ABSTRACT
I. Introduction 287
II. Proton MR Imaging 289
A. Venous Injection of Gd-Chelates and Inhalation of
Aerosolized Gadolinium 289
B. Venous Injection of Gd-Chelates and Oxygen Inhalation 291
C. Spin Labeling MR Perfusion and Oxygen Inhalation 291
III. Hyperpolarized Gas Imaging 293
A. Perfusion Imaging Using Encapsulated Polarized Helium
and Inhalation of 3He 293
B. Venous Injection of Superparamagnetic Contrast Agent
During 3He Inhalation 293
IV. Proton Perfusion MR Imaging (Spin Labeling or Gd
Injection) and 3He Ventilation Imaging 295
V. Indirect Measure of Ventilation/Perfusion Ratio by the Assessment of Alveolar Oxygen 296
References 300
I. Introduction
The major function of the lung is to permit gas exchange between the airways and
the blood. Integral to this task is the matching of local alveolar ventilation and
pulmonary blood flow. Both lungs together receive a total of 4 L/min alveolar
ventilation and 5 L/min of blood flow, for an overall ventilation/perfusion ratio of 0.8. However, even in the normal individual, ventilation and blood flow are not
equally distributed among all alveoli. Some alveoli have more ventilation than
perfusion (increased ventilation/perfusion ratio), whereas others have more perfusion than ventilation (decreased ventilation/perfusion ratio). The changes in ventilation/perfusion ratios are small in the healthy person and are in part due to the effects of gravity on the distribution of pulmonary blood flow and venti-
lation between the base and the apex of the lung. The reason there is a regional
difference in ventilation is due to the normal gradient in pleural surface pressure
caused by gravity and interactions with the chest wall. In addition, more gravity-
dependent regions of the lung receive more blood flow per unit volume. The
intravascular pressure in the lower regions of the lung is greater because of hydro-
static effects in blood flow. Blood vessels in the more dependent portions of the
lung are, therefore, more distended leading to greater perfusion. It is important to
recognize that in many types of lung disease there are alterations in ventilation,
perfusion, and the ventilation/perfusion ratio, which can become significant. In clinical practice, ventilation and perfusion testing is used for two major
reasons: detection of pulmonary emboli and assessment of regional lung function.
Ventilation and perfusion lung scanning to assess regional lung function is often
performed before surgery involving resection of a part of the lung, usually one or
more lobes. By visualizing which areas of the lung receive ventilation and per-
fusion, the physician can determine how much the area to be resected is contri-
buting to the overall lung function. It is also possible to predict postoperative
pulmonary function, which is a guide to postoperative respiratory problems
and impairment. Ventilation and perfusion to broncho-pulmonary segments is
matched in a healthy individual. In pulmonary embolic disease, segmental
reduction in perfusion occurs with maintenance of normal ventilation. This
leads to the mismatch of perfusion and ventilation in the broncho-pulmonary
segment. In parenchymal lung disease, matched ventilation and perfusion
defects are usually observed.
