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24 S. I. Wahl and K. M. Zinn
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iodinated agents, and it can cause discomfort during
injection in a substantial minority of patients, interfering
with study completion and adequacy.
15
CO2femoral arteriography may result in an incomplete distal study in up
to 50 to 75% of patients.
16,17
Because of the limitations of CO2as an alternative to
iodinated contrast, interest has centered on utilizing gadolinium based agents (e.g., gadodiamide) for angiography. Gadodiamide has been used alone or in conjunction
with conventional contrast or CO
for studies of the pe-
2
ripheral and renal arteries and the inferior vena cava. It
appears to be equivalent to dilute iodinated contrast in its
attenuation properties and may decrease the incidence
of renal failure in at-risk patients.
18–21
■ Conclusion
An understanding of the basic concepts of anatomy,
physiology filming, and injection technique is key to obtaining a good diagnostic angiogram. Evolving digital
technology has enabled the acquisition of diagnostic
studies quickly and safely. Modification of standard techniques, judiciously applied, will serve to benefit patients
and enhance the quality of the study.
REFERENCES
1. Cohen MI, Vogelzang RI. A comparison of techniques for improved
visualization of the arteries of the distal lower extremity. AJR Am J
Roentgenol 1986;147:1021–1024.
2. Crummy AB, Strother CM, Lieberman RP, et al. Digital video subtraction angiography for evaluation of peripheral vascular disease.
Radiology 1981;141:33–37.
3. Crummy AB, Steighorst MF, Turski PA, et al. Digital subtraction
angiography: current status and use of intra-arterial injection. Radi-
ology 1982;145:303–307.
4. Katzen BT. Current status of digital angiography in vascular imaging. Radio Clin North Am 1995;33:1–14.
5. Kadir S. Diagnostic Angiography. Philadelphia: WB Saunders; 1986.
6. Katayama H, et al. Adverse reactions to ionic and nonionic contrast
media: a report from the Japanese Committee on the Safety of
Contrast Media. Radiology 1990;175:621–628.
7. Lasser EC, et al. Pretreatment with corticosteroids to alleviate reactions to intravenous contrast material. N Engl J Med 1987;317:
845–849.
8. Lasser EC, Berry CC, Mishkin MM, et al. Pretreatment with cortecosteroids to prevent adverse reactions to nonionic contrast media.
AJR Am J Roentgenol 1994;162:523–526.
9. Lautin EM, Freeman NJ, Schoenfeld AH, et al. Radiocontrast-associated renal dysfunction: incidence and risk factors. AJR Am R
Roentgenol 1991;157:49–58.
10. Cruz C, Hricak H, Samhouri F, et al. Contrast media for angiography: effect on renal function. Radiology 1986;158:109–112.
11. Barrett BJ, Carlisle EJ. Metaanalysis of the relative nephrotoxicity of
high-osmolality and low-osmolality iodinated contrast media. Radi-
ology 1993;188:171–178.
12. Bettman MA. The evaluation of contrast-related renal failure. Am J
Roentgenol 1991;157:66–68.
13. Golman K, Almen T. Contrast media-induced nephrotoxicity: survey and present state. Invest Radiol 1985;21:S92–S97.
14. Kerns SR, Hawkins IF. Carbon dioxide digital subtraction angiography: expanding applications and technical evolution. AJR Am J
Roentgenol 1995;164:735–741.
15. Hawkins IF, Caridi JG, Kerns SR. Plastic bag delivery system for
hand injection of carbon dioxide. Am J Radiol 995;165:1487–1489.
16. Diaz LP, Pabion IP, Garcia JA, Lopez M. Assessment of CO
raphy in arterial occlusive disease of the lower extremities. J Vasc
Interv Radiol 2000;1:163–169.
17. Rolland Y, Duvauferrier R, Lucas A, et al. Lower limb angiography:
a prospective study comparing carbon dioxide with iodinated contrast material in 30 patients. AJR Am J Roentgenol 1998;171:333–337.
18. Kaufman JA, et al. Renal insufficiency: gadopentetate dimeglumine
as a radiographic contrast agent during peripheral vascular interventional procedures. Radiology 1996;198:579–581.
19. Kaufman JA, et al. Gadolinium-based contrast agents as an alternative at vena cavography in patients with renal insufficiency: early
experience. Radiology 1999;212:280–284.
20. Spinosa DJ, Angle JF, Hagspiel KD, Hartwell GD, Matsumoto AH.
Lower extremity arteriography with use of iodinated contrast material or gadodiamide to supplement CO
with renal insufficiency. J Vasc Interv Radiol 1996;11:35–43.
21. Spinosa DJ, Matsumoto AH, Angle JF, Hagspiel KD. Use of
gadopentetate dimeglumine as a contrast agent for percutaneous
transluminal renal angioplasty and stent placement. Kidney Int
1998;53:503–507.
angiography in patients
2
arteriog-
2
I.Osborn and R. SnyderAnApproach to Sedation and Monitoring
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4
■■■
An Approach to Sedation and Monitoring
IRENE OSBORN AND RANDALL SNYDER
As the complexity and duration of diagnostic vascular and
interventional procedures have increased, it has become
more important to provide adequate comfort for the patient. Interventional radiologists must consider that patients are anxious about the procedure, will be in a supine
position on a hard and generally uncomfortable procedure table, and are usually in a cold environment. Interventional radiologists also must address the pain or possibility of pain during the procedure. Monitoring the
patient during the procedure, especiallyif conscious sedation is administered, and monitoring during recovery are
essential. Patients must refrain from moving for much of
the procedure, and cooperate in specific breath-holding
maneuvers during some manipulations and during most
image acquisitions. Thus, the role of sedation and monitoring is key.
Personnel attending to patients undergoing interventional procedures should have sufficient knowledge of
the pharmacology, indications, and contraindications for
relevant sedative agents and the ability to recognize and
initiate treatment for any adverse reaction to those agents
as well as have basic skills in emergency airway management and resuscitation. In addition to the supervising
physician, the team should include a health professional
whose primary job is monitoring the patient and who is
available to the patient during the procedure and postprocedure recovery phase, until the patient has been
transferred to appropriate recovery room personnel.
Intravenous conscious sedation is a minimally depressed level of consciousness induced by the administration of pharmacologic agents, in which the patient retains conscious and independent ability to maintain a
patent airway and to respond to physical and or verbal
1
stimulation. Deep sedation is a controlled state of depressed consciousness or unconsciousness from which
the patient is not easily aroused; it is accompanied by a
partial or complete loss of protective reflexes, including
the ability to maintain a patent airway independently and
loss of appropriate response to physical stimulation or
verbal command. During conscious sedation, a patient
may start out anxious or agitated but will become tranquil, cooperative, and oriented; during the deepest level
of conscious sedation, the patient may be somnolent,
responding to commands only. When the patient enters
a state of deep sedation, the patient may respond only to
loud auditory stimuli or may exhibit sluggish or absent
response to a loud stimulus or glabellar tap. Deep sedation is monitored under the care of an anesthesia, rather
than a procedural, team.
led state of unconsciousness in which there is a loss of
protective reflexes, including the ability to maintain a
patent airway independently and to respond appropriately to physical stimulation or verbal command.
Before intravenous conscious sedation is instituted, a
pertinent history and physical examination must be performed or they must be available from the day of the
procedure. A specific allergic history is also necessary. An
assessment of baseline vital signs, level of consciousness,
motor function, electrocardiogram (ECG) and oximetry
also should be done. In children, weight is needed to
determine dose determination. During conscious sedation, intravenous access must be continually maintained.
Monitoring of physiologic measurements should be performed and recorded at least every 15 minutes during the
procedure and into the recovery phase. Physiologic parameters to be measured include level of consciousness,
2
General anesthesia is a control-
25
26 I. Osborn and R. Snyder
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respiratory function, oximetry, blood pressure, and heart
rate and rhythm. Supplemental oxygen and suction must
be immediately available and a defibrillator with a crash
cart that has backup emergency power also should be
available immediately. These general principles apply to
adults. Requirements for pediatric sedation and analgesia
in children are somewhat more specific.
3
■ Pharmacologic Agents
Widely used agents for anxiety in the interventional suite
include the benzodiazepines midazolam (Versed) and diazepam (Valium). Lorazepam (Ativan) is another benzodiazepine that can be used for sedation. Like most other
agents, benzodiazepines are usually administered intravenously. These anxiolytic agents are important components of intraprocedural sedation, but they can be used
as the initial premedication soon after the patient has
arrived in the angiography suite and consent has been
obtained. Benzodiazepines provide anxiolysis and amnesia and have anticonvulsant properties; they are also reversible. These agents can cause respiratory depression
and hypotension, and may interact with administered
opioids (Table 4-1).
For analgesia, three commonly used opioids are morphine, meperidine (Demerol), and fentanyl. Opioids provide analgesia and sedation; they are reversible, but they
may cause respiratory depression, nausea, and hypotension. Benzodiazepines and opioids may interact. Additional agents that may be useful include droperidol, a
major tranquilizer that supplements sedation and produces a calm appearance, but is not administered as a sole
agent. Droperidol is useful for the prevention and treatment of nausea. Ketamine, administered intravenously
over 2 to 3 minutes, may be used for additional analgesia.
Ketamine may cause hallucinations and is generally given
after benzodiazepines are first administered and the patient is reassured. Ketamine is a powerful analgesic that
can be used in painful procedures such as percutaneous
biliary drainage.
Reversal agents includeflumazenil (Romazicon), which
is used to reverse the effects of benzodiazepines, and naloxone (Narcan), which can reverse the effects of opioids.
■ Physiologic Monitoring: Vital Signs
Monitoring during conscious sedation by an independent
observer who is trained in patient care and monitoring,
such as a registered nurse or physician assistant, is manda-
1
tory.
Although physiologic monitoring is essential and
required, it is not a substitute for direct patient observation. Patients undergoing conscious sedation should be
quiet but easily arousable. Respirator y depression or apnea, hypotension, contrast reactions, and cardiac arrythmia and arrest can occur, and it is essential that the interventional radiologist be prepared to handle these
potential adverse events. We also usually monitor patients
undergoing procedures that do not use intravenous conscious sedation because patients may have comorbid conditions and because even benign procedures may engen-
TABLE 4-1. Commonly Used Agents for Conscious Sedation
“Metabolism,”
Agent Class/indication Usual IV dosage contraindications (min.) Half life
Diazepam Benzodiazepine: anxiolysis 5–10 mg Liver 30 21 h
(0.1 mg/kg)
Midazolam Benzodiazepine: anxiolysis 2–5 mg Liver 10 2.5 h
(0.1 mg/kg) max Hypotension
Lorazepam Benzodiazepine: anxiolysis 2.5 mg Elderly 20 h duration of effect
Morphine sulfate Opioid: analgesia 5–10 mg Liver 20 3–4 h duration of effect
(0.15 mg/kg) Hypotension
Meperidine Opioid: analgesia 25–75 mg Liver 20 2–4 h duration of effect
(1.0 – 1.5 mg/kg) Hypotension
Fentanyl Opioid: analgesia 1–2 microgram/kg Liver 5–10 2–4 h
Benadryl Antihistamine 25–50 mg ↑ intraocular pressure
Naloxone Reversal agent 0.1–0.2 mg Hypertension 3 20 min.
Flumazenil Reversal agent 0.2 – 1mg max 3–5 10 min. duration of effect
Ketamine Analgesia 0.25 mg/kg Increased intracranial 5–7 2 h
Inapsine Anti-nausea agent 1.25–2.5 mg Elderly debilitated pts. 20 4h
(Droperidol)
IV, intravenous
common Peak
CNS depressants
pressure, Anxiety
An Approach to Sedation and Monitoring 27
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der physiologic responses (e.g., a peripherally inserted
central catheter line in the right ventricle can induce an
arrythmia).
The blood pressure may be taken manually, but, more
commonly, automatic noninvasive arterial blood pressures are used. These automated devices also allow observation of the ECG, heart rate, respiratory rate, and pulse
oximetry.
The width of the blood pressure cuff should be approximately 40% of the upper arm circumference, with
the length approximately 80% of the arm circumfer-
4
ence.
Falsely elevated blood pressures can be obtained
with cuffs that are either too narrow or too loose.
The normal respiratory rate for adults is between 12
and 20 breaths per minute. Bradypnea may be associated
with conscious sedation and analgesia and may signal
oversedation. Tachypnea often is related to preprocedure
anxiety, but it may have physiologic causes as well.
Pulse oximetry
Pulse oximetry uses spectrophotometry to measure
changes in light absorption of blood. The pulse oximeter
is an essential monitoring device that noninvasively measures arterial hemoglobin oxygen saturation (SaO
pulse rate. Oxygen is highly bound to hemoglobin, and
one hemoglobin module may bind zero to four molecules
of oxygen.
5
For example, when four oxygen molecules are
bound, the hemoglobin molecule is 100% saturated;
when one is bound, it is 25% saturated. SaO
2
saturated hemoglobin to the total number of hemoglobin
modules, and it is a fairly good reflection of arterial oxygenation. SaO
oxygen (PaO
ciation cur ve. PaO
is directly related to the partial pressure of
2
) via the well-known oxyhemoglobin disso-
2
is a reflection of the amount of oxy-
2
gen dissolved in the blood and normally ranges from 80 to
100 mm of mercury when pH and body temperature are
normal. This is equivalent to an SaO
of 95 to 100%, the
2
normal range seen on the pulse oximeter. Pulse oximetry
is a reliable indicator of SaO
and has become the stand-
2
ard of care for patients receiving conscious sedation and
analgesia. If the SaO
falls below 95%, immediate assess-
2
ment of the patient’s ventilation and status should be
performed. Pulse oximetry providesan indication of early
hypoxemia. Because procedural positioning oreven draping of the patient may make it difficult to observe the
patient’s ventilatory status, it is an essential tool in providing uninterrupted continuous monitoring of the patient.
Capnography allows monitoring end expiratory carbon dioxide and can be used for early detection of hypoventilation and airway obstruction. Its use is becoming
increasingly encouraged for monitoring patients undergoing conscious sedation.
), and
2
is the ratio of
Electrocardiogram
The final, but by far the most important, monitoring
parameter is the ECG. We monitor the ECG in virtually
all patients undergoing vascular and interventional procedures, even those performed without conscious sedation. A procedure should not be started until an adequate
tracing is obtained. ECG monitoring requires the proper
placement of at least two sensing electrodes and a third
ground or reference electrode. Because the ECG is a
small (1 mV) electrical signal, its measurement is susceptible to electrical interference from other sources, especially power cords, electrocautery, and patient motion. It
is important that electrodes have adequate contact gel
and be applied to clean, dry skin.
The most commonly monitored lead is lead II because
the P wave is easily seen and has the greatest voltage in
this lead. This orientation allows enhanced detection of
dysrhythmias as well as diagnosis of inferior wall ischemia.
Lead V5 is monitored for the detection of myocardial
ischemia because the bulk of left ventricular myocardium
lies beneath it. If only a three-lead system is available, a
modified V5 lead can be obtained by placing the right
arm lead under the right clavicle, the left arm lead in the
V5 position, and the left leg lead in its usual position
while monitoring lead I.
6
A preprocedural rhythm strip should be obtained as
standard protocol so that any potential change in rhythm
or ST segment that is appreciated can be compared with
a baseline later. If the patient has significant cardiac disease, a five-lead system may be used which will provide 80
to 96% sensitivity for the detection of intraprocedural
cardiac events. Lead II alone has a sensitivity of only 18 to
33%. Prompt treatment of dysrhythmias or tachyarrhythmias is often necessary in the inter ventional suite. A
working knowledge of ACLS may be helpful in these
situations.
■ American Society of Anesthesiologist
Classification
In 1940 the American Society of Anesthesiologists (ASA)
developed a physical status classification for patients receiving anesthesia and surgery to standardize physical
and clinical status for statistical analysis and outcomes
research for hospital records (Table 4-2). This classification was further modified by Dripps et al in 1961, and it
remains the standard classification used by anesthesiologists and others to characterize the patients’ physical
status preprocedurally or preoperatively.
not intended as such, the ASA physical status classification has since been shown to correlate well with the
perioperative mortality rate.
7
Although it was
28 I. Osborn and R. Snyder
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TABLE 4-2. Physical Status Classification of the
American Society of Anesthesiologists*
a
Class
1 Healthy patient 0.06–0.08
2 Mild systemic disease 0.27–0.4
3 Severe systemic disease, not incapacitating 1.8–4.3
4 Severe systemic disease that is a constant 7.8–23
5 Moribund, not expected to live 24 h 9.4–51
a
Refs: 7, 8
** An E is added to ASA status to designate an emergency operation.
Description rate (%)
threat to life
irrespective of operation
Mortality
Underlying disease is only one of many factors that contribute to perioperativecomplications. ASAphysical status
classification is a useful tool in planning anesthetic management and level of intraprocedural monitoring and it
can be used by the interventionalist to categorize potential
procedural risk.
8
In some institutions, ASA classification is
used as criterion for calling an anesthesia consultation for
procedural management in interventional radiology.
Patients receiving intravenous conscious sedation
should recover in an area where continuous monitoring
and resuscitative equipment are available. Patients should
be monitored to full recovery by a return to baseline
mental status and vital signs. Many institutions utilize an
objective scoring system, such as the Aldrete or modified
Aldrete score.
9,10
The Aldrete system scores patients from
0 to 2 on five parameters: activity, respiration, circulation
(blood pressure), consciousness, and skin color. Return
to physiologic baseline or an adequate pre-set score on
these parameters is required for discharge from the procedure area (intensive monitoring, phase I) to a less intensive recovery area (phase II). Additional discharge criteria from the phase II area may include the ability to
tolerate the fluids taken orally, minimal or absent nausea
or vomiting, lack of dizziness when sitting, a complete
return to the baseline level of consciousness, absence of
pain, and ability to urinate.
4
If short-acting sedation
agents have been used, some patients may be discharged
from this area in as little as 1 hour, but other procedural
factors may mandate longer observation (e.g., the presence of an arterial puncture site or the need to watch for
bleeding after deep organ biopsy). When short-acting reversal agents are used during the procedure, it is essential
to observe the patient for a sufficient time to allow observation of the reemerging effects of the longer-acting
agonists; this is typically 2 hours if short-acting benzodiazepines and opioids have been used. Patients should
be cautioned not to drive or operate heavy machinery for
at least 24 hours after the administration of analgesic
sedative agents. Patients should not be sent home with
pain that cannot be controlled with oral analgesics, and
they should have 24 hour emergency contacts and written
postoperative instructions before discharge.
REFERENCES
1. American College of Radiology. Standard for use of intravenous
conscious sedation. 1998 ACR Standards. Reston, VA: ACR; 1998
2. Ramsey MAE, Savage TM, Simpson TRJ, Goodwin R. Controlled
sedation with alphaxalone-alphadolone. BMJ 1974;2:656–659.
3. American College of Radiology. Standard for pediatric sedation/
analgesia. 1998 ACR Standards. Reston, VA: ACR, 1998.
4. Watson DS. Conscious Sedation/Analgesia. St. Louis: Mosby; 1998.
5. Westmiller SW, Hoffman LA, Wiseman M. Understanding transtracheal oxygen delivery. Nursing 1989; 12:43–47.
6. Morgan GE, Mikhail MS. Clinical Anesthesiology. Norwalk: Appleton
& Lange; 1992.
7. Dripps RD, Lamont A, Eckenhoff JE. The role of anesthesia in
surgical mortality. JAMA 1961;178:261.
8. Muravchick S. The Anesthetic Plan: From Physiologic Principles to Clini-
cal Strategies. St. Louis: Mosby Year Book; 1991.
9. Aldrete JA, Kroulik D. A postanesthetic recovery score. Anesth Analg
1970;49(6):924–934.
10. Aldrete JA. Modifications to the postanesthesia score for use in
ambulatory surgery. J Perianesth Nurs 1998;13(3):148–155.
J.A. KaufmanMRand CT Angiography
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5
■■■
Magnetic Resonance and Computed
Tomographic Angiography
JOHN A. KAUFMAN
The choice of an imaging modality for patients with vascular disease is no longer straightforward. Conventional
angiography has benefited greatly from the development
of safer contrast agents, sophisticated cathetersand guidewires, and digitally enhanced image acquisition. Innovations in cross-sectional imaging technology, however, have
resulted in a number of less invasive vascular imaging modalities. It is now conceivable that ina singleinstitution the
following menu could exist for vascular imaging: filmscreen angiography, intraarterial digital subtraction angiography (IADSA), intravenous DSA (IVDSA), magnetic
resonance angiography (MRA), computed tomographic
angiography (CTA), duplex and color-flow ultrasound,
contrast enhanced ultrasound, transesophageal/transluminal ultrasound, three-dimensional ultrasound, intravascular ultrasound, and isotope angiography. How does one
choose the correct test, or is there such a thing? This
chapter focuses on two of these modalities: CTA and MRA
and when each should be used.
The ideal vascular imaging modality would be rapid,
accurate, painless, risk free, applicable to all patients, easy
to interpret, and inexpensive. Because of its proven accuracy and wide acceptance by clinicians, traditional angiography is the standard against which all new modalities must be compared. Traditional angiography provides
reliable diagnostic images and is applicable to large numbers of patients, but it is associated with some degree of
discomfort and risk.
noninvasive imaging modalities that can provide accurate
angiographic information quickly in large numbers of
patients. Both modalities have different strengths and
weaknesses, which in many cases are complementary.
The clinical evaluation and application of MRA and CTA
1
MRA and CTA hold promise as
are ongoing, yet there is no doubt that these techniques
will have important roles in vascular imaging in the future.
■ Basic Techniques
MR imaging/MRA
Principles of MR imaging
One need only understand that MR imaging (MRI) is
based on the detection of radiofrequency signals emitted
by protons spinning within a powerful magnetic field. In
the bore of an MR scanner, protons in tissue preferentially
align the axes of their spins with the magnetic field (longi-
tudinal magnetization). By applying a radiofrequency pulse
to the protons, the spins can be deflected momentarily, or
“tipped,” out of alignment with the magnetic field, in a
plane perpendicular to the magnetic field (transverse mag-
netization). The degree of deflection is determined by
characteristics of the radiofrequency pulse and is described as the flip angle. As the protons realign with the
magnetic field (relaxation), a signal (echo) is created. Images are created that emphasize the vectors of the longitudinal (T1) or transverse (T2) relaxation of the spins as
they realign. In general, short echo times reflect the
longitudinal magnetization (T1 weighted), and long echo
times the transverse magnetization (T2 weighted). By varying how often and how forcefully the protons are tipped,
as well as when the echo is sampled, the same tissue may
2
appear alternatively bright or dark. The complex behavior of protons in magnetic fields provides the basis for the
29
30 J. A. Kaufman
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wide variety of pulse sequences and imaging strategies
available in MRI.
MRI technology
Most first-generation of MR scanners were constructed
using a central tunnel or bore lined with coiled superconducting wires. Generally, the higher the field strength
(measured in Tesla [T], and ranging from 0.3 to 1.5 T for
most clinical scanners), the larger the actual scanner and
the higher the signal-to-noise ratio in the images. Although these scanners remain in active clinical use, newer
“open” magnet designs are becoming available. These
scanners offer the practical advantages of accommodating
larger patients than the older devices as well as presenting
a less confined scanning environment to claustrophobic
patients. Of great interest to vascular and interventional
radiologists is the possibility of performing MR-guided
procedures in magnets with open designs that permit access to the patient during scan acquisition. Sophisticated
targeting and device tracking technology is required in
this environment, and special equipment construction is
necessary for working within the strong magnetic field.
Nevertheless, this technology offers great promise, particularly in patients in whom pathologic processes can be
imaged only by using MRI.
3
Principles of MRA
One of the most common non-contrast enhanced applications of MRI in vascular and interventional radiology is
MRA. Early in the clinical application of MRI, it was noted
that flowing blood sometimes appeared bright on images
of the brain and body. The brightness of the blood suggested that angiographic information could be obtained
without catheterization and injection of iodinated contrast agents. In a landmark article in Science in 1985,
Wedeen and co-workers published the first true angiographic images created using MR techniques.
4
The initial
clinical applications of MRA were in the intracranial arterial circulation and the extracranial carotid arteries. Now
MRA is performed in all areas of the body, with the most
common nonneurological applications being lowerextremity arteries and veins, the abdominal aorta, the
renal arteries, the portal veins, and the thoracic aorta.
5–14
The two most common MRA techniques are time-offlight (TOF) and phase-contrast (PC) MRA. Both techniques can be used to acquire data on a slice-by-slice basis
[two-dimensional (2-D)] or as a volume acquisition
[three-dimensional (3-D)]. In general, thinner images
can be obtained using 3-D techniques, but signal-to-noise
is better in 2-D images. Acquisitions also may be cardiac
gated, so that images are obtained at the same level
throughout the cardiac cycle or at different levels at the
same point in the cardiac cycle. Images obtained at one
level throughout the cardiac cycle can be displayed in a
movie loop (cine), which is useful in situations such as
evaluting cardiac valvular competence. Gating to obtain
images at multiple levels at the same point in the cardiac
cycle minimizes pulsatility artifact and signal loss in multiphasic flow.
TOF/MRA
The most widely available MRA technique for body applications is TOF MRA. With this technique, stationar y tissues are subjected to multiple and frequent radiofrequency pulses; so the spins do not have time to realign
(relax) with the magnetic field, resulting in saturation of
the tissues and only a weak signal. Blood that flows into
the area being imaged contains spins that have not yet
been subjected to the radiofrequency pulses. These fresh,
or unsaturated, spins emit a strong signal when they first
enter the area being imaged and thus appear bright (Fig.
5-1). If the blood is exposed to the radiofrequency pulses
for any length of time, however, it will become saturated
and indistinguishable from the surrounding stationary
tissues. This problem is particularly troublesome when
using 3-D or volume imaging or when vessels are in the
plane of a 2-D slice (Fig. 5-2).
Flow from all directions into a slice or volume will be
visualized during MRA unless measures are taken to select flow in the direction of interest. With TOF MRA this
visualization usually can be accomplished by saturating
the spins in unwanted blood with radiofrequency pulses
before they enter the area being imaged. These radiofrequency pulses are applied in a broad presaturation slab
positioned outside the area of interest (Fig. 5-3). For
example, with 2-D TOF MRA of the lower extremity arteries, source images are acquired in the axial plane; to
FIGURE 5-1. Schematic diagram of time-of-flight magnetic
resonance angiography. Signal from background tissues is
suppressed by repetitive radiofrequency pulses. Signal is emitted from the fresh spins in blood as it enters the imaging
volume or slice.
A B
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FIGURE 5-2. Saturation of signal from in-plane flow. A: Coronal maximum
intensity projection of a two-dimensional time-of-flight magnetic resonance
angiogram of the popliteal arteries and proximal tibial arteries. The source
images were acquired in the axial plane. There is loss of signal in the
horizontal (in-plane) portion of the right anterior tibial artery origin (
which could be due to either a stenosis or artifactual signal loss. B: Conventional angiogram of the same patient shows that this artery is normal
arrow
). C: Schematic diagram illustrating saturation of in-plane flow.
C
(
arrow
),
FIGURE 5-3. Saturation slabs are critical in time-of-flight imaging to eliminate unwanted signal from flowing blood. In this
schematic, arterial blood entering the slice from the left contains fresh spins that can be imaged. The spins in venous
blood are saturated when they pass through a saturation slab
outside of the imaging slice, so that no signal is emitted when
they enter the slice.
32 J. A. Kaufman
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FIGURE 5-4. Gadolinium-enhanced, three-dimensional magnetic resonance angiogram of a normal aortic arch. Note that the subclavian artery flow (
imaging volume, does not become saturated, and retains excellent signal because of the gadolinium. The pulmonary arteries and veins (
vena cava (S) is faintly seen as a result of rapid shunting of gadolinium through the cerebral
circulation and enhancement of the jugular veins and subsequently the superior vena cava.
curved arrows
arrows
), which reverses direction within the
) are visible within the chest. The superior
eliminate signal from the veins, a presaturation slab is
placed inferior to the slice of interest (i.e., toward the
feet), thus saturating spins in the venous blood before
they flow into the slice. To obtain a 2-D TOF MR venogram of the lower extremities, the presaturation slab is
repositioned superior (i.e., toward the head) to the slice
of interest to saturate the spins in the arterial blood
before they flow into the slice. Care must be taken when
imaging tortuous vessels, however, as they may loop into
the presaturation slab and appear occluded on the MRA.
Flowing blood produces the brightest signal when the
spins all move at the same velocity, in the same direction,
and perpendicular to the plane of image acquisition. Unfortunately,blood flowis complexin vivoas aresult ofsuch
diverse factors as shear along the vessel wall; changes in
velocity and acceleration resulting from cardiac activity,
elastic recoil of arterial walls,and stenoses;and turbulence
caused by tortuosity of blood vessels, aneurysms, or stenoses. Each of these factors results in some loss of signal
from blood with all MRA techniques. Strategies to reduce
this signal loss include techniques such as flow compensation and cardiac gating, but all incur a time penalty.
Gadolinium-enhanced MRA
One of the most effective strategies to increase intravascular signal with TOF sequences is the addition of an MR
contrast agent, such as gadolinium, as described by
Prince.
15
This agent has been applied most successfully to
3-D acquisitions, although it can be used with 2-D imaging
as well. Gadolinium-based contrast agents act to shorten
the T1 of blood and tissues. In conventional MRI, a dose
of 0.1 mmol/kg (0.2 mL/kg) is administered intravenously before the patient is placed in the scanner to evaluate the soft tissues. For vascular imaging, a relatively large
volume ofgadolinium contrast (up to0.4 mmol/kg, or 0.8
mL/kg) is injected rapidly during the acquisition of a 3-D
volume. The arteries are preferentially enhanced, much
in the same fashion that arteries are visualized during
rapid venous injection of iodinated contrast agents with
IVDSA. This technique eliminates loss of signal resulting
15
from slow, turbulent, or in-plane flow.
Large fields of
view can be covered without signal loss in blood that remains within the imaging volume (Fig. 5-4). This technique has become widely used in body MRA.
PC MRA
PC MRA is both more complex and less widely available
than TOF sequences because the demands on scanner
hardware, particularly gradient coils, are greater with PC
than with TOF MRA. The fundamental principles of PC
MRA are that gradients var y in strength within the magnetic field, and the phase of a spin will vary in proportion
to the strength and duration of an applied magnetic
gradient (Fig. 5-5). If equal and opposite gradients are
applied, a spin that remains stationary will deflect in an
equal and opposite manner. When the vectors of these
opposite and equal deflections are subtracted, the net
value (or phase shift) is zero. When a moving spin is
exposed to the same equal and opposite gradients, the
spinning proton moves to a physically different place
within the magnet during the time between application
of the gradients. The strength of the gradients varies
depending on their location within the magnetic field. As
a result, the vectors of the deflections caused by the
gradients are opposite but not equal. Subtraction of these
two opposite but unequal vectors results in a measurable
net phase shift, which then can be displayed as signal,
whereas stationary background tissues with a net phase
shift of “0” have no signal. The larger the phase shift (the
greater the distance that the spin moves or the larger the
FIGURE 5-5. Schematic drawing of the principle of phase-con-
https://t.me/med1917
trast angiography. When stationary spins are exposed to equal
and opposite gradient pulses, the deflections of the spins are
equal and opposite; therefore, there is no net signal. When the
same gradient pulses are applied to a moving spin, the deflections are opposite but not equal because of variation in gradient
strength within the magnet. Therefore, a net signal is created
that can be imaged. (After Dumoulin CL, Souza SP, Pelc NJ.
Phase sensitive flow imaging. In: Gottschalk A, ed.
Resonance Angiography: Concepts and Applications.
EJ, Siebert JE, Haacke EM, St Louis: Mosby, 1993).
Magnetic
Potchen
applied gradient), the stronger the signal. PC MRA is by
definition a purely subtractive technique in that only
moving spins will be visualized, with no need to saturate
background tissues as in TOF techniques.
The direction of flow is an important consideration in
PC MRA because phase shifts occur only in the direction
of the phase encoding gradient. Blood flow in most areas
of the body is complex, rather than unidirectional. Consider the right renal artery, which arises from a slightly
anterior location on the abdominal aorta, curves posteriorly behind the inferior vena cava, and then curves inferiorly toward the renal hilum. Imaging only right-to-left
flow in this artery would result in incomplete visualization
of the proximal and distal portions of the vessel. By acquiring phase shifts in all three directions (right to left,
superior to inferior, and anterior to posterior), a complete study of the vessel can be performed (Fig. 5-6).
Unfortunately, one pair of gradients is required per direction to create phase shifts. With each set of gradients
used, the time required to perform the study increases.
Fortunately, flow in three directions can be imaged with
four, rather than six, gradients using a technique known
as Hadamard multiplexing. Nevertheless, the time required to complete a PC acquisition is long relative to
TOF MRA.
One of the most critical elements of PC MRA is the
velocity of the blood flow to be imaged. The goal of PC
imaging is to maximize the phase shifts in flowing spins
(large shifts result in brighter-appearing flow in the final
images). Phase shift is directly proportional to several constants (such as the strength of the magnetic field) and to
the variables of velocity of the spins and the amplitude of
the gradients. Thus, larger gradients are required to im-
MR and CT Angiography 33
age slow flow and smaller gradients to image faster flow.
The parameter that describes this relationship is velocity
encoding (V
), usually calibrated in centimeters per sec-
enc
ond. For example, normal main renal artery flow is approximately 100 cm/sec; so one would selecta V
enc
slightly
higher than the expected flow (perhaps 110 cm/sec) to
optimize visualization of flow in this portion of the vessel.
Flow at other velocities (such as in the peripheral intrarenal arteries) will not image as well. Furthermore, if the
flow is slower than expected, perhaps because of severe
occlusive disease in the artery or poor cardiac output,
imaging at a high V
will result in poor visualization of
enc
the main renal arter y flow. Currently, there is no widely
available method for quickly determining which V
enc
will
result in the best image for a particular vessel in a particular patient. This limitation of PC MRA is one of the major
impediments to more widespread application of this technique.
The information acquired during PC MRA can be processed in several ways. The most common technique is
phase difference, in which phase shifts are assigned a pixel
value and color based on thevalue of the subtracted phase
angle (Fig. 5-7). The intensity of the pixel reflects the size
of the phase shift, with the most intense pixels representing flow closest to the value of the V
. Directional
enc
information can be provided easily by displaying the pixels as either black or white; spins moving in the direction
of the velocity gradients appear white, and those moving
in the opposite direction appear black. Spins moving in a
direction at right angles to the velocity gradients have no
signal. A common artifact is the abrupt transition of one
pixel color to another that occurs when the velocity of
flow exceeds the V
, termed aliasing, and is easily recog-
enc
nized by the irregular interface between the two colors
caused by the pixel edges (Fig. 5-8).
Limitations
All MRA techniques are subject to certain limitations.
Patients with pacemakers, intraocular or intraaural metallic foreign bodies, or claustrophobia cannot undergo imaging. Most MR units carefully screen patients with both
a questionnaire and a brief interview with the technologist before scanning. Patients who are hemodynamically
unstable require careful monitoring during scanning using specialized equipment that is compatible with strong
magnetic fields. Turbulent flow (such as immediately distal to a stenosis) is difficult to image with non-gadolinium-enhanced techniques, as the spins are extremely disorganized in orientation and appear as areas of signal
loss. Lastly, uncooperative patients or those with dementia may not be able to remain still during image acquisition, introducing motion artifacts that can render a study
uninterpretable.
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