Optimization Of CT ScanProtocol In Acute Abdomen
Scan Protocols• core of every CT examination.• protocols should be appropriate for the clinical indication• should include all aspects of the exam such• positioning,• nursing instructions,• scan parameters( including radiation dose)• reconstruction/reformatting instructions,
How do you design a CT protocol• components – Scanning parameters – What do to the patient • eg contrast when , how , and how we doing – Dose information – filming – network instruction – billing code
Scanning parameters• CT machine• kVp• mAs• Slice collimation• Slice thickness• Interscan spacing• Reconstruction algorithm for different tissues
Scanning parameters• multislice CT is better than single slice• MSCT : – High quality – Wider range of examination – Thinner slices – Shorter scan time – Multiphases protocol – Better reconstruction ( isotropic voxel)
kVp• Between 80-140• Higher kVp: in routine CT abdomen• Lower KVp: CTA, perfusion studies• Manual versus automatic KVp selection: – Care kV, Siemens machine
Tube current• mAs selected should result in diagnostic quality images• Most body CT and even head CT: Use AEC
Tube current• For all patients less than 20 years old, set the minimum mA to 80 for all studies.
Collimation• Narrow collimation and small reconstruction intervals can help detect calculi in the biliary system and genitourinary tract.• Affects – Total scan time – Noise / Low contrast resolution – Thinnest available recons• Some configurations (esp. narrow collimations) are less dose efficient (vendor-specific)
• Slice thickness: Acquire thins, reconstruct thick: Less noise• Scan coverage: scan length• Rotation speed: Keep fastest…for most regions to allow breath hold tech and more coverage
Increment• is the distance between the reconstructed images in the Z direction.• When the chosen increment is smaller than the slice thickness, the images are created with an overlap.
Increment• is useful to reduce partial volume effect, giving you better detail of the anatomy and high quality 2D and 3D post-processing .• can be freely adapted from 0.1 - 10 mm.
CT Image suitable for diagnostic purpose :– Low noise– High contrast resolution– Sharpness of image– Absence of artifacts
Pediatric protocols• should be adjusted regarding exposure parameters• Protocol optimization reducing radiation dose: – mAs according to patient size and weight – Implementation of automatic control sysyem
Oral Contrast• Type of contrast• Volume of contrast• Timing of contrast
oral contrast Types• Water neutral: negative contrast used in most cases• Water soluble positive contrast – Ominipaque 350 – Gastrografin agent (2% – 4%)– Diluted barium suspension (1% – 2%) e.g., EZCAT
oral contrast Volume• Upper abdomen: – Minimum 700-1000 ml of contrast – divided into 3 cups (approximately 250 – 300 ml) – 1st cup,30 minutes before exam – 2nd cup,15 minutes before exam – 3rd cup , 5 minutes before exam
oral contrast Volume• Abdomen-Pelvis: – Minimum 1000 ml – divided into 4 cups – 1st cup ,1 hour before exam – 2nd – 4th cups every 15 minutes – Start exam 5 minutes after the 4th cup
oral contrast• Use in – Suspected appendicitis – Fistula – Leakage of contrast anatomosis gastric bypass – Perforation• Not used in – High intestinal obstruction – Ureteric colic – Intestinal bleeding – Vascular cuases
Rectal contrast• may be used in – appendicitis – diverticulitis – leak or perforation – colonography – penetrating injury
IV Contrast• opacifies abdominal vasculature and• provides useful information regarding enhancement of the parenchymal organs and intestine• 100-120 mL of iodinated contrast material injected• rate of 3-5 mL per second is adequate
IV Contrast• is recommended in most cases.• Exceptions:• include evaluation of suspected ureteral colic, retroperitoneal hge or• contraindication to contrast
IV contrast• Normal creatinine level , should be within a month• High creatinine level , to be discuss with ordering physician• Look for – renal disease , hypertension, diabetes ,malignancy• Consider using a lower osmolar agent (Visipaque) in patients with diabetes and renal insufficiency•
Diabetics taking Metformin (glucophage)• should be stopped for 2 days after CT & creatinine checked prior to restarting Metformin• If creatinine is normal (< 1.5), I.V. contrast may be given;• If creatinine > 1.5,do not administer I.V. contrast.• Contact clinician &reschedule patient.• Contact referring clinician to obtain lab values.
Premedication Allergy pateints• Oral: 50 mg p.o. of prednisone 13 h., 7 h. and 1 h. prior to procedure and• 50 mg p.o. of Benadryl 1 h. prior to procedure. These patients should be accompanied to the hospital; they should not drive after taking Benadryl• IV: 200mg hydrocortisone 6h and 2h prior to procedure and 50 mg p o of Benadryl 1h prior to procedure
Technical aspect of acute abdomen CT Imaging• IV contrast should be given at 3-5 ml/sec• total of 100-120 mL,• followed by saline• Use SMART PREP or threshold tech
IV access CTAs :• high rates of injection,• a large bore IV, 18 g or larger is required• Do not use hand/forearm veins• Antecubital only.
IV access CTAs :• During power injections, the site must be closely monitored during the first 15 to 20 seconds to prevent extravasation• Some PICC catheters are designed for use with power injectors,• Check the label of any catheter for maximum flow rate and pressure.• Adjust the settings on the power injector accordingly.
Contrast extravasation• most are small & self limited. Ice pack and elevate for 20 mins. If swelling/pain resolved patient can be discharged – Advise patient to contact MD or go to E.R. if swelling/pin worsen• Skin sloughing is rare, can require a referral to plastic surgeon
Contrast extravasation• Compartment syndrome : with large volumes in the forearm/hand. – pain with extension of fingers. – May lose pulses – become cold/discolored. – requires referral to plastic/orthopedic/hand surgeon.
Renal Function/Creatinine levels• Patients with pre-existing renal failure or Diabetes Mellitus should have creatinine levels checked when the exam is non- emergent• In general, a creatinine of 1.8 or less is acceptable for non-ionic contrast use
Renal Function/Creatinine levels• For Creatinine levels above 1.8 there are several options: – 1. Withhold contrast if indication for contrast use is equivocal – 2. Administer N acetylcysteine (Mucomyst) – 3. Use a reduced dosage. – 4. If the patient is on dialysis with no renal function, they can be given contrast, preferably prior to dialysis. – 5. If the patient is on dialysis with borderline function, the nephrologist should be consulted prior to contrast use.
Contrast Allergy• Patients with prior severe/life threatening reactions should avoid contrast if at all possible• For other prior reactions, pre-medicate withoral prednisone 50mg 13 hrs,7 hrs & 1 hr priorto injection and oral benadryl 50 mg 1 hr prior
GeneralHints• Topogram : AP, 512 or 768 mm.• Patient positioning: Patient lying in supine position, arms positioned comfortably above the head in the head-arm rest lower legs supported.• Patient respiratory instructions: inspiration• Scout : AP and lateral
GeneralHints• Limit scan to intended anatomic area to cut dose by 10% – Abdomen: • Just above diaphragm – Inferior pubic symphysis – Chest: • Routine: Apex to adrenals • PE or benign clinical reasons: Apex to lung bases
Common causes of acute abdomen Practical aspect
Appendicitis• most common causes of acute abdominal pain• Most :1000 cc oral contrast before about 1 hour before• Others give oral & rectal• Scanning after 70 second from IV injection , might need delayed scan
Inflamed appendix Normal appendixThe appendix (arrows) is fluid-filled and distended withperiappendiceal fat-stranding.
Acute Pyelonephritis• Fever, chills, and flank tenderness.• referred for CT when symptoms are poorly localized or suspected complications .• nephrographic phase (70–90 seconds after injection) or• excretory phase (5 minutes after injection).
enlarged with perinephric stranding patchy, striated nephrograms
Ureteral Stones• continuous breath-hold acquisition from kidneys to bladder base.• Narrow (3-mm) collimation and small reconstruction intervals (also 3 mm) are essential for optimal detection of small calculi• Prone scans may be needed to differentiate a ureterovesical junction stone from a recently passed stone
51-year-old woman obstructing calculus in the midureterright hydronephrosis
Acute Pancreatitis• Contrast:• Patient should drink water as the oral contrast, OPACIFICATION AND DISTENTION OF DUODENUM IS VERY HELPFUL• IV contrast at 4-5mL/sec for 120 mL
Acute Pancreatitis• RS=0.5, narrow collimation , thin reconstructions, apply radiation protection facilities in the machine ( ASIR , Care dose )• scan entire pancreas in single breath hold for all phases.
Acute Pancreatitis• Noncontrast – Liver dome to iliac crests• Arterial phase – Initiate scan at 25 sec. Use “SMART PREP” Aorta (150HU) to monitor those with poor cardiac output. Top to bottom of liver. Ideally obtain excellent pancreatic parenchymal arterial opacification with minimal contrast in portal vein.
Acute Pancreatitis• Portal venous phase – 80 sec delay. Scan the entire abdomen in this acquisition (top of the liver to sp).• Delayed 3 minute scan through liver and kidneys.• Coronal and sagittal reformat of portal venous phase
Diverticulitisrectal contrast: is highly accurate for diagnosis Most use 400–800 mL of 3% iodinated contrastIV contrast : helpful in detection & characterization of pericolonic inflammation recommended in most patients.
62-year-old manAcute diverticulitisfocal thickening and pericolonic stranding
Small Bowel Obstruction• common cause of acute abdomen• adhesions most common (64%–79%)• hernia (15%–25%),• tumor (10%–15%)
Small Bowel Obstructionhigh-grade small bowel obstruction :• best performed without oral contrast.• large amounts of fluid in bowel acts as a natural contrast agent,• when combined with IV contrast ,allows opacification of bowel wall & masses
Small Bowel Obstruction low-grade obstruction:• oral contrast• improves accuracy in detection of inflammation & abscesses• optimize identification of a transition zone
Small bowel obstruction from inguinal hernia in a 55-year-old man
Ischemic Bowel• present with symptoms ranging from relatively minor discomfort to acute abdominal pain, which makes clinical diagnosis difficult• vascular occlusion or thrombosis, whether from arterial or venous disease, and hypoperfusion
Ischemic Bowel• rapid (4-5 mL/sec) IV contrast for optimal vascular opacificationi• IV contrast is essential for depiction of the thickened, edematous bowel wall, which can easily be appreciated against the obstructed, fluid-filled intestine• Arterial & venous phases are essential• Water can be used as alternative for bowel lumen
Gastrointestinal Perforation• If possible, oral & IV contrast should be used• to help localize perforation & characterize complications• Such as peritonitis and abscess formation.
• rapid (4-5 mL/sec) IV bolus contrast for optimal vascular opacification• Narrow collimation• high-quality 3D images• Oral contrast material is not administered , can interfere with reconstruction
AORTIC ANEURYSM• Study should only be performed in hemodynamically stable patients.• Hemodynamically unstable patients with high degree of suspicion of aortic pathology should go directly to OR.• If becomes unstable in CT, a quick noncon scan may be diagnostic.
Ruptured abdominal aortic aneurysm in an elderly man
AORTIC DISSECTION• Contrast: – No oral contrast – IV contrast at 4-5mL/sec with 125 mL
AORTIC DISSECTION• Scan method: – RS=0.5, narrow collimation , thin reconstructions, apply radiation protection facilities in the machine ( ASIR , Care dose ) – Non contrast – show intramural hematoma not well seen with contrast. • Top of arch to iliac crests
AORTIC DISSECTION• Arterial: Use HiRes HD mode, SMART PREP over aortic arch with threshold 150 HU, Apices to SP• Portal Venous – 80 sec delay from dome of liver to SP to assess organ perfusion.• Coronal and sagittal reformat of arterial phase• Coronal and sagittal MIP of arterial phase
LOWER EXTREMITY RUN-OFF• Contrast:• IV contrast at 4-5mL/sec for 125 mL (consider increasing to 150 for very tall patients)
LOWER EXTREMITY RUN-OFF• Scanning method – RS=0.5, narrow collimation , thin reconstructions, apply radiation protection facilities in the machine ( ASIR , Care dose ) – Noncontrast: From diaphragmatic hiatus through toes – Arterial:
LOWER EXTREMITY RUN-OFF• SMART PREP over knees – trigger scan at first blush of contrast. Do not use ROI!• From diaphragmatic hiatus through toes• Coronal and sagittal reformat of arterial phase• Coronal MIP of arterial phase
Sharing protocol files• Once protocols are made – Educating the CT technologists – Saving CT protocols on individual scanners – Ensuring protocols for head go to head section only – Trial run in few cases – Review of images‐
Sharing protocol files• Having hard copy protocol books by body region in all scanner suites – Scan length – Scan phases or passes – Contrast injection details• Shared drive access to protocols with in the intranet from any internal personal computer – Electronic copies of protocols with version date and protocol types
Conclusions• Optimize the patient preparation• Choose the best scanning protocol for individual patient• Optimize the dose profile for the patient• proper technique and protocol is essential for optimizing the CT examination and maximizing diagnostic accuracy
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