Computed Tomography - CSB 056

Transcription

1 Computed Tomography - CSB 056 Week 1 - Some intra-abdominal fat is ideal to differentiate organs better - Dual tube/dual energy imaging (2 tubes and 2 detector banks) - CT used: if lesion is large enough to distort normal tissue outline, if lesion has different density to surround and if there is a pathological change to the normal texture of a structure - CT is expensive, lots of iodine needed to be purchased and lots of computer storage needed - Technical info includes exposure, slices, windows, contrast administered and field of view - CT number = k x (µp - µw) / (µw) - Patient prep might include oral contrast - Prepare the table, establish IV access and load pressure injector - Protocol selected, patient details entered, scout image - Rest of acquisition performed then processed - IV contrast demonstrates vasculature due to contrast bolus - Contrast also helps identify pathological processes - Low density means the mass is surrounded by oedema - Image is orientated viewing from feet (may be positioned head first or feet first) Week 1 CT parameters - Determine if patient is suitable for contrast and the timing of the contrast with the acquisition - Patient position in terms of scan plane and image orientation - Topogram à may be AP and lateral, length of the topo and exposure factors - Acquisition slice thickness, usually relatively small - Dose modulation and timing also play a role - Post processing including image display, planes and volume display - Protocol may select most of these parameters - Scout scan à to plan, enables planned angulation of gantry, enables assessment of patient positioning - Pseudo coronal of the skull, 20 degree gantry angle with patient prone and chin extended. Shows air fluid lines. However, is very uncomfortable and not often used - Scan plane (pt position): axial slice acquisition, helical data is interpolated to get this. - Exposure factors: impact on dose, scan time is dictated by gantry rotation time. Selectable from a defined range of values - Pitch: describes the CT table movement in relation to the rotation of the gantry. Travel distance per rotation of the tube. The multiplied by beam width. Higher pitch has less dose - Slice thickness: helical scans acquire a volume of data. Typically small slices acquired and reconstructed as thicker - Scan extent: if you were to keep everything the some but adjust the pitch, the volume of tissue scanned will change. Higher pitch will cover more volume and therefore change the scan extent. Want the scan extent to be the necessary anatomy only

2 - Table position and image orientation: operator defined and controllable. Select craniocaudal or caudiocranal - Subject contrast: inherent to tissue contrast, can be increased through the administration of contrast - Scan speed: time acquisition so contrast is at the phase of enhancement. Dependent on where you re looking at and if its venous or arterial. Contrast entered in venous system must enter heart and then aorta before it is disseminating through the body - Algorithms and kernels: also called a convolution filter or kernel. Applied to the raw data to represent the voxel values. Example is edge enhancement used for the lung to accentuate difference between high and low densities - Window widths and levels: influences the displayed appearance of the image but does not alter the actual CT numbers of the voxels. Adjustment of the scale used in the grey scale. Use smaller windows to accentuate low subject contrast, use larger windows for high subject contrast - BONE WINDOW à 400 WL and 2000 WW - LIVER à 150 WW to show more detail in liver (low subject contrast) - BRAIN à 40 to 60 WL and 100 to 150 WW - LUNG à -600 WL and 1500 WW - MEDIASTINUM AND ABDOMEN à 50 to 80 WL and 300 to 380 WW - Unenhanced scan shows low density lesions on standard windows, must be done - Low density in a standard brain scan will be ventricular (cysts or obstruction) or oedema (abscesses, tumor or infarcts) - High density lesions on standard windows and unenhanced scans will be calcifications or hemorrhages - Data management is a challenge in CT because there is so much information acquired from each scan - Summary series used to minimize the amount of info - Post processing slice width, MPR, Maximum intensity projections (MIPS), shaded surface display (SSD) and volume rendering - 6mm slice summary rather than the acquired 1mm slices - May also do coronal and sagittal acquisitions with 20 or so images - 3D presentation such as MIP, SSD and volume rendering - MIP is a 3D technique which displays the voxels of the highest value, good for looking for pulmonary emboli - SSD shows just the voxels on the edge of a structure, useful for bony structures and fractures - Volume rendering is 3D where there is a semitransparent view of all voxels contributing to the image - Week 2 CT Physics revision - Fan beam is being surpassed by cone shape that covers more anatomy in the z axis - Detectors record differential attenuation values - High frequency generator used, compact with capacity at 20 to 100 kw - 60kW can provide the machine with 120 to 140 kvp - High performance tube with 0.5 and 1 mm focal spots to maintain high spatial resolution

3 - Faster RPM (10,000 compared to standard 4,000), high heat loading capacity by thicker and wider anode - Beam restriction determines slice thickness and dose distribution profile - Most common slice thickness is 0.5mm to 10mm depending on part imaged - MDCT slice thickness controlled by collimation and detector configuration - Pre collimator hardens the beam, post collimator removes scatter - SFOV à determined by size of fan beam and number of detectors that collect data - Detector characteristics: high QDE, low or no after glow, high scatter suppression and high stability - After glow means light is stopped from producing the instant radiation stops being incident - Also, capture and absorption efficiency, fast response time and large dynamic range are desired - CT numbers: numbers used to define relative attenuation co-efficients of each voxel - Performing CT à select protocol, position patient, acquire scout, plan slices, acquire data, computer builds image, view reconstruction, post process as needed - Remember to zero the system to indicate where the landmark is - Tube and detectors are stationary in a scout, low dose, low spatial and contrast res with couch moving slowly - Image processing parameters: lvp, ma, gantry rotation time, beam collimation, number of detectors - Also, rate of detector measurements, algorithm, window and image viewing settings - Prospective à builds images as you scan - Fourier transformation: 17 th century mathematician - Discrete fourier transformation (DFT) - Fast fourier transformation (FFT) - Adaptive statistical iterative reconstruction: Week 2 Dose and Contrast - CT contributes to more than 50% of medical dose even though it is only 17% of xray examinations - Very topical with high profile case studies recently (mainly in the US) - Image gently and image wisely campaigns - Justification, optimization and limitation are essential - CTDI = computer tomography dose index, exposed dose at standardized conditions kv, 100mAs, 360 acquisition and 100mm scan length are the parameters - Is characteristic for a scanner and dependent on tube filtration - CTDI volume meausres the exposure parameters set, the preffered dose expression in CT and is used as a reference level - DLP = dose length product, exposed patient dose at clinical conditions - Specific to the patient for that study, in mgy/cm with effective dose taken into account - DLP impacted by CTDI, correction factor for kv and mas, number of slices and the thickness of slices - Dose reduction methods à adjust kv and mas based on patient weight, manufactures suggest tube current modulation

4 - Dose modulation: based on a topogram, less mas is used on areas that are less dense to reduce the total exposure - Shields used for eyes, breasts and thyroid (bismuth shields) - Shielding should be applied after scout so it doesn t impact dose modulation - In some machines CT only operates in 270 degrees to avoid direct to the eyes, eliminating the need for bismuth shield - Adaptive iterative reconstruction: about providing a diagnostic image not aiming for the best possible resolution. You select the noise level that is acceptable, allowing the machine to significantly lower the dose - Disadvantage of this is that it takes longer to produce the image and it does appear different to standard images - Decreasing slice thickness increases dose, as with a pitch below 1 - Patient positioning correct will help limit the slices needed and therefore the dose to the patient - Pitch also is crucial in dose saving, not always selected by the operator. May be selected automatically dependent on slice thickness and spacing - Oral contrast used to enhance bowel for abdominal imaging, consumed over 1 hour before imaging - Barium based compound but less concentrated than fluoroscopy procedures - May also use gastrografin or water instead - Longer period of waiting time for pelvic imaging - Rectal contrast used for fistulas - Intrathecal contrast used occasionally (injected into subarachnoid space, used in a myelogram) - Myelogram CT done for patients unsuitable for MR, metal hardware or artefacts in the patient - IV iodinated contrast used very commonly - Contrast enters the heart from the veins and highlights the arteries - Differentiates normal vessels from abnormal masses - Allows abnormality to become apparent - Determine vascular nature of a mass - And allows visualization of blood vessels for a CTA - Very safe medication but there is still life threatening risk - Requires informed consent form, not advised for renal impairment or strong allergic history - 70% of reactions will occur within 5 minutes of the injection - Delayed reactions are frequent and may include headache, redness and welling - Cardiovascular reaction à peripheral vasodilation, decreased BP followed by cardiac arrest - Injection site is high risk due to high pressure à tissue damage, thrombus formation and extravasion possible - Will be painful for patient and some machines have a pressure monitor - In case of extravasion stop injection and apply ice - Contrast I toxic to the skin and may lead to chronic/acute inflammation, necrosis, ulceration or compartment syndrome (compresses other internal things) - egfr must be above 60

5 - Metformin must be stopped 48 hours prior due to the risk of interaction with contrast leading to lactic acidosis - Contrast induced nephropathy risk factors à kidney disease, dehydration, hypotension, over 60, cardiovascular disease, nephrotoxic drugs, chemo, organ transplant, multiple myeloma - PRECAUTIONS à Make sure to carefully screen all patients with questions, have emergency equipment and protocol on hand. Observe patient 15 mins post contrast - Timing is important with IV contrast to optimize contrast location - Timing may change dependant on patient - Venous access through cannulation - Pressure injector used, usually power. Can deliver pricise flow rate and flow with a consistent technique - A maximum pressure is set and if it is exceed it will stop, is pressure limiting - Dual injectors allows follow up with saline to reduce artefact due to bolus - Timed from the start of injection to the start of the scan acquisition - Time for best demonstration for the pathology or organ of interest - Multiphase image: imaging an organ during more than 1 phase of contrast enhancement (example kidneys or a lesion in the liver) - Alternatively, a split bolus can be used to reduce patient dose by acquiring less images - Standard for abdo is portal venous enhancement, 60 seconds - Arterial enhancement is around 25 seconds - Timing depends on cardiac output, rate of injection and the volume of injection - This may be initiated manually or automatically where a density of a certain region reaches threshold (bolus tracking) - A low dose slice at the level of the vessel of interest, either aorta or pulmonary vessels - Typical injection rates à brain 50mls by hand, adbo 75 to 100mls at 2.5 to 3.0 ml/s, CAP and CTPA 100mls at around 3ml/s - Cardiac angio uses rates faster than 5ml/s for maximum enhancement - Hot flash and metallic taste exacerbated by high injection rates - Enhancement is dependent on rate and volume of contrast so pricise control required for CTA - IV contrast in brain is used to show any breach of the blood-brain barrier - Blood brain barrier is the endothial cells lining the brain vessels to prevent perfusion into the actual brain tissue - Normally contrast will not penetrate this barrier unless there is rupture - Normal brain with contrast will have parenchyma s with increased density, choroid plexus vessels and dural folds enhanced - Abnormal brain will show lesions enhanced by the contrast Week 3 Image Processing CT head - Determine if contrast is needed - Lateral scout scan, non con and then con scan if needed - CT brain indications à trauma, headache, CVA, TIA, SOL, hydrocephalus, cranial nerve palsy, visual field defects, aneurysm (usually berry) - Post surgery done where there is metal implants and MRI cant be removed, patients are unable to communicate pain or issues - Cerebellar ataxia, where patient has loss of balance and co-ordination

6 - Wide range of patient presentations from walk in to acute emergencies - Check pregnancy, fast from solids for 2 hours, remove opacities - Fasting empties the stomach in case there is reaction to contrast (nausea and vomiting) - Scout scan, patient supine head first, immobilized in cradle with Velcro strap - MSP and OML 90 degrees to plane of table - Centre plan through EAM, 18 to 25cm FOV on scout - Show from foramen magnum to the vertex, slice thickness less than 1mm - For contrast, 50mls injected with hand injection - Pituaitary tumours have no BBB, neither does inflammatory disease so will enhance - Cerebral oedema will have low CT values, vasogenic around a tumour, cytotoxic usually involves cortex - Difficulties à claustrophobia, patient movement causing streaj artifacts (short scan time by increasing ma), iodine allergy, ventilated patients, children and trauma - Trauma are not sedated as it may mask neurological signs but they are often uncooperative, often reconstructed on a bone algorithm to look for fracture - Algorithm is actually changing the CT numbers of the voxels unlike windowing, uses edge enhancement - Trauma images realigned, the 1mm slices stacked together to 5mm to get a better SNR and better images - Beam hardening associated with high density absorber in the field, similar to streak artefact - In head streak from amalgam in the teeth which is very high density - Ring artefact from faulty detector channel, section of the detector not receiving information - Trauma often combines a CT and face acquisition, and potentially a C spine as part of the trauma series. Week 4 CT Head continued - GCS - CT shows lesion size, location, configuration, density, ventricular change, blood presence - Trauma à hemorrhage, contusion, fracture - Tumours à intra-axial such as gliomas and metastases, extra axial such as meningioma and calavarial neoplasms such as pagets or osteomas - Hydrocephalus look for low density ventricles enlarged - Treated with VP shunts - Extradural hemorrhage between periosteum and dura mater, takes very large pressure blood - Due to acute arterial bleed, produces convex shape with very high density of fresh blood - Usually due to middle meningeal artery bleed, will raise ICP - Subdural is from a slower venous bleed, may be from relatively minor trauma - Often causes mass effect. More of a potential space so it follows a curve of the cranial vault - Can be acute high density or chronic low density - Chronic symptoms include headaches, agitation, confusion and drowsiness - Sub arachnoid (SAH), bleeding into CSF ventricular system - See increased density in where you normal see ventricles - Caused by ruptured aneurysm or avm