resonance tomography performed?
This depends essentially on the type of equipment and the purpose of the individual examination. Usually, the patient is placed on a gurney and pushed into a "tube" with a diameter of about 60 cm. During the examination, taps can be heard which are caused by the electromagnetic switches (gradient fields). In order to exclude excessive noise, the patient wears hearing protection (headset, ear-plugs, small pillows at the ears). The average examination lasts about 15-30 minutes. It is sometimes necessary to administer a contrast agent.
What must you do if you are to undergo magnetic resonance tomographic examination?
- Do you have a pacemaker, insulin ump, artificial heart valve or other implants?
- Do you know of any metal parts remaining in your body after surgery or accident? (for example after broken bones, clips, spirals, metal splinters in the eye, gunshots?)
- Do you have any allergies or intolerance to medications?
- Did any problems arise after other examinations with contrast agents?
- Do you suffer from a disease of the kidney?
If any of these apply to you, a magnetic resonance tomography may not be possible, or only possible without contrast agent!
Angiography (vascular imaging):
Angiography is a routine procedure with which vessels supplying the brain or spinal cord can be imaged. The classical angiography via catheter has been replaced in part in recent years by computed tomographic or magnetic resonance tomographic angiography techniques which are non-invasive. The catheter angiography is used these days in a computer-supported form, the so-called digital subtraction angiography, in which only arteries and veins are imaged and bone is not visible and does not cover the vessel. Modern angiography equipment can make images at two planes simultaneously (view from the side and from the front) (Fig. 1).
Modern biplane angiography. Thanks to simultaneous recording of frontal and lateral series, the quantity of contrast agent needed can be cut in half and the time is shorter.
Usually, angiography is used in preparation of an intervention, that is it precedes a minimally-invasive or surgical therapy, for which planning requires very precise knowledge of the details of the vascular system.
Diagnostic angiography is mainly used when one of the following diseases is suspected
- Narrowing of the neck arteries which supply the brain (stenosis of the vertebral or carotid artery) or vessels in the brain itself (anterior, middle or posterior cerebral artery, basilar artery).
- Occlusion of the carotid or cerebral arteries, for example in stroke, venous and venous sinus thromboses, clarification of unexplained cerebral bleeding if magnetic resonance or computed tomography does not definitely reveal an aneurysm, vascular malformations (arterio-venous vascular malformation, arterio-venous fistula).
- Inflammatory diseases of the cerebral vessels (so-called vasculitis).
- Vessel-rich tumours prior to embolisations (obliteration of tumour vessels).
On the day before the procedure, laboratory tests are performed of the function of thyroid and kidneys in order to rule out a disease to these organs. In preparation of angiography, the groin - usually on the right side - is shaved and desinfected. Then the patient is covered with sterile cloths and local anaesthetic is applied. The femoral artery in the groin is punctured with a hollow needle, through which a guide wire and than a so-called introduction sheath - a temporary access - is pushed into the artery. The process is largely pain-free (Fig. 2).
The sheath has already been positioned in the right groin artery (arrow, left side of the picture. A catheter 90 cm long can now be introduced and pushed into the artery to be examined (right side of the picture).
Under fluoroscopy, the catheter is navigated through the pelvic and aortic trunk artery into the vessel relevant to diagnosis of the disease. In addition to the less-often examined spinal cord arteries, these are usually the anterior and posterior neck arteries which supply the brain (Fig. 3, so-called carotid artery and vertebral artery), which arise from the aorta immediately above the heart (from the aortic arch).
3D reconstruction image of the carotid arteries from a left oblique view (upper image).
Aortic arch model with the course of the guiding catheter in the right carotid artery (lower left half of the image) and view of several differently-formed catheters for navigation in the carotid arteries.
Arteries and veins of the brain can be imaged from the position of the catheters in the carotid arteries (Fig. 4). If further details are to be examined, single brain arteries can be selected and made visible in the X-ray image by pushing a small catheter (microcatheter) about 1 mm thick through the larger guiding catheter. The patient must hold his breath for about 10 seconds while the contrast agent is injected so that the blood vessels can be clearly imaged without movement artefacts. At the same time, there is a feeling of warmth or heat in the neck or head, sometimes with flashes before the eyes, which lasts seconds
When the examination has been completed, the catheter and then the introducer sheath are removed, pressure is applied to the puncture site until it no longer bleeds, and a pressure bandage is applied for several hours. The patient must remain quietly in bed for this time, after which the puncture site has become sealed with a stable tissue clot.
Three-dimensional image of the vessels in one half of the brain, obtained by rotation angiography. The arteries (left half of the image) and veins (right half of the image) can be visualised independent of each other.
As complications, allergies to the contrast agent may occur, which are fatal in extremely rare cases (in one of 10 to 20,000 examinations). The average risk for persistent damage due to embolisms in the form of, for example, paralysis, is 0.3% of the examinations. The risk for patients with serious arteriosclerosis may be as high as nearly 1%.
Angiography enables imaging and evaluation of, for example, narrowing in the neck arteries (stenoses), vascular occlusion (for example in stroke), aneurysms in the neck or brain vessels, vascular malformations (fistulas, arterio-venous malformations) or tumours. This may be necessary for diagnosis and planning of possibly required operations or minimal-invasive treatment (see chapter on Endovascular Interventions).
A. Temporary Carotid Occlusion Test
In this test, the internal carotid artery is occluded on one side for 20 minutes with a balloon. At the same time, an ultrasonic examination of the cerebral vessels (TCD = transcranial Doppler sonography) is performed to evaluate whether sufficient blood flows in the cerebral vessels dependent on the occlusion. This must be sufficient to prevent a stroke which may occur due to insufficient blood flow. The patient is continuously neurologically examined.
The temporary occlusion test is necessary if there is to be surgical or endovascular closure of a vessel. The test enables considerably better estimate of the risk of stroke and its results influence planning of the intervention.
B. Wada Test
In the Wada test, medication is injected into a vessel which causes the brain in the area supplied by the vessel to "go to sleep" for a few minutes. Usually, this medication is administered through a normal catheter into the internal carotid. Rarely, and only in very carefully defined examinations, the medication is introduced superselectively into a cerebral vessel through a microcatheter. These examinations are performed under local anaesthesia, since the aim is to examine the effect of the medication administration on brain function. These examinations are performed especially to clarify epilepsy prior to a planned operation.
In recent years, these examinations have continuously become less frequent, since functional magnetic resonance (fMRI) combined with neuropsychological testing has largely replace these invasive examinations, which are now justified only in special cases.
C. Venous blood collection in the Sinus cavernosus
Thanks to modern imaging with thin-layer and dynamic MR examinations of the pituitary, most pituitary tumours can be detected, even those extremely small in diameter (microadenoma). Nonetheless, in rare cases, imaging is not able to provide definite proof of a tumour, although for example the hormone constellation indicates a tumour. In order to clarify whether the hormone-active tumour is located in the pituitary, it is possible to position a microcatheter in a vein immediately next to the pituitary (cavernous sinus). Blood is drawn through this catheter before and after medical stimulation of the pituitary from which the presence of a tumour can be determined.
Myelography is used to examine the spinal canal. X-ray dense contrast agent is injected into the "nerve water" (cerebro-spinal fluid), which thus makes the fluid space visible. This is performed through a puncture in the lower spine, a procedure by which neurologists also examine the cerebro-spinal fluid. Local anaesthetic can be administered for the puncture. Usually contrast imaging is combined with a computer-tomographic examination after the X-ray agent has dispersed, since the two methods together provide the best diagnostic results. In order to be able to examine all segments of the spine, the contrast agent is transported to the area to be examined by changing the position of a tilting-table.
Usually, spinal column diagnostics with magnetic resonance tomography or computer tomography are sufficient, for example in the case of tumour or disc prolapse, so that many myelographies can be avoided these days. If myelography is performed, it is usually in preparation of surgery. Its great advantage is that functional examinations of the spine, such as slipped disc under static stress and with testing of the movement which causes pain, can be simulated. Additional weights, held by the patient with outstretched arms, are used for this loading myelography. This enables examination of diseases caused by a lack of stability in the spine.
Headache may be a side effect of the examination, but this usually subsides after a short time. Complications like infections or injury to nerve structures are very rare.