A Comparative Analysis Of The Sensitivity Of Ct And Mri In Stroke Patients

Controversy exists about the optimal imaging modality in patients with stroke. This work was carried out to optimally compare CT and MRI images in patients with stroke. ( A case study of  National Hospital, Abuja). A retrospective study of 54 stroke patients who underwent both CT scans and MRI scans at National Hospital, Abuja between January 2010 to March 2013 was conducted. Tables and T-test for hypothesis was used to analyse the data. CT is significantly more sensitive than MRI in the detection of acute hemorrhagic stroke (92% vs. 48%  p<0.05). MRI is significantly more sensitive than CT in the detection of ischemic stroke (88% vs. 28% p<0.05). Generally, MRI is more sensitive than CT in the detection of all strokes( 70.37% vs. 62.96%) that is when other non-hemorrhagic and non-ischemic stroke symptoms are included. However, the T-test for hypothesis shows that there is no significant difference in the the sensitivity of CT and MRI in stroke patients. (p<0.05). CT is more sensitive than MRI in the diagnosis of hemorrhagic stroke while MRI is more sensitive than CT in the diagnosis of ischemic stroke. However, MRI is more sensitive than CT in the diagnosis of all stroke though they shows no significant difference in sensitivity from the T-test.

CHAPTER ONE

CHAPTER TWO

CHAPTER THREE

CHAPTER FOUR

CHAPTER FIVE

 

Computed tomography (CT) scan is a medical imaging procedure that utilizes computer processed x-rays to produce tomographic images or ‘slices’ of specific areas of the body. These cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines. CT produces a volume of data that can be manipulated through a process known as “windowing”, in order to demonstrate various bodily structures based on their ability to attenuate the x-ray beam.1

Magnetic Resonance Imaging (MRI) scan of the head is an imaging test that uses powerfull magnets and radio waves to create detailed images of the brain and surrounding nerve tissue. It also provides clear images of parts of the brain that are difficult to see clearly on CT scans.3

Indications for a brain MRI scan include birth defect of the brain, bleeding in the brain (subarachnoid or intracranial haemorrhage, brain infections, brain tumours, hormonal disorders such as acromegaly, galactorrhea and cushing syndrome, multiple sclerosis, stroke. An MRI scan of the head can also determine the cause of muscle weakness or numbness and tingling, changes in thinking or behaviour, hearing loss, headaches (when certain other symptoms or signs are present), speaking difficulties, vision problems.2

Magnetic Resonance Imaging (MRI) machines make use of the fact that body tissue contains lots of water, and hence protons (1H nuclei), which get aligned in a large magnetic field.3 Each water molecule has two hydrogen nuclei or protons. When a person is inside the powerful magnetic field of the scanner, the average magnetic moment of many protons becomes aligned with the direction of the field. A radio frequency current is briefly turned on, producing a varying electromagnetic field. This electromagnetic field has just the right frequency, known as the resonance frequency, to be absorbed and flip the spin of the protons in the magnetic field. After the electromagnetic field is turned off, the spins of the protons return to thermodynamic equilibrium and the bulk magnetization becomes re-aligned with the static magnetic field. During this relaxation, a radio frequency signal (electromagnetic radiation in the RF range) is generated, which can be measured with receiver coils.

These fields, generated by passing electric currents through gradient coils, make the magnetic field strength vary depending on the position within the magnet. Because this makes the frequency of the released radio signal also dependent on its origin in a predictable manner, the distribution of protons in the body can be mathematically recovered from the signal, typically by the use of the inverse Fourier transform. The 3D images obtained in MRI can be rotated along arbitrary orientations and manipulated by the doctor to be better able to detect tiny changes of structures within the body.3

Magnetic Resonance Imaging (MRI) contains strong magnets, and because of this metal objects are not allowed into the room with the MRI scanner. Patients that are not allowed to undergo an MRI scan are patients with metallic implants in thier body and they include patients with brain aneurysm clips, certain types of artificial heart valves, heart defibrillator or pace maker, inner ear (cochlear) implants, recently placed artificial joints, certain types of vascular stents, those that have worked with metal in the past without going for a test check for metal pieces in thier eyes. Patients with kidney disease or dialysis may not be able to reciene contrast.2

Sensitivity can be defined as the proportion of actual positives which are correctly identified as such (eg the percentage of sick people who are correctly identified as having the condition).4

Sensitivity of a CT scan or an MRI scan of the head in stroke patients refers to the ability of the CT scan or the MRI scan to correctly identify those patients with stroke.5

The use of X-rays, a type of ionizing radiation, by a computed tomography (CT) allows for examination of tissues composed of elements of a higher atomic number than the surrounding tissues. MRI, on the other hand, uses non-ionizing radio frequency (RF) signals to acquire images and is best suited for soft tissue (although MRI can also be used to visualize bones, teeth8 and even fossils9).

Since CT scans use ionizing radiation (X-rays) to produce images, there is a risk of damage to DNA that can subsequently cause cancer. In 2007, it was estimated that 0.4% of current cancers in the United States are due to CTs performed in the past, although in the future this may increase to as high as 1.5–2% based on past rates of CT usage.8 Unlike CT, MRI does not use ionizing radiation, although it is associated with other risks.8

Contrast in CT images is generated purely by X-ray attenuation, while a variety of properties may be used to generate contrast in MR images. By variation of scanning parameters, tissue contrast can be altered to enhance different features in an image (see Applications for more details). Both CT and MR images may be enhanced by the use of contrast agents. Contrast agents for CT contain elements of a high atomic number relative to tissue, such as iodine or barium, while contrast agents (such as gadolinium and manganese) for MRI have paramagnetic properties that are used to alter tissue relaxation times. Commonly used MRI contrast agents may be contraindicated in people with significant permanent or transient kidney dysfunction.8

Computed Tomography (CT) and MRI scanners are able to generate multiple two-dimensional cross-sections (tomographs, or "slices") of tissue and three-dimensional reconstructions. MRI can generate cross-sectional images in any plane (including oblique planes). In the past, CT was limited to acquiring images in the axial plane (or near axial), and so the images were called Computed Axial Tomography scans (CAT scans). However, the development of multi-detector CT scanners with near-isotropic resolution allows the CT scanner to produce data that can be retrospectively reconstructed in any plane with minimal loss of image quality. For purposes of tumor detection and identification in the brain, MRI is generally superior.8 However, in the case of solid tumors of the abdomen and chest, CT is often preferred as it suffers less from motion artifacts. Furthermore, CT usually is more widely available, faster, and less expensive.

Magnetic Resonance Imaging (MRI) is also best suited for cases when a patient is to undergo several exams in the short term, since it does not expose the patient to the hazards of ionizing radiation. However MRI is usually contraindicated if the patient has any type of medical implant, such as vagus nerve stimulators, implantable cardioverter-defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, metallic foreign bodies (e.g. shrapnel or shell fragments), or metallic implants such as surgical prostheses. These devices can malfunction or heat up during an MRI scan, so CT scans are considered the safer option for these patients.

Magnetic Resonance Imaging (MRI) provides more sensitivity in the evaluation of the cavernous sinus and the orbital apex.8

One advantage of brain CT over a brain MRI is in the evaluation of intracerebral calcifications.8

A stroke, or cerobrovascular accident (CVA), is a rapid loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia (lack of blood flow) caused by blockage (thrombosis, arterial embolism), or a haemorrhage.6  As a result, the affected area of the brain cannot function, which might result in an inability to move one or more limbs on one side of the body, inability to understand or formulate speech, or an inability to see one side of the visual field.7

Strokes can be classified into two major categories: ischemic and hemorrhagic. Ischemic stroke are those that are caused by the interuption of the blood supply while haemorrhagic strokes are the ones which results from rupture of a blood vessel or an abnormal vascular structure.8

Stroke remains the third most important cause of mortality in industralized countries, this has prompted research for improvement in both diagnostic and therapeutic strategies for patients with signs of stroke.9

Imaging has become a conerstone of stroke management, translating pathophysiological knowledge to everyday decision-making.10