Imaging techniques and patient safety part 1: brain imaging

Imaging techniques and patient safety part 1: brain imaging.

CT Technique. From the scout image, the technologist normally selects two imaging ranges The first range is from the base of skull through the petrous pyramids. The second range of slices will be from the end of the first range and extending to the superior aspect of the brain tissue. The slice thickness in the first range is normally thinner (5mm) in the comparison to the second range (10mm) , to accommodate for artifacts created by the petrous pyramids. Angulations of axial slices is to be angled parallel with orbito - meatal line..

CT technique trauma protocol. ** axial images at 2.5-mm sections in standard algorithm, ** axial images at 5-mm sections in standard algorithm, **axial images at 2.5-mm sections in bone algorithm, **coronal and sagittal MPR . **axial images at 0.625-mm sections that can be used to make 3D reconstruction..

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CT brain commonly used window and level. W/L Structure 80/35 brain 2000/300 bone 250/50 acute subdural blood 40/40 Grey-white differentiation Early ischemia 400/40 Soft tissue.

CT windowing. The density of a tissue is represented using the Hounsfield scale , with water having a value of zero Hounsfield units (HU ), tissues denser than water having positive values, and tissues less dense than water having negative values By convention, low-density tissues are assigned darker ( blacker ) colors and high-density structures are assigned brighter (whiter) colors . To display/view CT scans, windowing is used to transform HU numbers into gray scale to values this allows different features of tissues to be seen by maximizing subtle differences among the tissues windowing is controlled by two parameters: window level (WL) and window width (WW).

Upper and lower grey level calculation. one can calculate the upper and lower grey levels i.e. values over x will be white and values below y will be black. the upper grey level (x) is calculated via WL + (WW ÷ 2) the lower grey level (y) is calculated via WL - (WW ÷ 2) For example: a CT brain with W:80 L:40, therefore, all values above +80 will be white and all values below 0 are black. A CT brain with a level of 0 HU and a width of 400 HU will have a range of −200 HU to +200 HU. Any tissue with a density of −200 HU or less will be black, and any tissue with a density of +200 HU or more will be white..

Hounsfield units Tissue 1000 Bone, calcium, metal 100 to 600 Iodinated CT contrast 30 to 500 Punctate calcifications 60 to 100 Intracranial hemorrhage 35 Gray matter 25 White matter 20 to 40 Muscle, soft tissue 0 Water −30 to −70 Fat − 1000 Air.

Bone windows: By shifting the gray scale to densities typical of bone, it allows the detection of abnormalities such as subtle fracture lines. meanwhile, they sacrifice detailed evaluation of structures less dense than bone (brain, CSF and blood vessels ). Brain windows: are useful for evaluation of brain hemorrhage, brain tissues and ventricles. On brain windows, bone and other dense or calcified structures (e.g., surgical clips , contrast nad calcified lesions) all appear bright white. Subdural windows: thin acute SDH may be difficult on routine CT. CT brain W/L is typically 80/35 HU so, any value higher than 80HU will be white and indistinguishable from other higher values. Acute blood has density around 100HU and bone much higher than this. Both will appear white look like a thicker skull.

CT windowing.

Subdural windows. olo.

The millisievert and milligray as measures of radiation dose and exposure.

Measuring radiation dosage. Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to intentional introduction of radiation sources . Background radiation has natural and artificial sources . These include naturally occurring radioactive materials (such as radon and radium ), as well as man-made medical X-rays, fallout from nuclear weapons testing . The annual background radiation in the UK is 2.7 mSv . Note for pediatric patients : Pediatric patients vary in size. Doses given to pediatric patients will vary significantly from those given to adults..

Measuring radiation dosage. When radiation passes through the body, some of it is absorbed. The x-rays that are not absorbed are used to create the image. The amount the patient absorbs contributes to the patient's radiation dose. The scientific unit of measurement for whole body radiation dose, called "effective dose," is the millisievert ( mSv ). Radiation doses that exceed a minimum (threshold) level can cause undesirable effects such as depression of the blood cell-forming process (threshold dose = 500 mSv ) or cataracts (threshold dose = 5,000 mSv )..

Radiation effect. CT scanners emit X rays . Different tissue types absorb X rays in varying amounts. Absorbed radiation can break chemical bonds in tissues, liberating charged ions (hence the term “ionizing radiation”) that can damage DNA and produce cancer should cells be unable to repair themselves. Nonionizing radiation —lower-energy radiofrequency waves such as those emitted by microwave ovens and cell phones—doesn’t break chemical bonds..

Medical Exposure. Radiation radiation exposure for medical purposes (diagnostic/treatment) is regulated under Ionising Radiation (Medical Exposure) Regulations 2000 (IRMER 2000). It provides guidance for three roles: Referrer (clinician) – provide adequate clinical details Practitioner (radiologist) – ensure scan is justified Operator (radiographer) – minimize amount of radiation.

Approximate effective radiation dose Comparable to natural background radiation for: Procedure 1.6 mSv 7 months Computed Tomography (CT)–Brain 3.2 mSv 13 months Computed Tomography (CT)–Brain, repeated with and without contrast material 1.2 mSv 5 Months Computed Tomography (CT)–Head and Neck 8.8 mSv 3 years Computed Tomography (CT)–Spine.

Benefit versus risk. The risk associated with medical imaging procedures refers to possible l ong-term or short-term side effects. Hospitals and imaging centers apply the principles of ALARA ( As Low As Reasonably Achievable ). This means they make every effort to decrease radiation risk. Exposure to 10 mSv ionizing radiation, such as from a CT chest or a CT abdomen, increases one’s lifetime attributable risk of cancer by 0.1 %, and this risk is cumulative with serial CT scans performed over one’s lifetime..

Age and Radiation Sensitivity. Age plays an important role in radiation sensitivity. Adults have less risk for radiation-induced health conditions, such as thyroid problems, than children. In patients age 60 and older, radiation exposure is not as significant an issue. The body tissues of older patients are less sensitive to the effects of radiation..

What can we do? Remember the following:.

1-Doctor’s role. 1- Assess the value of the requested CT, and the possibility of being replaced by another study. 2-Adequate monitoring of the ongoing examination to avoid the repeat of an exam and determine the necessity of a contrast media . 3-minimize amount of radiation.

2-Low-dose head computed tomography. To perform “half-dose” and “quarter-dose” head CT scans, we decreased the tube current (and thus the dose in mA) by approximately 50% and 75%, respectively, compared with standard head CT. decreasing the tube current by 50 %–75% will essentially decrease the radiation dose by 50 %–75%. However, reduction in tube current also increases image noise This technique usually used for those patients in need of multiple follow up head scans ..

2-Low-dose head computed tomography. Inclusion criteria to obtain a low-dose head CT scan Dose reduction to half Inclusion Criteria 1 ) shunted hydrocephalic patients presenting for routine follow-up, presenting to the emergency room over concern for ventriculoperitoneal shunt malfunction, or receiving a routine postop scan for catheter placement verification; 2) all patients receiving procedure needing neuronavigation ; 3 ) postop craniotomy patients ; 4 ) follow-up of a known ICH.

2-Low-dose head computed tomography. Dose reduction to quarter Inclusion Criteria routine postop CT scans (& follow-up) for craniosynostosis patients after either endoscopic or open repair where the skull bones were the primary object of radiographic evaluation; 2 ) follow-up of a bone-only skull lesion.

3-CT Safety During Pregnancy. 1- Alternatives to CT: In a pregnant patient, other imaging exams, such as ultrasound or magnetic resonance imaging (MRI), that do not involve x-rays, may be used if they provide the information your doctor needs . 2-Nonurgent x-rays should be avoided in weeks 10 to 17, the period of greatest CNS sensitivity. 3-Plain films result in negligible dose to the fetus when the fetus is not in the field of view. It is recommended that a radiation shield be applied over the gravid uterus for these examinations. 3-Plain films through the gravid uterus, such as radiographs of the abdomen and lumbar spine, result in a fetal dose of 1 mGy to 3.5 mGy . The background radiation dose to the fetus for 9 months of pregnancy is 0.5 to 1 mGy.

3-CT Safety During Pregnancy. 5- CT When the fetus is not in the field of view, the fetal radiation dose is negligible . The fetal dose for a CT head is 0 mGy and for a CT chest is 0.2 mGy or less depending on the trimester of pregnancy. Shielding the gravid uterus does little to reduce the radiation dose to the fetus for CT, but it is often done for reassurance 6- Concern about the harmful effects of x-rays to the fetus from CT is only considered for those examinations where the gravid uterus is in the field of view. The typical fetal radiation dose for a routine CT of the abdomen and pelvis is 25 mGy ..

3-CT Safety During Pregnancy. In the fetus, teratogenic effects , such as prenatal death, small head size, mental retardation, intrauterine growth restriction, and organ malformations. These teratogenic effects do not occur below a threshold of 50 mGy to 100 mGy and only occur in the first 15 weeks postconception . Above 100 to 150 mGy , the risk is serious enough that one may discuss therapeutic abortion with the mother. Since the fetal dose from a CT scan through the abdomen and pelvis is < 50 mGy , teratogenic effects are not a concern to the fetus when exposed to a single exam. They become a concern when multiple exams or multiphase exams are performed..

3-CT Safety During Pregnancy. Stochastic effects are due to DNA damage. In both the mother and fetus, this is carcinogenic. The probability of the effect rather than the severity increases with increasing dose, and there is no threshold below which there is no risk. The fetus is more sensitive to carcinogenesis than children and adults and also has a longer life expectancy in which cancers may manifest. increased risk of cancer in children when the fetus is exposed in utero to doses as low as 10 mGy ..

3-CT Safety During Pregnancy. You should not refuse a CT exam necessary for diagnosing your potentially serious or urgent illness because of fear of radiation. The goal is to take care of the mother, who has a much greater chance of developing a serious illness, such as appendicitis . The radiologist (a doctor with expertise in medical imaging) and the CT technologist will adjust the CT exam techniques to lower the radiation dose to your baby if they know you are pregnant..

Table 1. Fetal absorbed doses from selected procedures* X-Ray Examinations Cervical spine (AP, lat) Chest X Ray (PA, lat) Thoracic Spine X Ray (AP, lat) Abdomen X Ray (AP) Lumbar Spine X Ray (AP, lat) Limited IVP Barium Enema CT Examinationst CT Head CT Pulmonary Angiogram CT Abdomen CT Abdomen Pelvis CT KUB Fetal Absorbed Dose (mGy) <0.001 0.002 003 1-3 1 6 7 Fetal Absorbed Dose (mGy) 0.2 4 25 10 Background for 9 months of pregnancyt 0.5-1 • There are several measurements for radiation dose. The absorbed dose is measured in the rad and the effective dose is measured in the rem. In the international system of units, the rad is mea- sured as the Gray (Gy) and the rem is as the Sievert (Sv). I rad = 10 mGy and I rem= 10 mSV.1 tCT doses to conceptus and background dose to the conceptus obtained from McCollough CH, Schueler BA, Atwell TD, et al. Radiation and pregnancy: when should we Radiographics. 2m7;27909-917; discussion 917-908 (Reduced dose protocol for CT KUB)..

Table 3. Guidelines for managing pregnant women exposed to diagnostic radiation* Gestational age Radiation dose < 50 mGy < 2 weeks post conceptiont > 50-100 mGy 2-15 weeks < 50 mGy post conceptiont 50-150 mGy > 150 mGy Any > 15 weeks post conception Adverse biological effects None Possible abortion Minimal risk Childhood cancer (childhocxi cancer death 0.06% 10 mGy) Absolute cancer risk (0.4 % 10 Small head size (0.5-1 % per 10 mGy) Mental retardation (0.4% per 10 mGy) Childhood cancer (childhocd cancer death 0.06% FEr 10 mGy) Absolute cancer risk (0.4 % per 10 Small head size (15% risk) Mental retardation (6% risk) Childhood cancer cancer death 0.06% per 10 mGy) Absolute cancer risk (0.4 % 10 Childhood cancer (childhmd cancer death 0.060/0 per I O mGy) Absolute cancer risk (0.4 % 10 Recommendation None None None Generally, therapeutic abortion is not recommended. However, it may be considered with exposures > 100 mGy in the setting of other risk factors, such as acute viral infection or exposure to teratogenic drugs. Counsel mother about possible therapeutic abortion. None •source: Wagner I-K. RG Lester and LR Saldana Expcxure of the Pregnant Patient to Diagnostic Radiations. A Guide to Medical Management. Second Edition. Physics Publishing, Madison WI, 1997. Chapter 10 Ill The pp I 68. tFor 2-8 wæks conception at > mGy, there is a risk of organ malformations, but organ malformations have never observed in humans at diagnostic levels. *Applegate K and LK Wagner. ACR Practice Guideline for Imaging Pregnant or Potentially Pregnant Adolescents and Women with Ionizing Radiation. In: Resolution 26: Americnn of Radiology, 2(m:1-15.

Notes. The background radiation dose to the fetus for 9 months of pregnancy that is 0.5 mGy to 1 mGy . A single-pass CT scan of the abdomen and pelvis with intravenous and oral contrast exposes the fetus to 25 mGy ionizing radiation or less. The only risk to the fetus is a small increased risk of childhood cancer . Pulmonary embolism occurs with an increasing frequency in pregnancy due to the hypercoagulable state of pregnancy The risk to the fetus from ionizing radiation in pregnant patients is low for CTPA and V/Q scans. The fetal dose for a CTPA is 0.003 mGy to 0.13 mGy.

Conclusion. pregnant patients may experience nonobstetrical emergencies . A healthy pregnancy requires a healthy mother , and delayed diagnosis threaten the mother and her fetus. Ionizing radiation from CT results in a potential risk of cancer in the mother and fetus. Ionizing radiation from CT may result in teratogenic effects to the fetus at high doses up to 15 weeks postconception . A CT scan that does not include the gravid uterus in the field-of-view results in negligible fetal dose. A standard single CT scan through the gravid uterus results in a fetal dose of 25 mGy or less . At this dose, there is no risk of teratogenic effects to the fetus. The only risk is a small the risk of cancer. Intravenous and oral contrast may be given as necessary for CT..

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